Greater mesophyll conductance and leaf photosynthesis in the field through modified cell wall porosity and thickness via AtCGR3 expression in tobacco

Mesophyll conductance (gm) describes the ease with which CO2 passes from the sub-stomatal cavities of the leaf to the primary carboxylase of photosynthesis, Rubisco. Increasing gm is suggested as a means to engineer increases in photosynthesis by increasing [CO2] at Rubisco, inhibiting oxygenation and accelerating carboxylation. Here, tobacco was transgenically up-regulated with Arabidopsis Cotton Golgi-related 3 (CGR3), a gene controlling methylesterification of pectin, as a strategy to increase CO2 diffusion across the cell wall and thereby increase gm. Across three independent events in tobacco strongly expressing AtCGR3, mesophyll cell wall thickness was decreased by 7%-13%, wall porosity increased by 75% and gm measured by carbon isotope discrimination increased by 28%. Importantly, field-grown plants showed an average 8% increase in leaf photosynthetic CO2 uptake. Up-regulating CGR3 provides a new strategy for increasing gm in dicotyledonous crops, leading to higher CO2 assimilation and a potential means to sustainable crop yield improvement.

Agrivoltaics mitigate drought effects in winter wheat

Climate change is expected to decrease water availability in many agricultural production areas around the globe. At the same time renewable energy concepts such as agrivoltaics (AV) are necessary to manage the energy transition. Several studies showed that evapotranspiration can be reduced in AV systems, resulting in increased water availability for crops. However, effects on crop performance and productivity remain unclear to date. Carbon-13 isotopic composition (δ13 C and discrimination against carbon-13) can be used as a proxy for the effects of water availability on plant performance, integrating crop responses over the entire growing season. The aim of this study was to assess these effects via carbon isotopic composition in grains, as well as grain yield of winter wheat in an AV system in southwest Germany. Crops were cultivated over four seasons from 2016-2020 in the AV system and on an unshaded adjacent reference (REF) site. Across all seasons, average grain yield did not significantly differ between AV and REF (4.7 vs 5.2 t ha-1 ), with higher interannual yield stability in the AV system. However, δ13 C as well as carbon-13 isotope discrimination differed significantly across the seasons by 1‰ (AV: -29.0‰ vs REF: -28.0‰ and AV: 21.6‰ vs REF: 20.6‰) between the AV system and the REF site. These drought mitigation effects as indicated by the results of this study will become crucial for the resilience of agricultural production in the near future when drought events will become significantly more frequent and severe.

Water deficit and potassium affect carbon isotope composition in cassava bulk leaf material and extracted carbohydrates

Cassava (Manihot esculenta Crantz) is an important root crop, which despite its drought tolerance suffers considerable yield losses under water deficit. One strategy to increase crop yields under water deficit is improving the crop's transpiration efficiency, which could be achieved by variety selection and potassium application. We assessed carbon isotope composition in bulk leaf material and extracted carbohydrates (soluble sugar, starch, and cellulose) of selected leaves one month after inducing water deficit to estimate transpiration efficiency and storage root biomass under varying conditions in a greenhouse experiment. A local and improved variety were grown in sand, supplied with nutrient solution with two potassium levels (1.44 vs. 0.04 mM K+) and were subjected to water deficit five months after planting. Potassium application and selection of the improved variety both increased transpiration efficiency of the roots with 58% and 85% respectively. Only in the improved variety were 13C ratios affected by potassium application (up to - 1.8‰ in δ13C of soluble sugar) and water deficit (up to + 0.6‰ in δ13C of starch and soluble sugar). These data revealed a shift in substrate away from transitory starch for cellulose synthesis in young leaves of the improved variety under potassium deficit. Bulk δ13C of leaves that had fully developed prior to water deficit were the best proxies for storage root biomass (r = - 0.62, r = - 0.70) and transpiration efficiency (r = - 0.68, r = - 0.58) for the local and improved variety respectively, making laborious extractions redundant. Results obtained from the youngest fully developed leaf, commonly used as a diagnostic leaf, were complicated by remobilized assimilates in the improved variety, making them less suitable for carbon isotope analysis. This study highlights the potential of carbon isotope composition to assess transpiration efficiency and yield, depending on the chosen sampling strategy as well as to unravel carbon allocation processes.

Addressing the Relationship between Leaf Nitrogen and Carbon Isotope Discrimination from the Three Levels of Community, Population and Individual

The carbon, nitrogen and water cycles of terrestrial ecosystems are important biogeochemical cycles. Addressing the relationship of leaf nitrogen (N) and carbon isotope discrimination (Δ) will enhance the understanding of the links between these three cycles in plant leaves because Δ can reflect time-integrated leaf-level water-use efficiency (WUE) over the period when the leaf material is produced. Previous studies have paid considerable attention to the relationship. However, these studies have not effectively eliminated the interference of environmental factors, inter-species, and inter-individual differences in this relationship, so new research is necessary. To minimize these interferences, the present work explored the relationship at the three levels of community, population, and plant individual. Three patterns of positive, negative and no relationship were observed across communities, populations, and individuals, which is dependent on environmental conditions, species, and plant individuals. The results strongly suggested that there is no general pattern for the relationship between leaf N and Δ. Furthermore, the results indicated that there is often no coupling between leaf-level long-term WUE and leaf N in the metabolic process of carbon, N and water in leaves. The main reason for the lack of this relationship is that most plants do not invest large amounts of nitrogen into photosynthesis. In addition, the present study also observed that, for most plant species, leaf N was not related to photosynthetic rate, and that variations in photosynthetic rates are mainly driven by stomatal conductance.

Domestication has reduced leaf water use efficiency associated with the anatomy of abaxial stomata in cotton

Crop domestication for increasing growth rates and yields appears to have altered the features of adaxial and abaxial stomata, but its effect on leaf water use efficiency (WUE) have not been experimentally verified. In this study, we characterized stomatal anatomy and carbon isotope discrimination (δ13C) in 32 wild and 36 domesticated genotypes of cotton grown under agricultural field conditions. The results showed that domesticated genotypes possessed lower WUE, as indicated by low or more negative δ13C compared with wild genotypes. Higher theoretical maximum stomatal conductance (gsmax) after domestication was accounted for by more stomata rather than significantly enlarged stomata. Specifically, abaxial stomatal density was higher whilst there was no change in the adaxial density. The size of both adaxial and abaxial stomata was greater due to larger guard cells but without there being any increase in pore size. However, there was a negative relationship between δ13C and stomatal size across wild and domesticated genotypes, especially on the abaxial leaf surface, because bigger stomata resulted in a lower maximum stomatal response rate to fluctuating canopy light, resulting in increased water loss. Overall, our results indicate that cotton domestication has resulted in substantial variation in stomatal anatomy, and that WUE and drought tolerance can potentially be improved in future breeding by decreasing the size of abaxial stomata to produce a faster stomatal response and hence a reduction in unnecessary water loss.

Differences in heat tolerance, water use efficiency and growth among Douglas-fir families and varieties evidenced by GWAS and common garden studies

Severe and frequent heat and drought events challenge the survival and development of long-generation trees. In this study, we investigated the genomic basis of heat tolerance, water use efficiency and growth by performing genome-wide association studies in coastal Douglas-fir (Pseudotsuga menziesii) and intervarietal (menziesii × glauca) hybrid seedlings. GWAS results identified 32 candidate genes involved in primary and secondary metabolism, abiotic stress and signaling, among other functions. Water use efficiency (inferred from carbon isotope discrimination), photosynthetic capacity (inferred from %N), height and heat tolerance (inferred from electrolyte leakage in a heat stress experiment) were significantly different among Douglas-fir families and varieties. High-elevation seed sources had increased water use efficiency, which could be a result of higher photosynthetic capacity. Similarly, families with greater heat tolerance also had higher water use efficiency and slower growth, suggesting a conservative growth strategy. Intervarietal hybrids showed increased heat tolerance (lower electrolyte leakage at 50 and 55 °C) and higher water use efficiency compared with coastal families, suggesting that hybridization might be a source of pre-adapted alleles to warming climates and should be considered for large-scale reforestation projects under increasingly arid conditions.

Estimation of intrinsic water-use efficiency from δ13C signature of C3 leaves: Assumptions and uncertainty

Carbon isotope composition (δ¹³C) has been widely used to estimate the intrinsic water-use efficiency (iWUE) of plants in ecosystems around the world, providing an ultimate record of the functional response of plants to climate change. This approach relies on established relationships between leaf gas exchange and isotopic discrimination, which are reflected in different formulations of ¹³C-based iWUE models. In the current literature, most studies have utilized the simple, linear equation of photosynthetic discrimination to estimate iWUE. However, recent studies demonstrated that using this linear model for quantitative studies of iWUE could be problematic. Despite these advances, there is a scarcity of review papers that have comprehensively reviewed the theoretical basis, assumptions, and uncertainty of ¹³C-based iWUE models. Here, we 1) present the theoretical basis of ¹³C-based iWUE models: the classical model (iWUE_sim), the comprehensive model (iWUE_com), and the model incorporating mesophyll conductance (iWUE_mes); 2) discuss the limitations of the widely used iWUE_sim model; 3) and make suggestions on the application of the iWUE_mes model. Finally, we suggest that a mechanistic understanding of mesophyll conductance associated effects and post-photosynthetic fractionation are the bottlenecks for improving the ¹³C-based estimation of iWUE.

A pleiotropic QTL increased economic water use efficiency in bread wheat (Triticum aestivum L.)

Wheat is one of the most important food crops in the world and drought can severely impact on wheat productivity. The identification and deployment of genes for improved water use efficiency (WUE) can help alleviate yield loss under water limitation. In this study, a high-density genetic linkage map of wheat recombinant inbred lines (Ningchun 4 x Ningchun 27) containing 8751 specific locus amplified fragment (SLAF) tags (including 14757 SNPs), with a total map distance of 1685 cM and an average inter-marker map distance of 0.19 cM was constructed by SLAF-seq technology. The economic yield WUE and nine related traits under three water treatments was monitored over four years. The results showed that loci conditioning WUE were also associated with grain carbon isotope discrimination (CID), flag leaf chlorophyll content, plant height, 1000-grain weight, grain weight per spike and grain number per spike. One locus on chromosome 2B explained 26.3% WUE variation in multiple environments. Under good soil moisture conditions before flowering, the high CID genotype QWue.acn-2B ^{Ningchun 27}, was associated with WUE, high grain weight per spike, and kilo-grain weight. Under rain-fed conditions, the low CID genotype QWue.acn-2B ^{Ningchun 4} tended to maintain more spike number and was associated with improved WUE and yield. The introduction of good chromosome fragments of QWue.acn-2B into elite lines by molecular marker assisted selection will boost up the cultivation of high-yield and water-saving wheat varieties.

Spatio-Temporal Variations in Carbon Isotope Discrimination Predicted by the JULES Land Surface Model

Stable carbon isotopes in plants can help evaluate and improve the representation of carbon and water cycles in land-surface models, increasing confidence in projections of vegetation response to climate change. Here, we evaluated the predictive skills of the Joint UK Land Environmental Simulator (JULES) to capture spatio-temporal variations in carbon isotope discrimination (Δ¹³C) reconstructed by tree rings at 12 sites in the United Kingdom over the period 1979-2016. Modeled and measured Δ¹³C time series were compared at each site and their relationships with local climate investigated. Modeled Δ¹³C time series were significantly correlated (p < 0.05) with tree-ring Δ¹³C at eight sites, but JULES underestimated mean Δ¹³C values at all sites, by up to 2.6‰. Differences in mean Δ¹³C may result from post-photosynthetic isotopic fractionations that were not considered in JULES. Inter-annual variability in Δ¹³C was also underestimated by JULES at all sites. While modeled Δ¹³C typically increased over time across the UK, tree-ring Δ¹³C values increased only at five sites located in the northern regions but decreased at the southern-most sites. Considering all sites together, JULES captured the overall influence of environmental drivers on Δ¹³C but failed to capture the direction of change in Δ¹³C caused by air temperature, atmospheric CO₂ and vapor pressure deficit at some sites. Results indicate that the representation of carbon-water coupling in JULES could be improved to reproduce both the trend and magnitude of interannual variability in isotopic records, the influence of local climate on Δ¹³C, and to reduce uncertainties in predicting vegetation-environment interactions.

isocalcR: An R package to streamline and standardize stable isotope calculations in ecological research

The use of stable isotopes to characterize ecosystem dynamics and infer leaf gas exchange processes has become increasingly prevalent over the last few decades within the ecological community. While advancements in theory and our understanding of the physiological processes controlling isotopic signatures in plants has been well-documented, no standardized tool currently exists to facilitate the computation of common isotope-derived plant physiological indices. Here, we present isocalcR, an R package intended to facilitate the use of stable isotope data from plant tissues by providing an integrated collection of functions and recommended reference data. The isocalcR R package contains a suite of functions that compute leaf carbon isotope discrimination (∆¹³ C), leaf intercellular [CO₂ ], the ratio of leaf intercellular to atmospheric [CO₂ ], the difference between atmospheric and leaf intercellular [CO₂ ], and intrinsic water use efficiency from carbon isotope signatures in leaf or wood tissue with minimal inputs from the user. isocalcR also implements and provides recommended input atmospheric [CO₂ ] (ppm) and atmospheric δ¹³ CO₂ (‰) data for the period 0-2021 C.E. A major goal of isocalcR is to provide a standardized, open-source tool to streamline the calculation of reproducible physiological indices from stable isotope signatures in plant tissues, incorporating the most up-to-date theory, while simultaneously eliminating potential errors associated with complex calculations. isocalcR can be used for any location globally as long as the user provides information regarding temperature and elevation to the main workhorse functions.

Increased bundle-sheath leakiness of CO2 during photosynthetic induction shows a lack of coordination between the C4 and C3 cycles

Use of a complete dynamic model of NADP-malic enzyme C₄ photosynthesis indicated that, during transitions from dark or shade to high light, induction of the C₄ pathway was more rapid than that of C₃ , resulting in a predicted transient increase in bundle-sheath CO₂ leakiness (ϕ). Previously, ϕ has been measured at steady state; here we developed a new method, coupling a tunable diode laser absorption spectroscope with a gas-exchange system to track ϕ in sorghum and maize through the nonsteady-state condition of photosynthetic induction. In both species, ϕ showed a transient increase to > 0.35 before declining to a steady state of 0.2 by 1500 s after illumination. Average ϕ was 60% higher than at steady state over the first 600 s of induction and 30% higher over the first 1500 s. The transient increase in ϕ, which was consistent with model prediction, indicated that capacity to assimilate CO₂ into the C₃ cycle in the bundle sheath failed to keep pace with the rate of dicarboxylate delivery by the C₄ cycle. Because nonsteady-state light conditions are the norm in field canopies, the results suggest that ϕ in these major crops in the field is significantly higher and energy conversion efficiency lower than previous measured values under steady-state conditions.

Unraveling the Physiological Mechanisms Underlying the Intracultivar Variability of Water Use Efficiency in Vitis vinifera "Grenache"

Selecting genotypes with a better capacity to respond and adapt to soil water deficits is essential to achieve the sustainability of grapevine cultivation in the context of increasing water scarcity. However, cultivar changes are very poorly accepted, and therefore it is particularly interesting to explore the intracultivar genetic diversity in water use efficiency (WUE). In previous studies, the cultivar "Grenache" has shown up to 30% variability in WUE. This research aimed to confirm the intracultivar variability and to elucidate the traits underlying this variability in the response to a water deficit by analyzing the growth rates, water relations, osmotic potential, leaf morphology, leaf gas exchange and carbon isotope discrimination in nine "Grenache" genotypes grown in pots during two seasons. The results showed lower differences in WUE and carbon isotope ratio than in previous field studies, but fairly good consistency in genotype ranking. Leaf mass area and osmotic potential did not underlie differences in stem water potential and in stomatal conductance. Overall, stomatal regulation and photosynthetic capacity seem to underlie differences in WUE among genotypes with an important environmental influence. These results confirm the ability to select clones with higher WUE and present an opportunity for the genetic improvement of WUE in grapevines.

The Potential of Low Phytic Acid1-1 Mutant in Maize (Zea mays L.): A Sustainable Solution to Non-Renewable Phosphorus

BACKGROUND

Phosphorus is an essential component of fertilizers and feed and in recent decades has become one of the main sustainability issues as a non-renewable resource. In plant seeds, the main reserve of phosphorus is phytic acid, a strong anti-nutritional factor for monogastrics and a pollutant of cultivated lands. The reduction of phytic acid in cereal seeds has become a major challenge in breeding programs to increase the nutritional quality of foods and feeds and to improve the environmental phosphorus sustainability in agriculture. In maize (Zea mays L.), four low phytic acid (lpa) mutations have been isolated and lpa1-1 is the most promising. However, the reduction of phytic acid in lpa1-1 leads to many adverse pleiotropic effects on the seed and in general on plant performance. A seed weight reduction and a consequent yield loss were previously described in this mutant.

METHOD

In this work, a field experiment to study seed weight and yield was conducted for two years in two different genetic backgrounds (B73 and B73/Mo17). Furthermore, the greater susceptibility of lpa1-1 to drought stress was also investigated: a dedicated field experiment was set up and measurements were carried out under optimal water conditions and moderate drought stress.

RESULTS

From the first experiment it emerges that under high-input conditions, lpa1-1 seems to have comparable or even better yield than the relative control. The main problem of this mutant remains the reduced field emergence (~40%). In the study of drought stress it was found that the increased sensitivity in the mutant is mainly caused by an altered stomatal regulation, but not by a less developed root system, as previously reported. When the stress occurred, the parameters measured did not significantly change in the wild-type, while they dropped in the mutant: the net photosynthesis decreased by 58%, the transpiration rate by 63% and the stomatal conductance by 67%.

CONCLUSIONS

Some possible solutions have been proposed, with the aim of developing a commercial variety, which remains the main goal to exploit the nutritional benefits of low phytic acid mutants.

Multienvironment QTL analysis delineates a major locus associated with homoeologous exchanges for water-use efficiency and seed yield in canola

Canola varieties exhibit variation in drought avoidance and drought escape traits, reflecting adaptation to water-deficit environments. Our understanding of underlying genes and their interaction across environments in improving crop productivity is limited. A doubled haploid population was analysed to identify quantitative trait loci (QTL) associated with water-use efficiency (WUE) related traits. High WUE in the vegetative phase was associated with low seed yield. Based on the resequenced parental genome data, we developed sequence-capture-based markers and validated their linkage with carbon isotope discrimination (Δ¹³ C) in an F₂ population. RNA sequencing was performed to determine the expression of candidate genes underlying Δ¹³ C QTL. QTL contributing to main and QTL × environment interaction effects for Δ¹³ C and yield were identified. One multiple-trait QTL for Δ¹³ C, days to flower, plant height, and seed yield was identified on chromosome A09. Interestingly, this QTL region overlapped with a homoeologous exchange (HE) event, suggesting its association with the multiple traits. Transcriptome analysis revealed 121 significantly differentially expressed genes underlying Δ¹³ C QTL on A09 and C09, including in HE regions. Sorting out the negative relationship between vegetative WUE and seed yield is a priority. Genetic and genomic resources and knowledge so developed could improve canola WUE and yield.

Effects of Light Spectral Quality on Photosynthetic Activity, Biomass Production, and Carbon Isotope Fractionation in Lettuce, Lactuca sativa L., Plants

The optimization of plant-specific LED lighting protocols for indoor plant growing systems needs both basic and applied research. Experiments with lettuce, Lactuca sativa L., plants using artificial lighting based on narrow-band LEDs were carried out in a controlled environment. We investigated plant responses to the exclusion of certain spectral ranges of light in the region of photosynthetically active radiation (PAR); in comparison, the responses to quasimonochromatic radiation in the red and blue regions were studied separately. The data on plant phenotyping, photosynthetic activity determination, and PAM fluorometry, indicating plant functional activity and stress responses to anomalous light environments, are presented. The study on carbon isotopic composition of photoassimilates in the diel cycle made it possible to characterize the balance of carboxylation and photorespiration processes in the leaves, using a previously developed oscillatory model of photosynthesis. Thus, the share of plant photorespiration (related to plant biomass enrichment with ¹³C) increased in response to red-light action, while blue light accelerated carboxylation (related to ¹²C enrichment). Blue light also reduced water use efficiency. These data are supported by the observations from the light environments missing distinct PAR spectrum regions. The fact that light of different wavelengths affects the isotopic composition of total carbon allowed us to elucidate the nature of its action on the organization of plant metabolism.

Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event

Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought-related mortality derived from measurements of tree-ring growth (ring width index; RWI) and carbon isotope discrimination (∆¹³ C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012-2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆¹³ C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆¹³ C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine-dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management-particularly in drier regions-may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.

Global decadal variability of plant carbon isotope discrimination and its link to gross primary production

Carbon isotope discrimination (Δ¹³ C) in C₃ woody plants is a key variable for the study of photosynthesis. Yet how Δ¹³ C varies at decadal scales, and across regions, and how it is related to gross primary production (GPP), are still incompletely understood. Here we address these questions by implementing a new Δ¹³ C modelling capability in the land-surface model JULES incorporating both photorespiratory and mesophyll-conductance fractionations. We test the ability of four leaf-internal CO₂ concentration models embedded in JULES to reproduce leaf and tree-ring (TR) carbon isotopic data. We show that all the tested models tend to overestimate average Δ¹³ C values, and to underestimate interannual variability in Δ¹³ C. This is likely because they ignore the effects of soil water stress on stomatal behavior. Variations in post-photosynthetic isotopic fractionations across species, sites and years, may also partly explain the discrepancies between predicted and TR-derived Δ¹³ C values. Nonetheless, the "least-cost" (Prentice) model shows the lowest biases with the isotopic measurements, and lead to improved predictions of canopy-level carbon and water fluxes. Overall, modelled Δ¹³ C trends vary strongly between regions during the recent (1979-2016) historical period but stay nearly constant when averaged over the globe. Photorespiratory and mesophyll effects modulate the simulated global Δ¹³ C trend by 0.0015 ± 0.005‰ and -0.0006 ± 0.001‰ ppm^-1 , respectively. These predictions contrast with previous findings based on atmospheric carbon isotope measurements. Predicted Δ¹³ C and GPP tend to be negatively correlated in wet-humid and cold regions, and in tropical African forests, but positively related elsewhere. The negative correlation between Δ¹³ C and GPP is partly due to the strong dominant influences of temperature on GPP and vapor pressure deficit on Δ¹³ C in those forests. Our results demonstrate that the combined analysis of Δ¹³ C and GPP can help understand the drivers of photosynthesis changes in different climatic regions.

Genome-wide association identifies candidate genes for drought tolerance in coast redwood and giant sequoia

Drought is a major limitation for survival and growth in plants. With more frequent and severe drought episodes occurring due to climate change, it is imperative to understand the genomic and physiological basis of drought tolerance to be able to predict how species will respond in the future. In this study, univariate and multitrait multivariate genome-wide association study methods were used to identify candidate genes in two iconic and ecosystem-dominating species of the western USA, coast redwood and giant sequoia, using 10 drought-related physiological and anatomical traits and genome-wide sequence-capture single nucleotide polymorphisms. Population-level phenotypic variation was found in carbon isotope discrimination, osmotic pressure at full turgor, xylem hydraulic diameter, and total area of transporting fibers in both species. Our study identified new 78 new marker × trait associations in coast redwood and six in giant sequoia, with genes involved in a range of metabolic, stress, and signaling pathways, among other functions. This study contributes to a better understanding of the genomic basis of drought tolerance in long-generation conifers and helps guide current and future conservation efforts in the species.

Sensitivity and threshold dynamics of Pinus strobus and Quercus spp. in response to experimental and naturally occurring severe droughts

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (Js), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016-17). Results showed that Js in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15 m3 m-3, whereas oak's (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m3 m-3-a finding consistent with pine's more isohydric strategy. Nevertheless, once oaks' moisture threshold was surpassed, Js declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks' growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species' contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

Azospirillum baldaniorum Sp245 Induces Physiological Responses to Alleviate the Adverse Effects of Drought Stress in Purple Basil

Azospirillum spp. are plant growth-promoting rhizobacteria (PGPR) that exert beneficial effects on plant growth and yield of agronomically important plant species. The aim of this study was to investigate the effects of a root treatment with Azospirillum baldaniorum Sp245 on hormones in xylem sap and physiological performance in purple basil (Ocimum basilicum L. cv. Red Rubin) plants grown under well-watered conditions and after removing water. Treatments with A. baldaniorum Sp245 included inoculation with viable cells (1·10⁷ CFU mL^–1) and addition of two doses of filtered culture supernatants (non-diluted 1·10⁸ CFU mL^–1, and diluted 1:1). Photosynthetic activity, endogenous level of hormones in xylem sap (salicylic acid, jasmonic acid, and abscisic acid), leaf pigments, leaf water potential, water-use efficiency (WUE), and drought tolerance were determined. Fluorescence and gas exchange parameters, as well as leaf water potential, showed that the highest dose of filtered culture supernatant improved both photosynthetic performance and leaf water status during water removal, associated with an increase in total pigments. Moreover, gas exchange analysis and carbon isotope discrimination found this bacterial treatment to be the most effective in inducing an increase of intrinsic and instantaneous WUE during water stress. We hypothesize that the benefits of bacterial treatments based on A. baldaniorum Sp245 are strongly correlated with the synthesis of phytohormones and the induction of plant-stress tolerance in purple basil.

CO2 diffusion in tobacco: a link between mesophyll conductance and leaf anatomy

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g_m). We investigated how g_m varies with leaf age and through a tobacco (Nicotiana tabacum) canopy by combining gas exchange and carbon isotope measurements using tunable diode laser spectroscopy. We combined these measurements with the anatomical characterization of leaves. CO₂ assimilation rate, A, and g_m decreased as leaves aged and moved lower in the canopy and were linearly correlated. This was accompanied by large anatomical changes including an increase in leaf thickness. Chloroplast surface area exposed to the intercellular airspace per unit leaf area (S_c) also decreased lower in the canopy. Older leaves had thicker mesophyll cell walls and g_m was inversely proportional to cell wall thickness. We conclude that reduced g_m of older leaves lower in the canopy was associated with a reduction in S_c and a thickening of mesophyll cell walls.

Carbon Assimilation, Isotope Discrimination, Proline and Lipid Peroxidation Contribution to Barley (Hordeum vulgare) Salinity Tolerance

Barley (Hordeum vulgare L.) exhibits great adaptability to salt tolerance in marginal environments because of its great genetic diversity. Differences in main biochemical, physiological, and molecular processes, which could explain the different tolerance to soil salinity of 16 barley varieties, were examined during a two-year field experiment. The study was conducted in a saline soil with an electrical conductivity ranging from 7.3 to 11.5 dS/m. During the experiment, a number of different physiological and biochemical characteristics were evaluated when barley was at the two- to three-nodes growing stage (BBCH code 32–33). The results indicated that there were significant (p < 0.001) effects due to varieties for tolerance to salinity. Carbon isotopes discrimination was higher by 11.8% to 16.0% in salt tolerant varieties than that in the sensitive ones. Additionally, in the tolerant varieties, assimilation rates of CO₂ and proline concentration were 200% and up to 67% higher than the sensitive varieties, respectively. However, in sensitive varieties, hydrogen peroxide and lipid peroxidation were enhanced, indicating an increased lipid peroxidation. The expression of the genes Hsdr4, HvA1, and HvTX1 did not differ among barley varieties tested. This study suggests that the increased carbon isotopes discrimination, increased proline concentration (play an osmolyte source role), and decreased lipid peroxidation are traits that are associated with barley tolerance to soil salinity. Moreover, our findings that proline improves salt tolerance by up-regulating stress-protective enzymes and reducing oxidation of lipid membranes will encourage our hypothesis that there are specific mechanisms that can be co-related with the salt sensitivity or the tolerance of barley. Therefore, further research is needed to ensure the tolerance mechanisms that exclude NaCl in salt tolerant barley varieties and diminish accumulation of lipid peroxides through adaptive plant responses.

Forest restoration treatments in a ponderosa pine forest enhance physiological activity and growth under climatic stress

As the climate warms, drought will increasingly occur under elevated temperatures, placing forest ecosystems at growing risk of extensive dieback and mortality. In some cases, increases in tree density following early 20th-century fire suppression may exacerbate this risk. Treatments designed to restore historical stand structure and enhance resistance to high-severity fire might also alleviate drought stress by reducing competition, but the duration of these effects and the underlying mechanisms remain poorly understood. To elucidate these mechanisms, we evaluate tree growth, mortality, and tree-ring stable-carbon isotope responses to stand-density reduction treatments with and without prescribed fire in a ponderosa pine forest of western Montana. Moderate and heavier cutting experiments (basal area reductions of 35% and 56%, respectively) were initiated in 1992, followed by prescribed burning in a subset of the thinned units. All treatments led to a growth release that persisted to the time of resampling. The treatments had little effect on climate-growth relationships, but they markedly altered seasonal carbon isotope signals and their relationship to climate. In burned and unburned treatments, carbon isotope discrimination (Δ¹³ C) increased in the earlywood (EW) and decreased in the latewood (LW) relative to the control. The sensitivity of LW Δ¹³ C to late-summer climate also increased in all treatments, but not in the control. Such increased sensitivity indicates that the reduction in competition enabled trees to continue to fix carbon for new stem growth, even when the climate became sufficiently stressful to stop new assimilation in slower-growing trees in untreated units. These findings would have been masked had we not separated EW and LW. The importance of faster growth and enhanced carbon assimilation under late-summer climatic stress became evident in the second decade post-treatment, when mountain pine beetle activity increased locally, and tree mortality rates in the controls of both experiments increased to more than twice those in their respective treatments. These findings highlight that, when thinning is used to restore historical forest structure or increase resistance to high-severity fire, there will likely be additional benefits of enhanced growth and physiological activity under climatic stress, and the effects may persist for more than two decades.

Spruce beetle outbreak was not driven by drought stress: Evidence from a tree-ring iso-demographic approach indicates temperatures were more important

Climate change has amplified eruptive bark beetle outbreaks over recent decades, including spruce beetle (Dendroctonus rufipennis). However, for projecting future bark beetle dynamics there is a critical lack of evidence to differentiate how outbreaks have been promoted by direct effects of warmer temperatures on beetle life cycles versus indirect effects of drought on host susceptibility. To diagnose whether drought-induced host-weakening was important to beetle attack success we used an iso-demographic approach in Engelmann spruce (Picea engelmannii) forests that experienced widespread mortality caused by spruce beetle outbreaks in the 1990s, during a prolonged drought across the central and southern Rocky Mountain region. We determined tree death date demography during this outbreak to differentiate early- and late-dying trees in stands distributed across a landscape within this larger regional mortality event. To directly test for a role of drought stress during outbreak initiation we determined whether early-dying trees had greater sensitivity of tree-ring carbon isotope discrimination (∆¹³ C) to drought compared to late-dying trees. Rather, evidence indicated the abundance and size of host trees may have modified ∆¹³ C responses to drought. ∆¹³ C sensitivity to drought did not differ among early- versus late-dying trees, which runs contrary to previously proposed links between spruce beetle outbreaks and drought. Overall, our results provide strong support for the view that irruptive spruce beetle outbreaks across North America have primarily been driven by warming-amplified beetle life cycles whereas drought-weakened host defenses appear to have been a distant secondary driver of these major disturbance events.

Using δ13C and δ18O to analyze loblolly pine (Pinus taeda L.) response to experimental drought and fertilization

Drought frequency and intensity are projected to increase throughout the southeastern USA, the natural range of loblolly pine (Pinus taeda L.), and are expected to have major ecological and economic implications. We analyzed the carbon and oxygen isotopic compositions in tree ring cellulose of loblolly pine in a factorial drought (~30% throughfall reduction) and fertilization experiment, supplemented with trunk sap flow, allometry and microclimate data. We then simulated leaf temperature and applied a multi-dimensional sensitivity analysis to interpret the changes in the oxygen isotope data. This analysis found that the observed changes in tree ring cellulose could only be accounted for by inferring a change in the isotopic composition of the source water, indicating that the drought treatment increased the uptake of stored moisture from earlier precipitation events. The drought treatment also increased intrinsic water-use efficiency, but had no effect on growth, indicating that photosynthesis remained relatively unaffected despite 19% decrease in canopy conductance. In contrast, fertilization increased growth, but had no effect on the isotopic composition of tree ring cellulose, indicating that the fertilizer gains in biomass were attributable to greater leaf area and not to changes in leaf-level gas exchange. The multi-dimensional sensitivity analysis explored model behavior under different scenarios, highlighting the importance of explicit consideration of leaf temperature in the oxygen isotope discrimination (Δ18Oc) simulation and is expected to expand the inference space of the Δ18Oc models for plant ecophysiological studies.

Fire deficits have increased drought sensitivity in dry conifer forests: Fire frequency and tree-ring carbon isotope evidence from Central Oregon

A century of fire suppression across the Western United States has led to more crowded forests and increased competition for resources. Studies of forest thinning or stand conditions after mortality events have provided indirect evidence for how competition can promote drought stress and predispose forests to severe fire and/or bark beetle outbreaks. Here, we demonstrate linkages between fire deficits and increasing drought stress through analyses of annually resolved tree-ring growth, fire scars, and carbon isotope discrimination (Δ¹³ C) across a dry mixed-conifer forest landscape. Fire deficits across the study area have increased the sensitivity of leaf gas exchange to drought stress over the past >100 years. Since 1910, stand basal area in these forests has more than doubled and fire-return intervals have increased from 25 to 140 years. Meanwhile, the portion of interannual variation in tree-ring Δ¹³ C explained by the Palmer Drought Severity Index has more than doubled in ca. 300-500-year-old Pinus ponderosa as well as in fire-intolerant, ca. 90-190-year-old Abies grandis. Drought stress has increased in stands with a basal area of ≥25 m² /ha in 1910, as indicated by negative temporal Δ¹³ C trends, whereas stands with basal area ≤25 m² /ha in 1910, due to frequent or intense wildfire activity in decades beforehand, were initially buffered from increased drought stress and have benefited more from rising ambient carbon dioxide concentrations, [CO₂ ], as demonstrated by positive temporal Δ¹³ C trends. Furthermore, the average Δ¹³ C response across all P. ponderosa since 1830 indicates that photosynthetic assimilation rates and stomatal conductance have been reduced by ~10% and ~20%, respectively, compared to expected trends due to increasing [CO₂ ]. Although disturbance legacies contribute to local-scale intensity of drought stress, fire deficits have reduced drought resistance of mixed-conifer forests and made them more susceptible to challenges by pests and pathogens and other disturbances.

The validity of carbon isotope discrimination as a screening criterion for grain yield in two barley landraces under deficit irrigation with saline water in southern Tunisia

Arid and semiarid regions with rain shortage and scarce good quality water must make use of low-quality water for irrigation. Consequently, improved plant cultivars for use in these areas should show adaptation capacities to confer drought and salt resistance and allow the cultivation under limited water availabiltiy. The present study was conducted to determine the effect of deficit irrigation with saline water on two local barley landraces, "Karkeni" and "Bengardeni". Plants were saline-irrigated with three watering regimes during tillering, heading, and grain filling stages. Biochemical traits, carbon isotope discrimination (Δ13C), mineral composition, grain yield (GY) and water use efficiency based on grain yield (WUEgy) were evaluated as performance indicators. Almost all of the studied traits (e.g. soluble carbohydrates, proline, ∆13C, Na concentration, and GY) were significantly affected by deficient saline-irrigation regimes at different growth stages. The hierarchical clustering analysis clearly showed that Δ13C placed very close to GY averaging two barley landraces, which was in accordance with the scatter plot result. Multiple linear regression performed between GY as the dependent variable and other traits studied as the independent variables indicated that WUEgy, Δ13C, and soluble carbohydrates significantly explained the variability in GY (R 2=95.64%). A significant positive correlation that observed between ∆13C and GY at three growth stages, indicated that ∆13C may be an important proxy component for indirect selection of yield potential in barley under deficient irrigation regimes with saline water. According to our result, "Karkeni" seems to be more efficient in terms of higher GY, WUEgy, proline and carbohydrate contents, K, Mg and Zn concentrations, as well as lower Δ13C and lipid peroxidation as compared with "Bengardeni", under low osmotic potential imposed by deficient irrigation treatments with saline water, "Karkeni" can thus be selected and used as a parent in order to obtain more tolerant plants against such stresses in future breeding programs.

Cell wall properties in Oryza sativa influence mesophyll CO2 conductance

Diffusion of CO₂ from the leaf intercellular air space to the site of carboxylation (g_m ) is a potential trait for increasing net rates of CO₂ assimilation (A_net ), photosynthetic efficiency, and crop productivity. Leaf anatomy plays a key role in this process; however, there are few investigations into how cell wall properties impact g_m and A_net . Online carbon isotope discrimination was used to determine g_m and A_net in Oryza sativa wild-type (WT) plants and mutants with disruptions in cell wall mixed-linkage glucan (MLG) production (CslF6 knockouts) under high- and low-light growth conditions. Cell wall thickness (T_cw ), surface area of chloroplast exposed to intercellular air spaces (S_c ), leaf dry mass per area (LMA), effective porosity, and other leaf anatomical traits were also analyzed. The g_m of CslF6 mutants decreased by 83% relative to the WT, with c. 28% of the reduction in g_m explained by S_c . Although A_net /LMA and A_net /Chl partially explained differences in A_net between genotypes, the change in cell wall properties influenced the diffusivity and availability of CO₂ . The data presented here indicate that the loss of MLG in CslF6 plants had an impact on g_m and demonstrate the importance of cell wall effective porosity and liquid path length on g_m .

Determination of leaf respiration in the light: comparison between an isotopic disequilibrium method and the Laisk method

Quantification of leaf respiration is important for understanding plant physiology and ecosystem biogeochemical processes. Leaf respiration continues in the light (R_L ) but supposedly at a lower rate than in the dark (R_Dk ). However, there is no method for direct measurement of R_L and the available methods require nonphysiological measurement conditions. A method based on isotopic disequilibrium quantified R_L (R_L13C ) and mesophyll conductance of young and old fully expanded leaves of six species. R_L13C was compared to R_L determined by the Laisk method (R_L Laisk ) on the very same leaves with a minimum time lag. R_L 13C and R_L Laisk were generally lower than R_Dk , and were not significantly affected by leaf ageing. R_L Laisk and R_L 13C were positively correlated (r²  = 0.35), and both were positively correlated with R_Dk (r²  ≥ 0.6). R_L Laisk was systematically lower than R_L 13C by 0.4 μmol m^-2  s^-1 . Using A/C_c instead of A/C_i curves, a higher photocompensation point Γ* (by 5 μmol mol^-1 ) was found but no influence on R_L Laisk estimates was observed. The results imply that the Laisk method underestimates actual R_L significantly, probably related to the measurement condition of low CO₂ and irradiance. The isotopic disequilibrium method is useful for assessing responses of R_L to irradiance and CO₂ , improving our mechanistic understanding of R_L .

Introgression of Physiological Traits for a Comprehensive Improvement of Drought Adaptation in Crop Plants

Burgeoning population growth, industrial demand, and the predicted global climate change resulting in erratic monsoon rains are expected to severely limit fresh water availability for agriculture both in irrigated and rainfed ecosystems. In order to remain food and nutrient secure, agriculture research needs to focus on devising strategies to save water in irrigated conditions and to develop superior cultivars with improved water productivity to sustain yield under rainfed conditions. Recent opinions accruing in the scientific literature strongly favor the adoption of a "trait based" crop improvement approach for increasing water productivity. Traits associated with maintenance of positive tissue turgor and maintenance of increased carbon assimilation are regarded as most relevant to improve crop growth rates under water limiting conditions and to enhance water productivity. The advent of several water saving agronomic practices notwithstanding, a genetic enhancement strategy of introgressing distinct physiological, morphological, and cellular mechanisms on to a single elite genetic background is essential for achieving a comprehensive improvement in drought adaptation in crop plants. The significant progress made in genomics, though would provide the necessary impetus, a clear understanding of the "traits" to be introgressed is the most essential need of the hour. Water uptake by a better root architecture, water conservation by preventing unproductive transpiration are crucial for maintaining positive tissue water relations. Improved carbon assimilation associated with carboxylation capacity and mesophyll conductance is important in sustaining crop growth rates under water limited conditions. Besides these major traits, we summarize the available information in literature on classifying various drought adaptive traits. We provide evidences that Water-Use Efficiency when introgressed with moderately higher transpiration, would significantly enhance growth rates and water productivity in rice through an improved photosynthetic capacity.

Water Stress Scatters Nitrogen Dilution Curves in Wheat

Nitrogen dilution curves relate a crop's critical nitrogen concentration (%N_c) to biomass (W) according to the allometric model %N_c = a W ^-b . This model has a strong theoretical foundation, and parameters a and b show little variation for well-watered crops. Here we explore the robustness of this model for water stressed crops. We established experiments to examine the combined effects of water stress, phenology, partitioning of biomass, and water-soluble carbohydrates (WSC), as driven by environment and variety, on the %N_c of wheat crops. We compared models where %N_c was plotted against biomass, growth stage and thermal time. The models were similarly scattered. Residuals of the %N_c - biomass model at anthesis were positively related to biomass, stem:biomass ratio, Δ¹³C and water supply, and negatively related to ear:biomass ratio and concentration of WSC. These are physiologically meaningful associations explaining the scatter of biomass-based dilution curves. Residuals of the thermal time model showed less consistent associations with these variables. The biomass dilution model developed for well-watered crops overestimates nitrogen deficiency of water-stressed crops, and a biomass-based model is conceptually more justified than developmental models. This has implications for diagnostic and modeling. As theory is lagging, a greater degree of empiricism might be useful to capture environmental, chiefly water, and genotype-dependent traits in the determination of critical nitrogen for diagnostic purposes. Sensitivity analysis would help to decide if scaling nitrogen dilution curves for crop water status, and genotype-dependent parameters are needed.

Effect of waterlogging on carbon isotope discrimination during photosynthesis in Larix gmelinii

Soil moisture is a major factor controlling carbon isotope discrimination (Δ¹³C), which has been demonstrated to decrease under dry conditions in many studies; however, few studies on Δ¹³C under waterlogging condition have been conducted. In this study, a pot experiment was conducted with Larix gmelinii, a major larch species in the east Siberian Taiga, to investigate the effect of waterlogging on Δ¹³C during photosynthesis. Assimilation rate and Δ¹³C_RD (instantaneous Δ¹³C calculated with Rayleigh distillation equation) decreased drastically soon after waterlogging, followed by recovery in their values, which was caused by a change in stomatal conductance. Thereafter, assimilation rate decreased gradually, whereas Δ¹³C_RD decreased more gently. These results were thought to be caused by the decrease in both stomatal conductance and carboxylation. Our results indicate that extreme wet events may cause a decrease in Δ¹³C, which is important information for detecting flooding events in the past using tree-ring isotope analyses and for studying impacts of flooding on plants in areas where waterlogging might occur.

Increased water-use efficiency translates into contrasting growth patterns of Scots pine and sessile oak at their southern distribution limits

In forests, the increase in atmospheric CO₂ concentrations (C_a ) has been related to enhanced tree growth and intrinsic water-use efficiency (iWUE). However, in drought-prone areas such as the Mediterranean Basin, it is not yet clear to what extent this "fertilizing" effect may compensate for drought-induced growth reduction. We investigated tree growth and physiological responses at five Scots pine (Pinus sylvestris L.) and five sessile oak (Quercus petraea (Matt.) Liebl.) sites located at their southernmost distribution limits in Europe for the period 1960-2012 using annually resolved tree-ring width and δ¹³ C data to track ecophysiological processes. Results indicated that all 10 natural stands significantly increased their leaf intercellular CO₂ concentration (C_i ), and consequently iWUE. Different trends in the theoretical gas-exchange scenarios as a response to increasing C_a were found: generally, C_i tended to increase proportionally to C_a , except for trees at the driest sites in which C_i remained constant. C_i from the oak sites displaying higher water availability tended to increase at a comparable rate to C_a . Multiple linear models fitted at site level to predict basal area increment (BAI) using iWUE and climatic variables better explained tree growth in pines (31.9%-71.4%) than in oak stands (15.8%-46.8%). iWUE was negatively linked to pine growth, whereas its effect on growth of oak differed across sites. Tree growth in the western and central oak stands was negatively related to iWUE, whereas BAI from the easternmost stand was positively associated with iWUE. Thus, some Q. petraea stands might have partially benefited from the "fertilizing" effect of rising C_a , whereas P. sylvestris stands due to their strict closure of stomata did not profit from increased iWUE and consequently showed in general growth reductions across sites. Additionally, the inter-annual variability of BAI and iWUE displayed a geographical polarity in the Mediterranean.

Stomatal Conductance and Morphology of Arbuscular Mycorrhizal Wheat Plants Response to Elevated CO2 and NaCl Stress

Stomata play a critical role in the regulation of gas exchange between the interior of the leaf and the exterior environment and are affected by environmental and endogenous stimuli. This study aimed to evaluate the effect of the arbuscular mycorrhizal (AM) fungus, Rhizophagus irregularis, on the stomatal behavior of wheat (Triticum aestivum L.) plants under combination with elevated CO₂ and NaCl stress. Wheat seedlings were exposed to ambient (400 ppm) or elevated (700 ppm) CO₂ concentrations and 0, 1, and 2 g kg^-1 dry soil NaCl treatments for 10 weeks. AM symbiosis increased the leaf area and stomatal density (SD) of the abaxial surface. Stomatal size and the aperture of adaxial and abaxial leaf surfaces were higher in the AM than non-AM plants under elevated CO₂ and salinity stress. AM plants showed higher stomatal conductance (g _s ) and maximum rate of g _s to water vapor (g _smax ) compared with non-AM plants. Moreover, leaf water potential (Ψ) was increased and carbon isotope discrimination (Δ¹³C) was decreased by AM colonization, and both were significantly associated with stomatal conductance. The results suggest that AM symbiosis alters stomatal morphology by changing SD and the size of the guard cells and stomatal pores, thereby improving the stomatal conductance and water relations of wheat leaves under combined elevated CO₂ and salinity stress.

Participation of Green Organs to Grain Filling in Triticum turgidum var durum Grown under Mediterranean Conditions

In wheat, flag leaf, stem, chaff and awns contribute to grain filling through photosynthesis and/or re-mobilization. Environmental and genetic effects on the relative contribution of each organ were examined by analyzing the consequences of sink-source manipulations (shading and excision) and by comparing carbon isotope discrimination (Δ) values in dry matter (at maturity) and sap (two weeks after anthesis) in six durum wheat genotypes grown in two contrasting seasons. The contribution of flag leaf, stem, chaff and awns to grain filling, estimated by sink-source manipulations, highly varied with the season. The contribution of ear photosynthesis and re-mobilization from the stem increased with post-anthesis water stress. They showed a large genetic variation that was, however, not clearly associated to morphological characteristics of ear and stem. Isotopic imprints of chaff on grain Δ were identified as a possible surrogate of the destructive and cumbersome sink-source manipulations to evaluate the contribution of carbon assimilated in ears or re-mobilized from stem. This might facilitate screening of genetic resources and allow the combining of favourable drought tolerance mechanisms in wheat.

Evaluation of Yield and Drought Using Active and Passive Spectral Sensing Systems at the Reproductive Stage in Wheat

Active and passive sensors are available for ground-based, high-throughput phenotyping in the field. However, these sensor systems have seldom been compared with respect to their determination of plant water status and water use efficiency related parameters under drought conditions. In this study, five passive and active reflectance sensors, including a hyperspectral passive sensor, an active flash sensor (AFS), the Crop Circle, and the GreenSeeker, were evaluated to assess drought-related destructive and non-destructive morphophysiological parameters (ground cover, relative leaf water content, leaf temperature, and carbon isotope discrimination of leaves and grain) and grain yield of twenty wheat (Triticum aestivum L.) cultivars. Measurements were conducted in a 2-year study, including a drought stress and a control environment under field conditions. A comparison of the active sensors at the heading, anthesis and grain-filling stages indicated that the Crop Circle provided the most significant and robust relationships with drought-related parameters (relative leaf water content and leaf and grain carbon isotope discrimination). In comparison with the passive sensor, the five water and normalized water indices (WI and NWI-1 to 4), which are only provided by the passive sensor, showed the strongest relationships with the drought stress-related parameters (r = -0.49 to -0.86) and grain yield (r = -0.88) at anthesis. This paper indicates that precision phenotyping allows the integration of water indices in breeding programs to rapidly and cost-effectively identify drought-tolerant genotypes. This is supported by the fact that grain yield and the water indices showed the same heritability under drought conditions.

Assessing Wheat Traits by Spectral Reflectance: Do We Really Need to Focus on Predicted Trait-Values or Directly Identify the Elite Genotypes Group?

Phenotyping, via remote and proximal sensing techniques, of the agronomic and physiological traits associated with yield potential and drought adaptation could contribute to improvements in breeding programs. In the present study, 384 genotypes of wheat (Triticum aestivum L.) were tested under fully irrigated (FI) and water stress (WS) conditions. The following traits were evaluated and assessed via spectral reflectance: Grain yield (GY), spikes per square meter (SM2), kernels per spike (KPS), thousand-kernel weight (TKW), chlorophyll content (SPAD), stem water soluble carbohydrate concentration and content (WSC and WSCC, respectively), carbon isotope discrimination (Δ13C), and leaf area index (LAI). The performances of spectral reflectance indices (SRIs), four regression algorithms (PCR, PLSR, ridge regression RR, and SVR), and three classification methods (PCA-LDA, PLS-DA, and kNN) were evaluated for the prediction of each trait. For the classification approaches, two classes were established for each trait: The lower 80% of the trait variability range (Class 1) and the remaining 20% (Class 2 or elite genotypes). Both the SRIs and regression methods performed better when data from FI and WS were combined. The traits that were best estimated by SRIs and regression methods were GY and Δ13C. For most traits and conditions, the estimations provided by RR and SVR were the same, or better than, those provided by the SRIs. PLS-DA showed the best performance among the categorical methods and, unlike the SRI and regression models, most traits were relatively well-classified within a specific hydric condition (FI or WS), proving that classification approach is an effective tool to be explored in future studies related to genotype selection.

An Assessment of Genetic Diversity and Drought Tolerance in Argan Tree (Argania spinosa) Populations: Potential for the Development of Improved Drought Tolerance

The argan tree (Argania spinosa) occurs in a restricted area of Southwestern Morocco characterized by low water availability and high evapotranspirative demand. Despite the adaptation of the argan tree to drought stress, the extent of the argan forest has declined markedly due to increased aridity, land use changes and the expansion of olive cultivation. The oil of the argan seed is used for cooking and as the basis for numerous cosmetics. The identification of argan tree varieties with enhanced drought tolerance may minimize the economic losses associated with the decline of the argan forest and constrain the spread of desertification. In this study we collected argan ecotypes from four contrasting habitats and grew them under identical controlled environment conditions to investigate their response to drought. Leaf gas exchange analysis indicated that the argan ecotypes showed a high degree of adaptation to drought stress, maintaining photosynthetic activity at low levels of foliar water content and co-ordinating photosynthesis, stomatal behavior and metabolism. The stomata of the argan trees were highly sensitive to increased leaf to air vapor pressure deficit, representing an adaptation to growth in an arid environment where potential evapotranspiration is high. However, despite originating in contrasting environments, the four argan ecotypes exhibited similar gas exchange characteristics under both fully irrigated and water deficit conditions. Population genetic analyses using microsatellite markers indicated a high degree of relatedness between the four ecotypes; indicative of both artificial selection and the transport of ecotypes between different provinces throughout centuries of management of the argan forest. The majority of genetic variation across the four populations (71%) was observed between individuals, suggesting that improvement of argan is possible. Phenotypic screening of physiological responses to drought may prove effective in identifying individuals and then developing varieties with enhanced drought tolerance to enable the maintenance of argan production as climate change results in more frequent and severe drought events in Northern Africa.

Basal area growth, carbon isotope discrimination, and intrinsic water use efficiency after fertilization of Douglas-fir in the Oregon Coast Range

Many hectares of intensively managed Douglas-fir (Pseudotsuga menziesii Mirb. Franco) stands in western North America are fertilized with nitrogen (N) to increase growth rates, but only about ⅔ of all stands respond. Understanding the mechanisms of response facilitates prioritization of stands for treatment. The primary objective of this study was to test the hypothesis that the short-term basal area growth response to a single application of 224 kg N ha^-1 as urea was associated with reduced stable carbon isotope discrimination (Δ¹³C) and increased intrinsic water use efficiency (iWUE) in a 20-yr-old plantation of Douglas-fir in the Oregon Coast Range, USA. Increment cores were measured to estimate earlywood, latewood, and total basal area increment over a time series from 1997 to 2015. Stable carbon isotope discrimination and iWUE were estimated using earlywood and latewood stable carbon isotope concentrations in tree-ring holocellulose starting seven years before fertilization in early 2009 and ending seven years after treatment. A highly significant (p<0.01) interaction effect between fertilization treatment and year was found for total basal area growth and earlywood basal area increment. Specifically, fertilized trees showed significant responses (p<0.05) in total basal area growth and earlywood basal area increment in the first (2009) and second (2010) growing seasons after fertilization in 2009. A marginally significant (p<0.10) fertilization effect was found for latewood basal area increment only in the first growing season after treatment. A significant treatment x year interaction was also found for Δ¹³C and iWUE in earlywood and latewood. Fertilization significantly reduced earlywood Δ¹³C and increased earlywood iWUE in the first and second growing seasons after fertilization. Only a marginally significant fertilization effect was detected for latewood Δ¹³C and iWUE in the second growing season after treatment. Previous studies of N fertilization of Douglas-fir forests have reported consistently increased growth and iWUE on low productivity sites treated with relatively high fertilization rates. This study suggested that these responses can also be observed on highly productive sites despite their lower frequency and apparently shorter duration. Other key mechanisms driving growth responses appear less important than iWUE, including an increase in LAI and shift from belowground to aboveground carbon allocation.

Bundle-sheath leakiness and intrinsic water use efficiency of a perennial C4 grass are increased at high vapour pressure deficit during growth

Bundle-sheath leakiness (ϕ) is a key parameter of the CO2-concentrating mechanism of C4 photosynthesis and is related to leaf-level intrinsic water use efficiency (WUEi). This work studied short-term dynamic responses of ϕ to alterations of atmospheric CO2 concentration in Cleistogenes squarrosa, a perennial grass, grown at high (1.6 kPa) or low (0.6 kPa) vapour pressure deficit (VPD) combined with high or low N supply in controlled environment experiments. ϕ was determined by concurrent measurements of photosynthetic gas exchange and on-line carbon isotope discrimination, using a new protocol. Growth at high VPD led to an increase of ϕ by 0.13 and a concurrent increase of WUEi by 14%, with similar effects at both N levels. ϕ responded dynamically to intercellular CO2 concentration (C i), increasing with C i Across treatments, ϕ was negatively correlated to the ratio of CO2 saturated assimilation rate to carboxylation efficiency (a proxy of the relative activities of Rubisco and phosphoenolpyruvate carboxylase) indicating that the long-term environmental effect on ϕ was related to the balance between C3 and C4 cycles. Our study revealed considerable dynamic and long-term variation in ϕ of C. squarrosa, suggesting that ϕ should be determined when carbon isotope discrimination is used to assess WUEi Also, the data indicate a trade-off between WUEi and energetic efficiency in C. squarrosa.

Physiological Traits Associated with Wheat Yield Potential and Performance under Water-Stress in a Mediterranean Environment

Different physiological traits have been proposed as key traits associated with yield potential as well as performance under water stress. The aim of this paper is to examine the genotypic variability of leaf chlorophyll, stem water-soluble carbohydrate content and carbon isotope discrimination (Δ(13)C), and their relationship with grain yield (GY) and other agronomical traits, under contrasting water conditions in a Mediterranean environment. The study was performed on a large collection of 384 wheat genotypes grown under water stress (WS, rainfed), mild water stress (MWS, deficit irrigation), and full irrigation (FI). The average GY of two growing seasons was 2.4, 4.8, and 8.9 Mg ha(-1) under WS, MWS, and FI, respectively. Chlorophyll content at anthesis was positively correlated with GY (except under FI in 2011) and the agronomical components kernels per spike (KS) and thousand kernel weight (TKW). The WSC content at anthesis (WSCCa) was negatively correlated with spikes per square meter (SM2), but positively correlated with KS and TKW under WS and FI conditions. As a consequence, the relationships between WSCCa with GY were low or not significant. Therefore, selecting for high stem WSC would not necessary lead to genotypes of GY potential. The relationship between Δ(13)C and GY was positive under FI and MWS but negative under severe WS (in 2011), indicating higher water use under yield potential and MWS conditions.

Effective Use of Water and Increased Dry Matter Partitioned to Grain Contribute to Yield of Common Bean Improved for Drought Resistance

Common bean (Phaseolus vulgaris L.) is the most important food legume in the diet of poor people in the tropics. Drought causes severe yield loss in this crop. Identification of traits associated with drought resistance contributes to improving the process of generating bean genotypes adapted to these conditions. Field studies were conducted at the International Center for Tropical Agriculture (CIAT), Palmira, Colombia, to determine the relationship between grain yield and different parameters such as effective use of water (EUW), canopy biomass, and dry partitioning indices (pod partitioning index, harvest index, and pod harvest index) in elite lines selected for drought resistance over the past decade. Carbon isotope discrimination (CID) was used for estimation of water use efficiency (WUE). The main objectives were: (i) to identify specific morpho-physiological traits that contribute to improved resistance to drought in lines developed over several cycles of breeding and that could be useful as selection criteria in breeding; and (ii) to identify genotypes with desirable traits that could serve as parents in the corresponding breeding programs. A set of 36 bean genotypes belonging to the Middle American gene pool were evaluated under field conditions with two levels of water supply (irrigated and drought) over two seasons. Eight bean lines (NCB 280, NCB 226, SEN 56, SCR 2, SCR 16, SMC 141, RCB 593, and BFS 67) were identified as resistant to drought stress. Resistance to terminal drought stress was positively associated with EUW combined with increased dry matter partitioned to pod and seed production and negatively associated with days to flowering and days to physiological maturity. Differences in genotypic response were observed between grain CID and grain yield under irrigated and drought stress. Based on phenotypic differences in CID, leaf stomatal conductance, canopy biomass, and grain yield under drought stress, the lines tested were classified into two groups, water savers and water spenders. Pod harvest index could be a useful selection criterion in breeding programs to select for drought resistance in common bean.

Physiological Traits Associated with Wheat Yield Potential and Performance under Water-Stress in a Mediterranean Environment

Different physiological traits have been proposed as key traits associated with yield potential as well as performance under water stress. The aim of this paper is to examine the genotypic variability of leaf chlorophyll, stem water-soluble carbohydrate content and carbon isotope discrimination (Δ(13)C), and their relationship with grain yield (GY) and other agronomical traits, under contrasting water conditions in a Mediterranean environment. The study was performed on a large collection of 384 wheat genotypes grown under water stress (WS, rainfed), mild water stress (MWS, deficit irrigation), and full irrigation (FI). The average GY of two growing seasons was 2.4, 4.8, and 8.9 Mg ha(-1) under WS, MWS, and FI, respectively. Chlorophyll content at anthesis was positively correlated with GY (except under FI in 2011) and the agronomical components kernels per spike (KS) and thousand kernel weight (TKW). The WSC content at anthesis (WSCCa) was negatively correlated with spikes per square meter (SM2), but positively correlated with KS and TKW under WS and FI conditions. As a consequence, the relationships between WSCCa with GY were low or not significant. Therefore, selecting for high stem WSC would not necessary lead to genotypes of GY potential. The relationship between Δ(13)C and GY was positive under FI and MWS but negative under severe WS (in 2011), indicating higher water use under yield potential and MWS conditions.

The key factor limiting plant growth in cold and humid alpine areas also plays a dominant role in plant carbon isotope discrimination

Many environmental factors affect carbon isotope discrimination in plants, yet the predominant factor influencing this process is generally assumed to be the key growth-limiting factor. However, to our knowledge this hypothesis has not been confirmed. We therefore determined the carbon isotope composition (δ(13)C) of plants growing in two cold and humid mountain regions where temperature is considered to be the key growth-limiting factor. Mean annual temperature (MAT) showed a significant impact on variation in carbon isotope discrimination value (Δ) irrespective of study area or plant functional type with either partial correlation or regression analysis, but the correlation between Δ and soil water content (SWC) was usually not significant. In multiple stepwise regression analysis, MAT was either the first or the only variable selected into the prediction model of Δ against MAT and SWC, indicating that the effect of temperature on carbon isotope discrimination was predominant. The results therefore provide evidence that the key growth-limiting factor is also crucial for plant carbon isotope discrimination. Changes in leaf morphology, water viscosity and carboxylation efficiency with temperature may be responsible for the observed positive correlation between Δ and temperature.

Comparative physiology of allopatric Populus species: geographic clines in photosynthesis, height growth, and carbon isotope discrimination in common gardens

Populus species with wide geographic ranges display strong adaptation to local environments. We studied the clinal patterns in phenology and ecophysiology in allopatric Populus species adapted to similar environments on different continents under common garden settings. As a result of climatic adaptation, both Populus tremula L. and Populus balsamifera L. display latitudinal clines in photosynthetic rates (A), whereby high-latitude trees of P. tremula had higher A compared to low-latitude trees and nearly so in P. balsamifera (p = 0.06). Stomatal conductance (g s) and chlorophyll content index (CCI) follow similar latitudinal trends. However, foliar nitrogen was positively correlated with latitude in P. balsamifera and negatively correlated in P. tremula. No significant trends in carbon isotope composition of the leaf tissue (δ(13)C) were observed for both species; but, intrinsic water-use efficiency (WUEi) was negatively correlated with the latitude of origin in P. balsamifera. In spite of intrinsically higher A, high-latitude trees in both common gardens accomplished less height gain as a result of early bud set. Thus, shoot biomass was determined by height elongation duration (HED), which was well approximated by the number of days available for free growth between bud flush and bud set. We highlight the shortcoming of unreplicated outdoor common gardens for tree improvement and the crucial role of photoperiod in limiting height growth, further complicating interpretation of other secondary effects.

Expression of functional traits during seedling establishment in two populations of Pinus ponderosa from contrasting climates

First-year tree seedlings represent a particularly vulnerable life stage and successful seedling establishment is crucial for forest regeneration. We investigated the extent to which Pinus ponderosa P. & C. Lawson populations from different climate zones exhibit differential expression of functional traits that may facilitate their establishment. Seeds from two populations from sites with contrasting precipitation and temperature regimes east (PIPO dry) and west (PIPO mesic) of the Oregon Cascade mountains were sown in a common garden experiment and grown under two water availability treatments (control and drought). Aboveground biomass accumulation, vegetative phenology, xylem anatomy, plant hydraulic architecture, foliar stable carbon isotope ratios (δ(13)C), gas exchange and leaf water relations characteristics were measured. No treatment or population-related differences in leaf water potential were detected. At the end of the first growing season, aboveground biomass was 74 and 44% greater in PIPO mesic in the control and drought treatments, respectively. By early October, 73% of PIPO dry seedlings had formed dormant buds compared with only 15% of PIPO mesic seedlings. Stem theoretical specific conductivity, calculated from tracheid dimensions and packing density, declined from June through September and was nearly twice as high in PIPO mesic during most of the growing season, consistent with measured values of specific conductivity. Intrinsic water-use efficiency based on δ(13)C values was higher in PIPO dry seedlings for both treatments across all sampling dates. There was a negative relationship between values of δ(13)C and leaf-specific hydraulic conductivity across populations and treatments, consistent with greater stomatal constraints on gas exchange with declining seedling hydraulic capacity. Integrated growing season assimilation and stomatal conductance estimated from foliar δ(13)C values and photosynthetic CO2-response curves were 6 and 28% lower, respectively, in PIPO dry seedlings. Leaf water potential at the turgor loss point was 0.33 MPa more negative in PIPO dry, independent of treatment. Overall, PIPO dry seedlings exhibited more conservative behavior, suggesting reduced growth is traded off for increased resistance to drought and extreme temperatures.

Temperature responses of mesophyll conductance differ greatly between species

The temperature responses of mesophyll conductance (gm ) were investigated for nine species using carbon isotope techniques combining tunable diode laser spectroscopy and gas exchange measurements. Species included the evergreen trees Eucalyptus pauciflora and Quercus engelmannii; the tropical evergreen tree Lophostemon confertus; as well as the herbaceous species Nicotiana tabacum, Oryza sativa, Triticum aestivum, Gossypium hirsutum, Glycine max and Arabidopsis thaliana. Responses varied from a two- to threefold increase in mesophyll conductance between 15 and 40 °C observed for N. tabacum, G. hirsutum, G. max and E. pauciflora to almost no change in L. confertus and T. aestivum. To account for the different temperature responses between species, we suggest that there must be variation in both the activation energy for membrane permeability and the effective pathlength for liquid phase diffusion. Stomatal conductance was relatively independent of increases in leaf temperature and concomitant increases in leaf to air vapour pressure difference. Two exceptions were Eucalyptus and Gossypium, where stomatal conductance increased with temperature up to 35 °C despite increasing leaf to air vapour pressure. For a given species, temperature responses of stomatal and mesophyll conductance were independent of one another.

Carbon isotope compositions (δ(13) C) of leaf, wood and holocellulose differ among genotypes of poplar and between previous land uses in a short-rotation biomass plantation

The efficiency of water use to produce biomass is a key trait in designing sustainable bioenergy-devoted systems. We characterized variations in the carbon isotope composition (δ(13) C) of leaves, current year wood and holocellulose (as proxies for water use efficiency, WUE) among six poplar genotypes in a short-rotation plantation. Values of δ(13) Cwood and δ(13) Cholocellulose were tightly and positively correlated, but the offset varied significantly among genotypes (0.79-1.01‰). Leaf phenology was strongly correlated with δ(13) C, and genotypes with a longer growing season showed a higher WUE. In contrast, traits related to growth and carbon uptake were poorly linked to δ(13) C. Trees growing on former pasture with higher N-availability displayed higher δ(13) C as compared with trees growing on former cropland. The positive relationships between δ(13) Cleaf and leaf N suggested that spatial variations in WUE over the plantation were mainly driven by an N-related effect on photosynthetic capacities. The very coherent genotype ranking obtained with δ(13) C in the different tree compartments has some practical outreach. Because WUE remains largely uncoupled from growth in poplar plantations, there is potential to identify genotypes with satisfactory growth and higher WUE.