Biochar addition and reduced irrigation modulates leaf morpho-physiology and biological nitrogen fixation in faba bean-ryegrass intercropping

Intercropping legume with grass has potential to increase biomass and protein yield via biological N2-fixation (BNF) benefits, whereas the joint effects of biochar (BC) coupled with deficit irrigation on intercropping systems remain elusive. A 15N isotope-labelled experiment was implemented to investigate morpho-physiological responses of faba bean-ryegrass intercrops on low- (550 °C, LTBC) or high-temperature BC (800 °C, HTBC) amended sandy-loam soil under full (FI), deficit (DI) and partial root-zone drying irrigation (PRD). LTBC and HTBC significantly reduced intrinsic water-use efficiency (WUE) by 12 and 14 %, and instantaneous WUE by 8 and 16 %, respectively, in faba bean leaves, despite improved photosynthetic (An) and transpiration rate (Tr), and stomatal conductance (gs). Compared to FI, DI and PRD lowered faba bean An, gs and Tr, but enhanced leaf-scale and time-integrated WUE as proxied by the diminished shoots Δ13C. PRD enhanced WUE as lower gs, Tr and guard cell length than DI-plants. Despite higher carbon ([C]) and N concentration ([N]) in faba bean shoots amended by BC, the aboveground C- and N-pool of faba bean were reduced, while these pools increased for ryegrass. The N-use efficiency (NUE) in faba bean shoots was reduced by 9 and 14 % for LTBC and HTBC, respectively, but not for ryegrass. Interestingly, ryegrass shoots had 52 % higher NUE than faba bean shoots. The N derived from atmosphere (% Ndfa) was increased by 2 and 9 % under LTBC and HTBC, respectively, while it decreased slightly by reduced irrigation. Quantity of BNF in faba bean aboveground biomass decreased with HTBC coupled with reduced irrigation, mainly towards decreased biomass and soil N uptake by faba bean. Therefore, HTBC might not be a feasible option to improve WUE and BNF in faba bean-ryegrass intercropping, but PRD is permissible as the clear trade-off between BC and PRD.

Contrasting physiological strategies explain heterogeneous responses to severe drought conditions within local populations of a widespread conifer

Understanding how trees prioritize carbon gain at the cost of drought vulnerability under severe drought conditions is crucial for predicting which genetic groups and individuals will be resilient to future climate conditions. In this study, we investigated variations in growth, tree-ring anatomy as well as carbon and oxygen isotope ratios to assess the sensitivity and the xylem formation process in response to an episode of severe drought in 29 mature white spruce (Picea glauca [Moench] Voss) families grown in a common garden trial. During the drought episode, the majority of families displayed decreased growth and exhibited either sustained or increased intrinsic water-use efficiency (iWUE), which was largely influenced by reduced stomatal conductance as revealed by the dual carbon‑oxygen isotope approach. Different water-use strategies were detected within white spruce populations in response to drought conditions. Our results revealed intraspecific variation in the prevailing physiological mechanisms underlying drought response within and among populations of Picea glauca. The presence of different genetic groups reflecting diverse water-use strategies within this largely-distributed conifer is likely to lessen the negative effects of drought and decrease the overall forest ecosystems' sensitivity to it.

Grain carbon isotopes indicate the ability of wheat plants to maintain enhanced intrinsic water-use efficiency even after short-term exposure to high temperatures and drought

Minimizing the impact of heat and drought on crop yields requires varieties with effective protective mechanisms. We tested the hypothesis that even a short-term high temperature amplifies the negative effects of reduced water availability on leaf gas-exchange, but can induce long-lasting improvement in plant water-use efficiency after the stress period. Accordingly, three common varieties of winter wheat (Triticum aestivum) were grown under field conditions. During the stem extension, the plants were exposed to distinct temperatures (daily maximum 26 vs. 38 °C), water availabilities (75% of field water capacity vs. permanent wilting point), and their combination for 14 days. All treatments reduced light-saturated rates of CO2 assimilation and transpiration, particularly when heat and drought were combined. Drought enhanced water-use efficiency (WUE) in all varieties (31.4-36.4%), but not at high temperatures (decrease by 17-52%). Intrinsic WUE (iWUE), determined from the stable carbon isotope composition of grains, was enhanced by 7.9-37% in all treatments and varieties; however, not all changes were significant. The combination of heat and drought tended to increase total protein content in grains but reduced spike productivity. Noticeably, the strongest decline in spike productivity was observed in Elan - the variety displaying the smallest enhancement of iWUE, while it was negligible in Pannonia which shows the most pronounced improvement of iWUE. We conclude that even several hot and dry days can improve iWUE for the rest of the vegetation season. This improvement, however, does not necessarily lead to increased crop productivity possibly due to physiological trade-offs.

Leaf trait responses to global change factors in terrestrial ecosystems

Global change influences plant growth by affecting plant morphology and physiology. However, the effects of global change factors vary based on the climate gradient. Here, we established a global database of leaf traits from 192 experiments on elevated CO2 concentrations (eCO2), drought, N deposition, and warming. The results showed that the leaf mass per area (LMA) significantly increased under eCO2 and drought conditions but decreased with N deposition, whereas eCO2 levels and drought conditions reduced stomatal conductance and increased and decreased photosynthetic rates, respectively. Leaf dark respiration (Rd) increased in response to global change, excluding N deposition. Leaf N concentrations declined with eCO2 but increased with N deposition. Leaf area increased with eCO2, N deposition, and warming but decreased with drought. Leaf thickness increased with eCO2 but decreased with warming. eCO2 and N deposition enhanced plant water-use efficiency (WUE), eCO2 and warming increased photosynthetic N-use efficiency (PNUE), while N fertilization reduced PNUE significantly. eCO2 produced a positive relationship between WUE and PNUE, which were limited under drought but increased in areas with high humidity and high temperature. Trade-offs were observed between WUE and PNUE under drought, N deposition, and warming. These findings suggest that the effects of global change factors on plants can be altered by complex environmental changes; moreover, diverse plant water and nutrient strategy responses can be interpreted against the background of their functional traits.

Data-driven approaches to improve water-use efficiency and drought resistance in crop plants

With the increasing population, there lies a pressing demand for food, feed and fibre, while the changing climatic conditions pose severe challenges for agricultural production worldwide. Water is the lifeline for crop production; thus, enhancing crop water-use efficiency (WUE) and improving drought resistance in crop varieties are crucial for overcoming these challenges. Genetically-driven improvements in yield, WUE and drought tolerance traits can buffer the worst effects of climate change on crop production in dry areas. While traditional crop breeding approaches have delivered impressive results in increasing yield, the methods remain time-consuming and are often limited by the existing allelic variation present in the germplasm. Significant advances in breeding and high-throughput omics technologies in parallel with smart agriculture practices have created avenues to dramatically speed up the process of trait improvement by leveraging the vast volumes of genomic and phenotypic data. For example, individual genome and pan-genome assemblies, along with transcriptomic, metabolomic and proteomic data from germplasm collections, characterised at phenotypic levels, could be utilised to identify marker-trait associations and superior haplotypes for crop genetic improvement. In addition, these omics approaches enable the identification of genes involved in pathways leading to the expression of a trait, thereby providing an understanding of the genetic, physiological and biochemical basis of trait variation. These data-driven gene discoveries and validation approaches are essential for crop improvement pipelines, including genomic breeding, speed breeding and gene editing. Herein, we provide an overview of prospects presented using big data-driven approaches (including artificial intelligence and machine learning) to harness new genetic gains for breeding programs and develop drought-tolerant crop varieties with favourable WUE and high-yield potential traits.

Trade-off of leaf-scale resource-use efficiencies along the vertical canopy of the subtropical forest

Leaf resource-use efficiencies are key indicators of plant adaptability to climate change, as they depend on both photosynthetic carbon assimilation and available resources. However, accurately quantifying the response of the coupled carbon and water cycles is challenging due to the canopy vertical variability in resource-use efficiencies, which introduces greater uncertainty into the calculations. Here we experimented to ascertain the vertical variations of leaf resource-use efficiencies along three canopy gradients of coniferous (Pinus elliottii Engelmann.) and broad-leaved (Schima Superba Gardn & Champ.) forests over one year in the subtropical region of China. The efficiency of water (WUE), and nitrogen (NUE) showed higher values in the top canopy level for the two species. The maximum efficiency of light (LUE) occurred in the bottom canopy level for both species. The impact of photosynthetic photon flux density (PPFD), leaf temperature (T_leaf), and vapor pressure deficit (VPD) on leaf resource-use efficiencies varied with canopy gradients in slash pine and schima superba. We also observed a trade-off between NUE and LUE for slash pine and between NUE and WUE for schima superba. Moreover, the variation in the correlation between LUE and WUE indicated a change in resource-use strategies for slash pine. These results emphasize the significance of vertical variations in resource-use efficiencies to enhance the prediction of future carbon-water dynamics in the subtropical forest.

Genomic diversity of aquaporins across genus Oryza provides a rich genetic resource for development of climate resilient rice cultivars

BACKGROUND

Plant aquaporins are critical genetic players performing multiple biological functions, especially climate resilience and water-use efficiency. Their genomic diversity across genus Oryza is yet to be explored.

RESULTS

This study identified 369 aquaporin-encoding genes from 11 cultivated and wild rice species and further categorized these into four major subfamilies, among which small basic intrinsic proteins are speculated to be ancestral to all land plant aquaporins. Evolutionarily conserved motifs in peptides of aquaporins participate in transmembrane transport of materials and their relatively complex gene structures provide an evolutionary playground for regulation of genome structure and transcription. Duplication and evolution analyses revealed higher genetic conservation among Oryza aquaporins and strong purifying selections are assisting in conserving the climate resilience associated functions. Promoter analysis highlighted enrichment of gene upstream regions with cis-acting regulatory elements involved in diverse biological processes, whereas miRNA target site prediction analysis unveiled substantial involvement of osa-miR2102-3p, osa-miR2927 and osa-miR5075 in post-transcriptional regulation of gene expression patterns. Moreover, expression patterns of japonica aquaporins were significantly perturbed in response to different treatment levels of six phytohormones and four abiotic stresses, suggesting their multifarious roles in plants survival under stressed environments. Furthermore, superior haplotypes of seven conserved orthologous aquaporins for higher thousand-grain weight are reported from a gold mine of 3,010 sequenced rice pangenomes.

CONCLUSIONS

This study unveils the complete genomic atlas of aquaporins across genus Oryza and provides a comprehensive genetic resource for genomics-assisted development of climate-resilient rice cultivars.

Yellowhorn Xso-miR5149-XsGTL1 enhances water-use efficiency and drought tolerance by regulating leaf morphology and stomatal density

Yellowhorn (Xanthoceras sorbifolium) is a unique edible woody oil tree species in China. Drought stress is the major yield-limiting factor of yellowhorn. MicroRNAs play an important role in regulating the response of woody plants to drought stress. However, the regulatory function of miRNAs in yellowhorn remains unclear. Here, we first constructed coregulatory networks integrated with miRNAs and their target genes. According to GO function and expression pattern analysis, we selected the Xso-miR5149-XsGTL1 module for further study. Xso-miR5149 is a key regulator of leaf morphology and stomatal density by directly mediating the expression of the transcription factor XsGTL1. Downregulation of XsGTL1 in yellowhorn led to increased leaf area and reduced stomatal density. RNA-seq analysis indicated that downregulation of XsGTL1 increased the expression of genes involved in the negative control of stomatal density, leaf morphology, and drought tolerance. After drought stress treatments, the XsGTL1-RNAi yellowhorn plants were less damaged and had higher water-use efficiency than the WT plants, while destruction of Xso-miR5149 or overexpression of XsGTL1 had the opposite effect. Our findings indicated that the Xso-miR5149-XsGTL1 regulatory module plays a critical role in controlling leaf morphology and stomatal density; hence, it's a potential candidate module for engineering enhanced drought tolerance in yellowhorn.

Liana functional assembly along the hydrological gradient in Central Amazonia

Soil hydrology, nutrient availability, and forest disturbance determine the variation of tropical tree species composition locally. However, most habitat filtering is explained by tree species' hydraulic traits along the hydrological gradient. We asked whether these patterns apply to lianas. At the community level, we investigated whether hydrological gradient, soil fertility, and forest disturbance explain liana species composition and whether liana species-environment relationships are mediated by leaf and stem wood functional traits. We sampled liana species composition in 18 1-ha plots across a 64 km² landscape in Central Amazonia and measured eleven leaf and stem wood traits across 115 liana species in 2000 individuals. We correlated liana species composition, summarized using PCoA with the functional composition summarized using principal coordinate analysis (PCA), employing species mean values of traits at the plot level. We tested the relationship between ordination axes and environmental gradients. Liana species composition was highly correlated with functional composition. Taxonomic (PCoA) and functional (PCA) compositions were strongly associated with the hydrological gradient, with a slight influence from forest disturbance on functional composition. Species in valley areas had larger stomata size and higher proportions of self-supporting xylem than in plateaus. Liana species on plateaus invest more in fast-growing leaves (higher SLA), although they show a higher wood density. Our study reveals that lianas use different functional solutions in dealing with each end of the hydrological gradient and that the relationships among habitat preferences and traits explain lianas species distributions less directly than previously found in trees.

Carbon sequestration and water yield tradeoffs following restoration of abandoned agricultural lands in Mediterranean mountains

Abandoned cropland areas have the potential to contribute to climate change mitigation through natural revegetation and afforestation programs. These programs increase above and belowground carbon sequestration by expanding forest cover. However, this potential to mitigate climate change often involves tradeoffs between carbon sequestration and water availability. Particularly in a water limited environments such as the Mediterranean region, any loss of recharge to groundwater or streamflow can have critical societal consequences. In this study, we used an ecohydrologic model, Regional Hydro-Ecological Simulation System (RHESSys), to quantify these tradeoffs for land management plans in abandoned cropland areas in Mediterranean mountains. Changes to Net Ecosystem Production (NEP), water yield and Water-Use Efficiency (WUE) under different land management and climate scenarios were estimated for Arnás, a catchment with similar geology, vegetation and climate to many of the locations targeted for land abandonment restoration in the Spanish Pyrenees. Results showed significant changes to both carbon and water fluxes related to land management, while changes related to a warming scenario were not significant. Afforestation scenarios showed the highest average annual carbon sequestration rates (112 g C·m^-2·yr^-1) but were also associated with the lowest water yield (runoff coefficient of 26%) and water use efficiency (1.4 g C·mm^-1) compared to natural revegetation (-27 g C·m^-2·yr^-1, 50%, 1.7 g C·mm^-1 respectively). Under both restoration scenarios, results showed that the catchment ecosystem is a carbon sink during mid-February to July, coinciding with peak monthly transpiration and WUE, while during the rest of the year the catchment ecosystem is a carbon source. These results contribute to understanding carbon and water tradeoffs in Mediterranean mountains and can help adapt restoration plans to address both carbon sequestration and water management objectives.

Overexpression of the Selaginella lepidophylla bHLH transcription factor enhances water-use efficiency, growth, and development in Arabidopsis

Abiotic stresses have the greatest impact on the growth and productivity of crops, especially under current and future extreme weather events due to climate change. Thus, it is vital to explore novel strategies to improve crop plant abiotic stress tolerance to feed an ever-growing world population. Selaginella lepidophylla is a desiccation-tolerant spike moss with specialized adaptations that allow it to tolerate water loss down to 4% relative water content. A candidate basic helix-loop-helix (bHLH) transcription factor was highly expressed at 4% relative water content in S. lepidophylla (SlbHLH). This SlbHLH gene was codon-optimized (SlbHLH_opt) and overexpressed in Arabidopsis for functional characterization. Overexpression of the SlbHLH_opt gene not only significantly increased plant growth, development, and integrated water-use efficiency, but also significantly increased seed germination and green cotyledon emergence rates under water-deficit stress and salt stress conditions. Under a 150 mM NaCl salt stress condition, SlbHLH_opt-overexpressing lines increased primary root length, the number of lateral roots, and fresh and dry biomass at the seedling stage compared to control lines. Interestingly, SlbHLH_opt-overexpressing lines also have significantly higher flavonoid content. Altogether, these results suggest that SlbHLH functions as an important regulator of plant growth, development, abiotic stress tolerance, and water-use efficiency.

Wood anatomy and tree-ring stable isotopes indicate a recent decline in water-use efficiency in the desert tree Moringa peregrina

The ability of desert plants to adapt to future climate changes and maximize their water-use efficiency will determine their survival. This study uses wood anatomy and δ¹³C and δ¹⁸O isotope analyses to investigate how Moringa peregrina trees in the Egyptian desert have responded to the environment over the last 10 years. Our results show that M. peregrina tree-ring widths (TRWs) have generally declined over the last decade, although individual series are characterized by high variability and low Rbars. Vessel lumen area percentages (VLA%) are low in wet years but increase significantly in dry years, such as the period 2017-2020. Stable δ¹³C isotope values decrease between 2010 (- 23.4‰) and 2020 (- 24.9‰), reflecting an unexpected response to an increase in drought conditions. The mean δ¹⁸O value (± standard error, SE) for the first ten rings of each tree from bark to pith (2020-2010) is 33.0 ‰ ± 0.85 with a range of 29.2-36.3‰, which indicates a common drought signal. The intrinsic water-use efficiency (iWUE) declines gradually with time, from 130.0 µmol mol^-1 in 2010 to 119.4 µmol mol^-1 in 2020. The intercellular carbon concentration (C_i) and C_i/C_a ratio increase over the same period, likely as a result of decreasing iWUE. The results show that M. peregrina trees seem to cool their leaves and the boundary air at the cost of saving water.

Effects of biochar amendment and reduced irrigation on growth, physiology, water-use efficiency and nutrients uptake of tobacco (Nicotiana tabacum L.) on two different soil types

Biochar has shown beneficial effects in agricultural production, yet the combined effects of biochar and reduced irrigation on crop growth and water-use efficiency (WUE) in diverse soil types have not been fully explored. A split-root pot experiment was conducted to investigate the effects of addition of 2% softwood (SWB) and wheat straw biochar (WSB) on growth, physiology, WUE and nutrients uptake of tobacco (Nicotiana tabacum L.) plants grown in a Ferrosol and an Anthrosol, respectively, under three irrigation treatments. The plants were either irrigated daily to 90% of water-holding capacity (FI), or irrigated with 70% volume of water used for FI to the whole root-zone (DI) or alternately to half root-zone (PRD). The results showed that plants grown in Anthrosol possessed greater leaf gas exchange rates, dry biomass and WUE while lower nutrients content compared to those grown in Ferrosol. Despite a negative effect on plant N content and WUE, WSB addition increased water-holding capacity, consequently improved leaf gas exchange, water uptake, biomass and K content resulting in an improved in the leaf quality of tobacco as exemplified by an increased leaf K content and a more appropriate N to K stoichiometric ratio. However, these effects were not evident upon SWB addition. Moreover, these responses to biochar addition were stronger in Ferrosol than in Anthrosol might be associated with its lower pH. Compared to FI, PRD slightly reduced photosynthetic rate but significantly decreased stomatal conductance, transpiration rate and leaf area, leading to a significant increase in intrinsic, instantaneous and plant WUE. Additionally, PRD was superior over DI in improving yield, WUE, N uptake under a same irrigation volume. It was concluded that WSB combined with PRD could be a promising practice to synergistically improve tobacco yield, quality and WUE by improving soil hydro-physical properties and nutrients bioavailability.

Physiological homeostasis for ecological success of Typha (Typha domingensis Pers.) populations in saline soils

The natural capacity of plants to endure salt stress is largely regulated by multifaceted structural and physio-biochemical modulations. Salt toxicity endurance mechanism of six ecotypes of Typha domingensis Pers. was evaluated by analyzing photosynthesis, ionic homeostasis, and stomatal physiology under different levels of salinity (0, 100, 200 and 300 mM NaCl). Typha populations were collected across different areas of Punjab, an eastern province in Pakistan. All studied attributes among ecotypes presented differential changes as compared to control. Different salt treatments not only affected gas exchange attributes but also shown significant modifications in stomatal anatomical changes. As compared to control, net photosynthetic rate, transpiration rate, total chlorophyll contents and carotenoids were increased by 111%, 64%, 103% and 171% respectively, in Sahianwala ecotype among all other ecotypes. Similarly, maximum water use efficiency (WUE), sub stomatal CO2 concentration, sodium (Na+) and chloride (Cl-) contents were observed in Sahianwala (191%, 93%, 168%, 158%) and Knotti (162%, 75%, 146%, 182%) respectively, as compared to the others ecotypes. Adaxial and abaxial stomatal areas remained stable in Sahianwala and Knotti. The highest abaxial stomatal density was observed in Gatwala ecotype (42 mm2) and maximum adaxial stomatal density was recorded in Sahianwala ecotype (43 mm2) at 300 mM NaCl salinity. The current study showed that Typha ecotypes responded varyingly to salinity in terms of photosynthesis attributes to avoid damages due to salinity. Overall, differential photosynthetic activity, WUE, and changes in stomatal attributes of Sahianwala and Knotti ecotypes contributed more prominently in tolerating salinity stress. Therefore, Typha domingensis is a potential species to be used to rehabilitate salt affected lands for agriculture and aquatic habitat.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00963-x.

Physiological and environmental control on ecosystem water use efficiency in response to drought across the northern hemisphere

Drought, a natural hydrometeorological phenomenon, has been more frequent and more widespread due to climate change. Water availability strongly regulates the coupling (or trade-off) between carbon uptake via photosynthesis and water loss through transpiration, known as water-use efficiency (WUE). Understanding the effects of drought on WUE across different vegetation types and along the wet to dry gradient is paramount to achieving better understanding of ecosystem functioning in response to climate change. We explored the physiological and environmental control on ecosystem WUE in response to drought using observations for 44 eddy covariance flux sites in the Northern Hemisphere. We quantified the response of WUE to drought and the relative contributions of gross primary production (GPP) and evapotranspiration (ET) to the variations of WUE. We also examined the control of physiological and environmental factors on monthly WUE under different moisture conditions. Cropland had a peak WUE value under moderate drought conditions, while grassland, deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), and evergreen needleleaf forest (ENF) had peak WUE under slight drought conditions. WUE was mainly driven by GPP for cropland, grassland, DBF, and ENF but was mainly driven by ET for EBF. Vapor pressure deficit (VPD) and canopy conductance (G_c) were the most important factors regulating WUE. Moreover, WUE had negative responses to air temperature, precipitation, and VPD but had a positive response to G_c and ecosystem respiration. Our findings highlight the different effects of biotic and abiotic factors on WUE among different vegetation types and the important roles of VPD and G_c in controlling ecosystem WUE in response to drought.

Investigating industrial water-use efficiency in mainland China: An improved SBM-DEA model

Improving water-use efficiency (WUE) is a crucial way of achieving green industrial production and sustainable development. Applying an improved Super-slacks-based measure model with undesirable outputs, this paper investigates industrial WUE in mainland China. The results show that: (1) Industrial WUE in China is improving with the efficiency value increasing from 0.9874 to 0.9962 in 2012-2015. (2) The regions of water absolute scarcity and the vulnerability show the highest industry-related WUE, whereas the water stressed region, water scarce region, and water abundant region failed to achieve efficiency during the observation period. (3) The overall index value using the conventional model was higher than that of the improved model, indicating the need for a more reasonable water-use structure and environmentally friendly discharge structure. This study provides a new perspective for measuring industrial WUE and advances related studies by (1) incorporating the actual structure of water used and wastewater discharged with weights assigned to input and output slacks according to marginal use cost of water and marginal treatment cost of wastewater; and (2) adding realistic constraints on the amount of water used and wastewater discharged to the model. The estimated provinces in mainland China can adjust their industrial water-use structures and wastewater-discharge structures based on the results of this study, and thus improve the industrial WUE.

Seasonal and individual event-responsiveness are key determinants of carbon exchange across plant functional types

Differentiation in physiological activity is a critical component of resource partitioning in resource-limited environments. For example, it is crucial to understand how plant physiological performance varies through time for different functional groups to forecast how terrestrial ecosystems will respond to change. Here, we tracked the seasonal progress of 13 plant species representing C₃ shrub, perennial C₃ and C₄ grass, and annual forb functional groups of the Colorado Plateau, USA. We tested for differences in carbon assimilation strategies and how photosynthetic rates related to recent, seasonal, and annual precipitation and temperature variables. Despite seasonal shifts in species presence and activity, we found small differences in seasonally weighted annual photosynthetic rates among groups. However, differences in the timing of maximum assimilation (A_net) were strongly functional group-dependent. C₃ shrubs employed a relatively consistent, low carbon capture strategy and maintained activity year-round but switched to a rapid growth strategy in response to recent climate conditions. In contrast, grasses maintained higher carbon capture during spring months when all perennials had maximum photosynthetic rates, but grasses were dormant during months when shrubs remained active. Perennial grass A_net rates were explained in part by precipitation accumulated during the preceding year and average maximum temperatures during the past 48 h, a result opposite to shrubs. These results lend insight into diverse physiological strategies and their connections to climate, and also point to the potential for shrubs to increase in abundance in response to increased climatic variability in drylands, given shrubs' ability to respond rapidly to changing conditions.

Physiological approach to decipher the drought tolerance of a soybean genotype from Brazilian savana

Drought is one of the major constraints for soybean production in Brazil. In this study we investigated the physiological traits of two soybean parental genotypes under progressive soil drying and rewetting. The plants were evaluated under full irrigation (control) conditions and under water deficit imposed by suspending irrigation until the plants reached predawn leaf water potentials (Ψam) of -1.0 MPa (moderate) and -1.5 MPa (severe). Physiological analyses showed that these genotypes exhibit different responses to water deficit. The Embrapa 48 genotype reached moderate and severe water potential two days after the BR16 genotype and was able to maintain higher levels of A, ETR and ΦPSII even under deficit conditions. This result was not related to changes in gs, 13C isotopic composition and presence of a more efficient antioxidant system. In addition, Fv/Fm values did not decrease in Embrapa 48 genotype in relation to irrigated condition showing that stress was not causing photochemical inhibition of photosynthesis. The greater reduction in the relative growth of the shoots, with concomitant greater growth of the root system under drought, indicates that the tolerant genotype is able to preferentially allocated carbon to the roots, presenting less damage to photosynthesis. Therefore, the physiological responses revealed that the tolerant genotype postponed leaf dehydration by a mechanism involving a more efficient use and translocation of water from root to shoot to maintain cell homeostasis and photosynthetic metabolism under stress.

Carbon and water relations in perennial Kernza (Thinopyrum intermedium): An overview

Perennial crops have been proposed as a more sustainable alternative to annual crops, because they have extended growing seasons, continuous ground cover, reduced nutrient leakage, and sequester more carbon in the soils than annual crops. One example is intermediate wheatgrass (Thinopyrum intermedium), a perennial crop that has been used as a cool-season forage throughout the USA and Canada and also across its native range in Eurasia. Since the 1980's, intermediate wheatgrass has been under domestication to improve seed fertility and grain yield. Commercial products are being sold under the trade name Kernza, owned by The Land Institute, located in Salina, Kansas, USA. This review provides a comprehensive framework about the physical and biological aspects involving the water and carbon cycles in Kernza plants. The main aspects we highlight here are based on previous findings regarding Kernza: i) the ability of maintaining a relatively high water-use efficiency throughout the whole growing season, which is beneficial to mitigate water stress, representing an important physiological mean to acclimate under severe, unfavorable weather conditions, and ii) its higher evapotranspiration (ET) and net carbon uptake rates, particularly when compared to annual counterparts. Only a thorough multifaceted assessment of the repercussion for carbon and water fluxes of a shift from annual crops to Kernza will allow assessing the perspectives of such novel perennial crop to support food security and a number of ecosystem services, particularly under future climates.

Balancing indicators for sustainable intensification of crop production at field and river basin levels

Adequate tools for evaluating sustainable intensification (SI) of crop production for agro-hydrological system are not readily available. Building on existing concepts, we propose a framework for evaluating SI at the field and river basin levels. The framework serves as a means to assess and visualise SI indicator values, including yield, water-use efficiency and nitrogen-use efficiency (NUE), alongside water and nitrogen surpluses and their effects on water quantity and quality. To demonstrate the SI assessment framework, we used empirical data for both the field level (the Static Fertilization Experiment at Bad Lauchstädt) and the river basin level (the Selke basin, 463 km²) in central Germany. Crop yield and resource use efficiency varied considerably from 1980 to 2014, but without clear trends. NUE frequently fell below the desirable range (<50%), exposing the environment to a large N surplus (>80 kg N ha^-1). For the catchment as a whole, the average nitrate-N concentration (3.6 mg L^-1) was slightly higher than the threshold of 2.5 mg L^-1 nitrate-N in surface water. However, weather and climate-related patterns, due to their effects on transport capacity and dilution, influenced water quantity and quality indicators more than agronomic practices. To achieve SI of crop production in the Selke basin, irrigation and soil moisture management are required to reduce yield variability and reduce N surpluses at field level. In addition, optimum application of fertiliser and manure could help to reduce the nitrate-N concentration below the set water quality standards in the Selke basin. In this way, there is scope for increase in yields and resource use efficiencies, and thus potential reduction of environmental impacts at basin level. We conclude that the framework is useful for assessing sustainable production, by simultaneously considering objectives related to crop production, resource-use efficiency and environmental quality, at both field and river basin levels.

Growth, wood anatomy and stable isotopes show species-specific couplings in three Mexican conifers inhabiting drought-prone areas

An improved understanding of how tree species will respond to warmer conditions and longer droughts requires comparing their responses across different environmental settings and considering a multi-proxy approach. We used several traits (tree-ring width, formation of intra-annual density fluctuations - IADFs, wood anatomy, Δ¹³C and δ¹⁸O records) to retrospectively quantify these responses in three conifers inhabiting drought-prone areas in northwestern Mexico. A fir species (Abies durangensis) was studied in a higher altitude and slightly rainier site and two pine species were sampled in a nearby, lower drier site (Pinus engelmannii, Pinus cembroides). Tree-ring-width indices (TRWi) of the studied species showed a very similar year-to-year variability likely indicating a common climatic signal. Wood anatomy analyses done over 3.5 million measured cells, showed that P. cembroides lumen area was much smaller than in the other two species and it remained constant along all the studied period (over 64 years). Instead, cell wall thickness was widest in P. engelmannii and this species presented the highest amount of intra-annual density fluctuations. Climate and wood anatomy correlations pointed out that lumen area was positively affected by winter precipitation for all studied species, while cell-wall thickness was negatively affected by this season's precipitation in all species but P. cembroides. Stable isotope analysis showed significantly lower values of Δ¹³C for P. cembroides and no significant δ¹⁸O differences between the three species, although they shared a common decreasing trend. With very distinct wood anatomical traits (smaller cells, compact morphology), P. cembroides stood out as the better adapted species in its current environment and could be less affected by future drier climate. P. engelmannii and A. durangensis showed high plasticity at wood anatomical level, allowing them to promptly respond to seasonal water availability but likely gives few advantages on future climate scenarios with longer and frequent drought spells.

A leucine-rich repeat-receptor-like kinase gene SbER2-1 from sorghum (Sorghum bicolor L.) confers drought tolerance in maize

BACKGROUND

ERECTA (ER) is a leucine-rich repeat-receptor-like kinase gene (LRR-RLK) encoding a protein isolated from Arabidopsis. Although the regulatory functions of ER genes have been widely explored in plant development and disease resistance, their roles in drought stress responses remain to be clarified.

RESULTS

In this study, we cloned and characterized two ER genes, SbER1-1 and SbER2-1, from the drought-tolerant model plant sorghum (Sorghum bicolor L.). Under drought stress, the two genes were expressed in the leaves and stems but not in the roots, and SbER2-1 transcript accumulation in the stem was increased. SbER2-1 was localized both on the plasma membrane and in the chloroplast. Moreover, SbER2-1 expression in Arabidopsis and maize conferred increased drought tolerance, especially in regard to water-use efficiency, increasing the net photosynthetic rate in maize under drought stress. Based on RNA-Seq analysis together with the physiological data, we conclude that the transgenic maize plants have upregulated phenylpropanoid metabolism and increased lignin accumulation under drought stress.

CONCLUSIONS

Our results demonstrate that SbER2-1 plays an important role in response to drought stress. Furthermore, photosynthetic systems and phenylpropanoid metabolism are implicated in SbER2-1-mediated drought stress tolerance mechanisms. The use of genetic engineering to regulate SbER2-1 expression in plants and to breed new varieties tolerant to drought is a research field full of potential.

A comparison of the Mediterranean diet and current food consumption patterns in Spain from a nutritional and water perspective

The promotion of responsible consumption is a key strategy to achieve environmental benefits, sustainable food security, and enhance public health. Countries like Spain are making efforts to reverse growing obesity and promote healthy diets, such as the recommended and traditional Mediterranean, recognized as a key strategy to improve the population's health with locally grown, traditional, and seasonal products like fruits, vegetables, olive oil, and fish. With a view to connecting water, agriculture, food security, nutrition and health, this research aims to investigate and compare the nutritional and water implications of the current food consumption of Spanish households with the recommended Mediterranean diet. Besides, we calculate their nutritional composition, compare their water footprints, and develop a new methodological approach to assess nutritional water productivity (i.e. the nutritional value per unit of embedded water). Results show that the current Spanish diet is shifting away from the recommended Mediterranean towards an alternative one containing three times more meat, dairy and sugar products, and a third fewer fruits, vegetables, and cereals. The Mediterranean diet is also less caloric, as it contains smaller amounts of proteins and fats and is richer in fiber and micronutrients. Due to the high-embedded water content in animal products, a shift towards a Mediterranean diet would reduce the consumptive WF about 750 l/capita day. Additionally, the Mediterranean diet has better water-nutritional efficiency than the current one: it provides more energy, fiber, and nutrients per liter of consumptive water. The study confirms the Mediterranean diet is a healthier and more sustainable diet with strong cultural heritage.

Introgression of Root and Water Use Efficiency Traits Enhances Water Productivity: An Evidence for Physiological Breeding in Rice (Oryza sativa L.)

BACKGROUND

Semi-irrigated aerobic cultivation of rice has been suggested as a potential water saving agronomy. However, suitable cultivars are needed in order to sustain yield levels. An introgression of water mining and water use efficiency (WUE) traits is the most appropriate strategy for a comprehensive genetic enhancement to develop such rice cultivars.

RESULTS

We report a novel strategy of phenotyping and marker-assisted backcross breeding to introgress water mining (root) and water use efficiency (WUE) traits into a popular high yielding cultivar, IR-64. Trait donor genotypes for root (AC-39020) and WUE (IET-16348) were crossed separately and the resultant F1s were inter-mated to generate double cross F1s (DCF1). Progenies of three generations of backcross followed by selfing were charatcerised for target phenotype and genome integration. A set of 260 trait introgressed lines were identified. Root weight and root length of TILs were 53% and 23.5% higher, while Δ13C was 2.85‰ lower indicating a significant increase in WUE over IR-64. Five best TILs selected from BC3F3 generation showed 52% and 63% increase in yield over IR-64 under 100% and 60% FC, respectively. The trait introgressed lines resembled IR64 with more than 97% of genome recovered with a significant yield advantage under semi-irrigated aerobic conditions The study validated markers identified earlier by association mapping.

CONCLUSION

Introgression of root and WUE into IR64, resulted in an excellent yield advantage even when cultivated under semi-irrigated aerobic condition. The study provided a proof-of-concept that maintaining leaf turgor and carbon metabolism results in improved adaptation to water limited conditions and sustains productivity. A marker based multi-parent backcross breeding is an appropriate approach for trait introgression. The trait introgressed lines developed can be effectively used in future crop improvement programs as donor lines for both root and WUE.

Mapping QTLs and Identification of Genes Associated with Drought Resistance in Sorghum

Water limits global agricultural production. Increases in global aridity, a growing human population, and the depletion of aquifers will only increase the scarcity of water for agriculture. Water is essential for plant growth and in areas that are prone to drought, the use of drought-resistant crops is a long-term solution for growing more food for more people with less water. Sorghum is well adapted to hot and dry environments and has been used as a dietary staple for millions of people. Increasing the drought resistance in sorghum hybrids with no impact on yield is a continual objective for sorghum breeders. In this review, we describe the loci, quantitative trait loci (QTLs), or genes that have been identified for traits involved in drought avoidance (water-use efficiency, cuticular wax synthesis, trichome development and morphology, root system architecture) and drought tolerance (compatible solutes, pre- and post-flowering drought tolerance). Many of these identified genes and QTL regions have not been tested in hybrids and the effect of these genes, or their interactions, on yield must be understood in normal and drought-stressed conditions to understand the strength and weaknesses of their utility.

Recycling of wastes from fish beneficiation by composting: chemical characteristics of the compost and efficiency of their humic acids in stimulating the growth of lettuce

Waste from the beneficiation of fish was composted with crushed grass aiming to characterize their chemical composition and investigate the possibility of the use of the final compost as source of humic acids (HA) able to stimulate the growth of lettuce. Compost presented pH value, C/N ratio, and electrical conductivity that allow its use as an organic fertilizer. The element content was present in the following order of abundance in the compost: P > Ca > N > Mg > K > Fe > Zn > Mn > Mo > Cu, and the humus composition was similar to that observed in others kind of organic residues composted. The high content of oxygen pointed out a high level of oxidation of HA, in line with the predominance of phenolic acidity in the functional groups. The 13C-NMR spectra showed marked resonances due to the presence of lipids and other materials resistant to degradation as methoxy substituent and N-alkyl groups. A concentration of 20 mg L-1 HA increased significantly both dry and wet root matter in lettuce but the CO2 assimilation, stomatal conductance, and number of lateral roots of the plants were not affected. However, increases of 64% in the water-use efficiency was observed due to the HA addition, probably related to the root morphology alteration which resulted in 1.6-fold increase of lateral root average length and due to the higher H+ extrusion activity. Reuse of residues from the fish beneficiation activity by composting may represent a safe tool to increase the value of recycled organic residues and generate HA with potential use as plant growth stimulants.

Perspectives on the basic and applied aspects of crassulacean acid metabolism (CAM) research

Due to public concerns about the decreasing supply of blue water and increasing heat and drought stress on plant growth caused by urbanization, increasing human population and climate change, interest in crassulacean acid metabolism (CAM), a specialized type of photosynthesis enhancing water-use efficiency (WUE) and drought tolerance, has increased markedly. Significant progress has been achieved in both basic and applied research in CAM plants since the beginning of this century. Here we provide a brief overview of the current status of CAM research, and discuss future needs and opportunities in a wide range of areas including systems biology, synthetic biology, and utilization of CAM crops for human benefit, with a focus on the following aspects: 1) application of genome-editing technology and high-throughput phenotyping to functional genomics research in model CAM species and genetic improvement of CAM crops, 2) challenges for multi-scale metabolic modeling of CAM systems, 3) opportunities and new strategies for CAM pathway engineering to enhance WUE and drought tolerance in C₃ (and C₄) photosynthesis crops, 4) potential of CAM species as resources for food, feed, natural products, pharmaceuticals and biofuels, and 5) development of CAM crops for ecological and aesthetic benefits.

Genotypic differences in agro-physiological, biochemical and isotopic responses to salinity stress in quinoa (Chenopodium quinoa Willd.) plants: Prospects for salinity tolerance and yield stability

Quinoa is an important nutritive crop that can play a strategic role in the development of marginal and degraded lands. Genotypic variations in carbon isotope composition (δ¹³C), carbon isotope discrimination (Δ¹³C), ratio of intercellular to atmospheric CO₂ concentration (Ci/Ca), intrinsic water use efficiency (iWUE), seed yield and grain protein contents were analyzed in 6 quinoa cultivars grown in the field under saline conditions (0, 10, 20 dS m^-1). Significant variations occurred in dry biomass, seed yield, plant height, number of branches, number of panicles, panicle weight, harvest index, N and C content. Some genotypes produced yields with values significantly higher than 2.04 t ha^-1 (Q12), with an average increased to 2.58 t ha^-1 (AMES22157). The present study indicates a large variation in Δ¹³C for salinity treatments (3.43‰) and small magnitude of variations among genotypes (0.95‰). Results showed that Δ might be used as an important index for screening, and selection of the salt tolerant quinoa genotypes with high iWUE. Quinoa genotypes differs in foliar ¹³C and ¹⁵N isotope composition, which reflected complex interactions of salinity and plant carbon and nitrogen metabolisms. Grain protein contents were found higher in Q19 and Q31 and lowest in Q26. The study demonstrates that AMES22157 and Q12, were salt tolerant and high yielder while the AMES22157 was more productive. This study provides a reliable measure of morpho-physiological, biochemical and isotopic responses of quinoa cultivars to salinity in hyper arid UAE climate and it may be valuable in the future breeding programs. The development of genotypes having both higher water use efficiency and yield potential would be a very useful contribution for producers in the dry region of Arabian Peninsula.

How do disturbances and climate effects on carbon and water fluxes differ between multi-aged and even-aged coniferous forests?

Disturbances and climatic changes significantly affect forest ecosystem productivity, water use efficiency (WUE) and carbon (C) flux dynamics. A deep understanding of terrestrial feedbacks to such effects and recovery mechanisms in forests across contrasting climatic regimes is essential to predict future regional/global C and water budgets, which are also closely related to the potential forest management decisions. However, the resilience of multi-aged and even-aged forests to disturbances has been debated for >60years because of technical measurement constraints. Here we evaluated 62site-years of eddy covariance measurements of net ecosystem production (NEP), evapotranspiration (ET), the estimates of gross primary productivity (GPP), ecosystem respiration (R_e) and ecosystem-level WUE, as well as the relationships with environmental controls in three chronosequences of multi- and even-aged coniferous forests covering the Mediterranean, temperate and boreal regions. Age-specific dynamics in multi-year mean annual NEP and WUE revealed that forest age is a key variable that determines the sign and magnitude of recovering forest C source-sink strength from disturbances. However, the trends of annual NEP and WUE across succession stages between two stand structures differed substantially. The successional patterns of NEP exhibited an inverted-U trend with age at the two even-aged chronosequences, whereas NEP of the multi-aged chronosequence increased steadily through time. Meanwhile, site-level WUE of even-aged forests decreased gradually from young to mature, whereas an apparent increase occurred for the same forest age in multi-aged stands. Compared with even-aged forests, multi-aged forests sequestered more CO₂ with forest age and maintained a relatively higher WUE in the later succession periods. With regard to the available flux measurements in this study, these behaviors are independent of tree species, stand ages and climate conditions. We also found that distinctly different environmental factors controlled forest C and water fluxes under three climatic regimes. Typical weather events such as temperature anomalies or drying-wetting cycles severely affected forest functions. Particularly, a summer drought in the boreal forest resulted in an increased NEP owing to a considerable decrease in R_e, but at the cost of greater water loss from deeper groundwater resources. These findings will provide important implications for forest management strategies to mitigate global climate change.

Physiological, vascular and nanomechanical assessment of hybrid poplar leaf traits in micropropagated plants and plants propagated from root cuttings: A contribution to breeding programs

Micropropagated plants experience significant stress from rapid water loss when they are transferred from an in vitro culture to either greenhouse or field conditions. This is caused both by inefficient stomatal control of transpiration and the change to a higher light intensity and lower humidity. Understanding the physiological, vascular and biomechanical processes that allow micropropagated plants to modify their phenotype in response to environmental conditions can help to improve both field performance and plant survival. To identify changes between the hybrid poplar [Populus tremula × (Populus × canescens)] plants propagated from in vitro tissue culture and those from root cuttings, we assessed leaf performance for any differences in leaf growth, photosynthetic and vascular traits, and also nanomechanical properties of the tracheary element cell walls. The micropropagated plants showed significantly higher values for leaf area, leaf length, leaf width and leaf dry mass. The greater leaf area and leaf size dimensions resulted from the higher transpiration rate recorded for this stock type. Also, the micropropagated plants reached higher values for chlorophyll a fluorescence parameters and for the nanomechanical dissipation energy of tracheary element cell walls which may indicate a higher damping capacity within the primary xylem tissue under abiotic stress conditions. The performance of the plants propagated from root cuttings was superior for instantaneous water-use efficiency which signifies a higher acclimation capacity to stressful conditions during a severe drought particularly for this stock type. Similarities were found among the majority of the examined leaf traits for both vegetative plant origins including leaf mass per area, stomatal conductance, net photosynthetic rate, hydraulic axial conductivity, indicators of leaf midrib vascular architecture, as well as for the majority of cell wall nanomechanical traits. This research revealed that there were no drawbacks in the leaf physiological performance which could be attributed to the micropropagated plants of fast growing hybrid poplar.

Ancestral QTL alleles from wild emmer wheat improve grain yield, biomass and photosynthesis across enviroinments in modern wheat

Wild emmer wheat (Triticum turgidum ssp. dicoccoides) is considered a promising source for improving drought resistance in domesticated wheat. Nevertheless, wild germplasm has not been widely used in wheat breeding for abiotic stress resilience. In the current study, a near isogenic line NIL-7A-B-2, introgressed with a drought-related QTL from wild emmer wheat on chromosome 7A, and its recurrent parent, bread wheat cv. BarNir, were investigated under four environments across 2 years-water-limited and well-watered conditions in a rain-protected screen-house (Year 1) and two commercial open field plots under ample precipitation (Year 2). NIL-7A-B-2 exhibited an advantage over BarNir in grain yield and biomass production under most environments. Further physiological analyses suggested that enhanced photosynthetic capacity and photochemistry combined with higher flag leaf area are among the factors underlying the improved productivity of NIL-7A-B-2. These were coupled with improved sink capacity in NIL-7A-B-2, manifested by greater yield components than its parental line. This study provides further support for our previous findings that introgression of wild emmer QTL alleles, using marker assisted selection, can enhance grain yield and biomass production across environments in domesticated wheat, thereby enriching the modern gene pool with essential diversity for the improvement of yield and drought resistance.

Intra-specific association between carbon isotope composition and productivity in woody plants: A meta-analysis

The study of intra-specific variations in growth and plant physiological response to drought is crucial to understand the potential for plant adaptation to global change. Carbon isotope composition (δ(13)C) in plant tissues offers an integrated measure of intrinsic water-use efficiency (WUEi). The intra-specific association between δ(13)C and productivity has been extensively studied in herbaceous crops, but species-specific information on woody plants is still limited and has so far provided contradictory results. In this work we explored the general patterns of the relationship between δ(13)C and growth traits (height, diameter and biomass) using a meta-analysis. We compiled information from 49 articles, including 176 studies performed on 34 species from 16 genera. We found a positive global intra-specific correlation between δ(13)C and growth (Gr=0.28, P<0.0001), stronger for biomass than for height, and non-significant for diameter. The extent of this intra-specific association increased from Mediterranean to subtropical, temperate and boreal biomes, i.e. from water-limited to energy-limited environments. Conifers and shrubs, but not broadleaves, showed consistent positive intra-specific correlations. The meta-analysis also revealed that the relationship between δ(13)C and growth is better characterized at juvenile stages, under near-optimal and controlled conditions, and by analyzing δ(13)C in leaves rather than in wood.

The response of mesophyll conductance to nitrogen and water availability differs between wheat genotypes

Increased mesophyll conductance (gm) has been suggested as a target for selection for high productivity and high water-use efficiency in crop plants, and genotypic variability in gm has been reported in several important crop species. However, effective selection requires an understanding of how gm varies with growth conditions, to ensure that the ranking of genotypes is consistent across environments. We assessed the genotypic variability in gm and other leaf gas exchange traits, as well as growth and biomass allocation for six wheat genotypes under different water and nitrogen availabilities. The wheat genotypes differed in their response of gm to growth conditions, resulting in genotypic differences in the mesophyll limitation to photosynthesis and a significant increase in the mesophyll limitation to photosynthesis under drought. In this experiment, leaf intrinsic water-use efficiency was more closely related to stomatal conductance than to mesophyll conductance, and stomatal limitation to photosynthesis increased more in some genotypes than in others in response to drought. Screening for gm should be carried out under a range of growth conditions.

Comparison of evapotranspiration components and water-use efficiency among different land use patterns of temperate steppe in the Northern China pastoral-farming ecotone

Water-use efficiency (WUE), which links carbon and water cycles, is an important indicator of assessing the interactions between ecosystems and regional climate. Using chamber methods with and without plant removal treatments, we investigated WUE and evapotranspiration (ET) components in three ecosystems with different land-use types in Northern China pastoral-farming ecotone. In comparison, ET of the ecosystems with grazing exclusion and cultivating was 6.7 and 13.4 % higher than that of the ecosystem with free grazing. The difference in ET was primarily due to the different magnitudes of soil water evaporation (E) rather than canopy transpiration (T). Canopy WUE (WUEc, i.e., the ratio of gross primary productivity to T) at the grazing excluded and cultivated sites was 17 and 36 % higher than that at the grazing site. Ecosystem WUE (WUEnep, i.e., the ratio of net ecosystem productivity to ET) at the cultivated site was 34 and 28 % lower in comparison with grazed and grazing excluded stepped, respectively. The varied leaf area index (LAI) of different land uses was correlated with microclimate and ecosystem vapor/carbon exchange. The LAI changing with land uses should be the primary regulation of grassland WUE. These findings facilitate the mechanistic understanding of carbon-water relationships at canopy and ecosystem levels and projection of the effects of land-use change on regional climate and productivity.

Growth of Arabidopsis thaliana and Eutrema salsugineum in a closed growing system designed for quantification of plant water use

The identification of genetic determinants for water-use efficiency (WUE) and their incorporation into crop plants is critical as world water resources are predicted to become less stable over the coming decades. However, quantification of WUE in small model species such as Arabidopsis is difficult because of low plant water loss relative to root zone evaporation. Furthermore, measurements of long-term WUE are labor-intensive and time-consuming. A novel high-throughput closed-container growing system for measuring plant WUE is described. The system eliminates nearly all water loss from the media and does not require irrigation throughout the duration of a typical experiment. Using the model species Arabidopsis thaliana and Eutrema salsugineum, it was confirmed that under growth chamber conditions, this system: (1) eliminates the need for irrigation for as much as 30 days with media water content remaining above 80% full capacity; (2) allows for quantification of WUE in plants with a leaf area as small as ca. 20 cm(2); (3) does not inhibit plant growth; and (4) does not alter media conditions outside of an acceptable range for these species. The growing system provides an efficient high-throughput system for quantifying plant water loss and WUE.

The evolution of drought escape and avoidance in natural herbaceous populations

While the functional genetics and physiological mechanisms controlling drought resistance in crop plants have been intensely studied, less research has examined the genetic basis of adaptation to drought stress in natural populations. Drought resistance adaptations in nature reflect natural rather than human-mediated selection and may identify novel mechanisms for stress tolerance. Adaptations conferring drought resistance have historically been divided into alternative strategies including drought escape (rapid development to complete a life cycle before drought) and drought avoidance (reducing water loss to prevent dehydration). Recent studies in genetic model systems such as Arabidopsis, Mimulus, and Panicum have begun to elucidate the genes, expression profiles, and physiological changes responsible for ecologically important variation in drought resistance. Similar to most crop plants, variation in drought escape and avoidance is complex, underlain by many QTL of small effect, and pervasive gene by environment interactions. Recently identified major-effect alleles point to a significant role for genetic constraints in limiting the concurrent evolution of both drought escape and avoidance strategies, although these constraints are not universally found. This progress suggests that understanding the mechanistic basic and fitness consequences of gene by environment interactions will be critical for crop improvement and forecasting population persistence in unpredictable environments.

Semi-determinate growth habit adjusts the vegetative-to-reproductive balance and increases productivity and water-use efficiency in tomato (Solanum lycopersicum)

Tomato (Solanum lycopersicum) shows three growth habits: determinate, indeterminate and semi-determinate. These are controlled mainly by allelic variation in the self-pruning (SP) gene family, which also includes the "florigen" gene single flower TRUSS (SFT). Determinate cultivars have synchronized flower and fruit production, which allows mechanical harvesting in the tomato processing industry, whereas indeterminate ones have more vegetative growth with continuous flower and fruit formation, being thus preferred for fresh market tomato production. The semi-determinate growth habit is poorly understood, although there are indications that it combines advantages of determinate and indeterminate growth. Here, we used near-isogenic lines (NILs) in the cultivar Micro-Tom (MT) with different growth habit to characterize semi-determinate growth and to determine its impact on developmental and productivity traits. We show that semi-determinate genotypes are equivalent to determinate ones with extended vegetative growth, which in turn impacts shoot height, number of leaves and either stem diameter or internode length. Semi-determinate plants also tend to increase the highly relevant agronomic parameter Brix × ripe yield (BRY). Water-use efficiency (WUE), evaluated either directly as dry mass produced per amount of water transpired or indirectly through C isotope discrimination, was higher in semi-determinate genotypes. We also provide evidence that the increases in BRY in semi-determinate genotypes are a consequence of an improved balance between vegetative and reproductive growth, a mechanism analogous to the conversion of the overly vegetative tall cereal varieties into well-balanced semi-dwarf ones used in the Green Revolution.

What is the potential for dark CO2 fixation in the facultative crassulacean acid metabolism species Talinum triangulare?

In obligate Crassulacean acid metabolism (CAM) plants, dark CO2 fixation is almost the sole route of CO2 fixation and, under drought, continues for long periods. In contrast, in plants of the facultative CAM species Talinum triangulare under experimental drought, dark CO2 fixation provides a small proportion of the daily assimilation observed in watered plants and occurs only for a few days, after which almost nil CO2 fixation is observed. Under field conditions, with a practically unlimited substrate volume, drought-induced CAM might operate for a longer period and make a higher contribution to daily CO2 fixation. Greenhouse-grown plants of T. triangulare were subjected to low and nearly constant soil water content; the operation of CAM was assessed through the measurement of nocturnal proton accumulation and dark CO2 fixation. Dark CO2 fixation appeared 19d after the onset of drought; its contribution during three months of experiment to daily CO2 assimilation ranged from 0.5 to 30.7% with a mean of 13.5%. Twenty days after the beginning of treatment, nocturnal proton accumulation increased six times and remained high for over three months. In spite of low soil water content, leaves did not engage in dark CO2 fixation all the time but dark CO2 fixation was large enough to produce an increase in relative (13)C composition of mature leaves compared to watered plants but not to the value in short-term drought experiments. Leaf anatomical characteristics may guarantee the achievement of higher rates of dark CO2 fixation but results evidence the occurrence of a limit to the expression of CAM that remains to be determined.

Assessing the effects of nitrogen deposition and climate on carbon isotope discrimination and intrinsic water-use efficiency of angiosperm and conifer trees under rising CO2 conditions

The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO2 , on the variability of carbon isotope discrimination (Δ(13) C), and intrinsic water-use efficiency (iWUE) of angiosperm and conifer tree species. Eighty-nine long-term isotope tree-ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long-term time series of Δ(13) C and iWUE. Δ(13) C and iWUE were related to the increasing concentration of atmospheric CO2 over the industrial period (1850-2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950-2000. We applied generalized additive models and linear mixed-effects models to predict the effects of climatic variables and nitrogen deposition on Δ(13) C and iWUE. Results showed a declining Δ(13) C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO2 values. The temporal variation in Δ(13) C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO2 concentrations. We defined linear mixed-effects models that were effective to describe the variation of Δ(13) C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (Nrate ) and long-term cumulative (Ncum ) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however, Δ(13) C and iWUE showed complex dependent interactions between different covariates. A significant association of Nrate with iWUE and Δ(13) C was observed in conifers and in the angiosperms, and Ncum was the only independent term with a significant positive association with iWUE, although a multi-factorial control was evident in conifers.

Integrated nitrogen, carbon, and water relations of a xylem-tapping mistletoe following nitrogen fertilization of the host

Xylem-tapping mistletoes transpire large volumes of water (E) while conducting photosynthesis (A) at low rates, thus maintaining low instantaneous wateruse efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitrogen dissolved at low concentration in host xylem water; however, low A/E also results in substantial heterotrophic carbon gain. In this study, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to determine whether mistletoe A/E would approach that of the host if mistletoes were supplied with abundant nitrogen. Fertilization significantly increased foliar N concentrations (N), net assimilation rates, and A/E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the other hand, when instantaneous water-use efficiency and A/N were calculated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budgets was constructed to analyze the relative benefits of nitrogen- and carbonparasitism. The model assumes constant E and includes feedbacks of tissue nitrogen concentration on photosyn-thesis. These results, combined with our earlier observation that net assimilation rates of mistletoes and their hosts are approximately matched (Marshall et al. 1994), support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain. However, the low ratio of A/E is interpreted not as a means of acquiring nitrogen, but as an inevitable consequence of an imbalance in C and N assimilation.

Carbon isotope discrimination, water-use efficiency, growth, and mortality in a natural shrub population

In order to scale up from the ecophysiological characters of individual plants to population-level questions, we need to determine if character patterns in natural populations are stable through time, and if the characters are related to growth and survival. We investigated these questions in a 3-year study for one character, integrated water-use efficiency (WUE) as estimated by carbon isotope discrimination (δ) in a population of the Great Basin shrub, Chrysothamnus nauseosus. WUE was a conservative character for a given plant within and across seasons, and a previously documented difference between two size classes (represented by juveniles and adults) was maintained; smaller juveniles had a lower WUE than larger adults. The lower WUE of juveniles was often accompanied by higher rates of photosynthesis and stomatal conductance as compared to adults even though juveniles generally had more negative xylem pressure potentials. Although many discussions of the role of WUE in natural populations have been based on the expectation that higher WUE (lower δ) is generally associated with less growth, we found no such relation-ship for juvenile plants in this population (i.e δ was not positively correlated with height increase). In addition, juvenile plant mortality was not correlated with δ. Although there were stable patterns of WUE for plants in this population, the positive correlation between WUE and size, and the lack of a negative correlation between WUE and height growth, make it unlikely that the WUE of an individual plant will be related in a simple manner to its growth and survival in the population.

Genetic differentiation in carbon isotope discrimination and gas exchange in Pseudotsuga menziesii : A common-garden experiment

Patterns of genetic variation in gas-exchange physiology were analyzed in a 15-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation that contains 25 populations grown from seed collected from across the natural distribution of the species. Seed was collected from 33°30' to 53°12' north latitude and from 170 m to 2930 m above sea level, and from the coastal and interior (Rocky Mountain) varieties of the species. Carbon isotope discrimination (Δ) ranged from 19.70(‰) to 22.43(‰) and was closely related to geographic location of the seed source. The coastal variety (20.50 (SE=0.21)‰) was not significantly different from the interior variety (20.91 (0.15)‰). Instead, most variation was found within the interior variety; populations from the southern Rockies had the highest discrimination (21.53 (0.20)‰) (lowest water-use efficiency). Carbon isotope discrimination (Δ), stomatal conductance to water vapor (g), the ratio of intercellular to ambient CO₂ concentration (c_i/c_a), and intrinsic water-use efficiency (A/g) were all correlated with altitude of origin (r=0.76, 0.73, 0.74, and -0.63 respectively); all were statistically significant at the 0.01 level. The same variables were correlated with both height and diameter at age 15 (all at P≤0.0005). Observed patterns in the common garden did not conform to our expectation of higher WUE, measured by both A/g and Δ, in trees from the drier habitats of the interior, nor did they agree with published in situ observations of decreasing g and Δ with altitude. The genetic effect opposes the altitudinal one, leading to some degree of homeostasis in physiological characteri tics in situ.

Photosynthesis and water-use efficiency in pinyon-juniper communities along an elevation gradient in northern New Mexico

We investigated plant ecophysiological response to fertilization of selected sites along an elevation gradient in a pinyon-juniper woodland. Plant density and species composition followed typical patterns for pinyon-juniper woodlands over this gradient, with a sparse juniper (Juniperus monosperma (Engelm.) Sarg.)-grassland community at the lowest elevation and gradually increasing total canopy cover and pinyon (Pinus edulis Engelm.) cover with elevation. Carbon isotope analysis showed that both tree species had higher water-use efficiency (WUE) at the lowest, and presumably driest, sites. Over most of the gradient, however, it appeared that changes in stand density compensated for changes in water availability. Contrary to initial hypotheses, the more drought-tolerant juniper did not demonstrate significantly greater WUE than pinyon, although it maintained positive carbon gain at lower predawn xylem pressure potentials than pinyon. In pinyon, both A _max and WUE increased with increasing N concentration in tissues. Pinyon needles also demonstrated declining nitrogen-use efficiency with age. There was no relationship between tissue N and either A _max or WUE measured at A _max in juniper, although δ¹³C analysis indicated that WUE increased in juniper with increased N availability. Results from this study suggest that plasticity in plant physiological processes could result in nonlinear responses of organic matter production to climate change, and therefore must be accounted for in ecosystem models.

Comparative ecophysiology of five species of Sedum (Crassulaceae) under well-watered and drought-stressed conditions

Gas exchange patterns, diurnal malic acid fluctuations, and stable carbon isotope ratios of five species of Sedum were investigated to assess the ecophysiological characteristics of three different photosynthetic pathways under well-watered and drought-stressed conditions. All five species have succulent leaves and stems and were examined under identical environmental conditions. When well-watered, Sedum integrifolium (Raf.) Nels. and S. ternatum Michx. displayed C₃ photosynthesis, S. telephioides Michx. and S. nuttallianum Raf. exhibited CAM-cycling, and S. wrightii A. Gray showed CAM. When grown under a less frequent watering regime, S. integrifolium and S. ternatum exhibited CAM-cycling, whereas S. telephioides and S. nuttallianum displayed CAM-cycling simultaneously with low-level CAM. Sedum wrightii retained its CAM mode of photosynthesis. In general, leaf δ¹³C values reflected these variations in photosynthetic pathways. While all values of water-use efficiency (WUE) were greater than those reported for most C₃ and C₄ species, no correlation of malic acid accumulation in the CAM and CAM-cycling (including low-level CAM) species with increased WUE was found. Sedum wrightii (CAM) had the highest WUE value at night, yet its 24-h WUE was not different from S. ternatum when the latter was in the C₃ mode. Thus, relative water-use efficiencies of these species of Sedum were not predictable based on photosynthetic pathways alone.

Genetic variation in and covariation between leaf gas exchange, morphology, and development in Polygonum arenastrum, an annual plant

We present evidence of genetic variation in and covariation between leaf-level gas exchange properties and leaf size among family lines of Polygonum arenastrum. This self-fertilizing annual had previously been shown to vary genetically in developmental phenology and in morphology (size of leaves, internodes, flowers and seeds) (Geber 1990). Significant family differences were found in photosynthetic carbon assimilation rate (A), lcaf conductance to water vapor (g), instantaneous water-use efficiency (WUE), and leaf carbon isotope discrimination (Δ). A strong positive genetic correlation between A and g suggested that there was stomatal limitation on A. In addition, higher g led to relatively greater increases in transpiration, E, than in assimilation, A, so that families with high rates of gas exchange had lower instantaneous WUE and/or higher carbon isotope discrimination values. Leaf size and gas exchange were genetically correlated. In earlier studies leaf size was found to be genetically correlated with developmental phenology (Geber 1990). The pattern that emerges is one in which small-leaved families (which also have small internodes, flowers, and seeds) tend to have high gas exchange rates, low WUE, rapid development to flowering and high early fecundity, but reduced life span and maximum (vegetative and reproductive) yield compared to large-leaved families. We suggest that this pattern may have arisen from selection for contrasting suites of characters adapted to environments differing in season length.

The effect of water and nitrogen amendments on photosynthesis, leaf demography, and resource-use efficiency in Larrea tridentata, a desert evergreen shrub

In the Chihuahuan Desert of southern New Mexico, both water and nitrogen limit the primary productivity of Larrea tridentata, a xerophytic evergreen shrub. Net photosynthesis was positively correlated to leaf N, but only in plants that received supplemental water. Nutrient-use efficiency, defined as photosynthetic carbon gain per unit N invested in leaf tissue, declined with increasing leaf N. However, water-use efficiency, defined as the ratio of photosynthesis to transpiration, increased with increasing leaf N, and thus these two measures of resource-use efficiency were inversely correlated. Resorption efficiency was not significantly altered over the nutrient gradient, nor was it affected by irrigation treatments. Leaf longevity decreased significantly with fertilization although the absolute magnitude of this decrease was fairly small, in part due to a large background of insect-induced mortality. Age-specific gas exchange measurements support the hypothesis that leaf aging represents a redistribution of resources, rather than actual deterioration or declining resource-use efficiency.

Stem photosynthesis in a desert ephemeral, Eriogonum inflatum : Characterization of leaf and stem CO2 fixation and H2O vapor exchange under controlled conditions

The gas exchange characteristics of photosynthetic tissues of leaves and stems of Eriogonum inflatum are described. Inflated stems were found to contain extraordinarily high internal CO₂ concentrations (to 14000 μbar), but fixation of this internal CO₂ was 6-10 times slower than fixation of atmospheric CO₂ by these stems. Although the pool of CO₂ is a trivial source of CO₂ for stem photosynthesis, it may result in higher water-use efficiency of stem tissues. Leaf and stem photosynthetic activities were compared by means of CO₂ fixation in CO₂ response curves, light and temperature response curves in IRGA systems, and by means of O₂ exchange at CO₂ saturation in a leaf disc O₂ electrode system. On an area basis leaves contain about twice the chlorophyll and nitrogen as stems, and are capable of up to 4-times the absolute CO₂ and O₂ exchange rates. However, the stem shape is such that lighting of the shaded side leads to a substantial increase in overall stem photosynthesis on a projected area basis, to about half the leaf rate in air. Stem conductance is lower than leaf conductance under most conditions and is less sensitive to high temperature or high VPD. Under most conditions, the ratio C _i /C _a is lower in stems than in leaves and stems show greater water-use efficiency (higher ratio assimilation/transpiration) as a function of VPD. This potential advantage of stem photosynthesis in a water limited environment may be offset by the higher VPD conditions in the hotter, drier part of the year when stems are active after leaves have senesced. Stem and leaf photosynthesis were similarly affected by decreasing plant water potential.

Photosynthetic acclimation and water-use efficiency of three species of understory herbaceous bamboo (Gramineae) in Panama

To assess the role of photosynthetic acclimation in the response of tropical understory herbs to treefall light gaps, photosynthetic response curves were determined for three species of herbaceous bamboo growing in treatments of sun and shade at Barro Calorado Island, Panama. Increased maximum photosynthetic capacity did not always accompany higher ramet production in the sun treatment. Pharus latifolius reproduced abundantly in both treatments, and produced more ramets and developed higher maximum photosynthetic capacity under higher irradiance. Streptochaeta spicata also produced a high percentage of reproductive ramets in both treatments and produced more ramets in the sun, did not show any significant differences in photosynthetic parameters between treatments. Streptochaeta sodiroana did not change maximum photosynthetic capacity in the sun, and had higher photosynthetic efficiency and lower mortality in the shade. Stable carbon isotope composition of leaves indicated that all three species developed higher water-use efficiency under higher irradiance. Photosynthetic flexibility may contribute to the ability of P. latifolius to reproduce in treefall gaps, whereas S. spicata and S. sodiroana may maintain the ability to fix carbon efficiently in low irradiance even when growing or persisting in gaps.