Environmental and physiological determinants of carbon isotope discrimination in terrestrial plants

Assessing geographic controls of hair isotopic variability in human populations: A case-study in Canada

Studying the isotope variability in fast-growing human tissues (e.g., hair, nails) is a powerful tool to investigate human nutrition. However, interpreting the controls of this isotopic variability at the population scale is often challenging as multiple factors can superimpose on the isotopic signals of a current population. Here, we analyse carbon, nitrogen, and sulphur isotopes in hair from 590 Canadian resident volunteers along with demographics, dietary and geographic information about each participant. We use a series of machine-learning regressions to demonstrate that the isotopic values in Canadian residents' hair are not only influenced by dietary choices but by geographic controls. First, we show that isotopic values in Canadian residents' hair have a limited range of variability consistent with the homogenization of Canadian dietary habits (as in other industrialized countries). As expected, some of the isotopic variability within the population correlates with recorded individual dietary choices. More interestingly, some regional spatial patterns emerge from carbon and sulphur isotope variations. The high carbon isotope composition of the hair of eastern Canadians relative to that of western Canadians correlates with the dominance of corn in the eastern Canadian food-industry. The gradient of sulphur isotope composition in Canadian hair from coast to inland regions correlates with the increasing soil pH and decreasing deposition of marine-derived sulphate aerosols in local food systems. We conclude that part of the isotopic variability found in the hair of Canadian residents reflects the isotopic signature associated with specific environmental conditions and agricultural practices of regional food systems transmitted to humans through the high consumption rate of intra-provincial food in Canada. Our study also underscores the strong potential of sulphur isotopes as tracers of human and food provenance.

Carbon isotope ratio of leaf litter correlates with litter production in a mangrove ecosystem in South China

As an important ecological process, litter production is generally recognized as being directly relevant to net primary productivity and carbon storage of mangrove ecosystems. In the present study, we made continuous, monthly assessment of litter production from 2010 to 2016 for five mangrove sites in Shenzhen Futian Mangrove Nature Reserve. Results showed that all mangrove locations displayed distinct seasonality in litter production, and that the alien species produced significantly more litters than the native species. Carbon isotope analysis revealed an interesting, strongly negative relationship between litter production and δ¹³C of leaf litter (δ¹³C_LL) among the five studied sites. Although it has long been known that δ¹³C of plant leaves correlates with water use efficiency and some components of plant productivity, the observed δ¹³C_LL-litter production linkage is novel, justifying future exploration of δ¹³C_LL as an potential indicator of litter production and net primary productivity in mangrove ecosystems.

Strong overestimation of water-use efficiency responses to rising CO2 in tree-ring studies

The carbon isotope ratio (δ¹³ C) in tree rings is commonly used to derive estimates of the assimilation-to-stomatal conductance rate of trees, that is, intrinsic water-use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO₂ concentrations (C_a ), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree-ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising C_a . We found that increases in tree size and stand age greatly influenced iWUE. The effect of C_a on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to C_a , mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO₂ effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO₂ effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.

Mesophyll CO2 conductance and leakiness are not responsive to short- and long-term soil water limitations in the C4 plant Sorghum bicolor

Breeding economically important C₄ crops for enhanced whole-plant water-use efficiency (WUE_plant ) is needed for sustainable agriculture. WUE_plant is a complex trait and an efficient phenotyping method that reports on components of WUE_plant , such as intrinsic water-use efficiency (WUE_i , the rate of leaf CO₂ assimilation relative to water loss via stomatal conductance), is needed. In C₄ plants, theoretical models suggest that leaf carbon isotope composition (δ¹³ C), when the efficiency of the CO₂ -concentrating mechanism (leakiness, ϕ) remains constant, can be used to screen for WUE_i . The limited information about how ϕ responds to water limitations confines the application of δ¹³ C for WUE_i screening of C₄ crops. The current research aimed to test the response of ϕ to short- or long-term moderate water limitations, and the relationship of δ¹³ C with WUE_i and WUE_plant , by addressing potential mesophyll CO₂ conductance (g_m ) and biochemical limitations in the C₄ plant Sorghum bicolor. We demonstrate that g_m and ϕ are not responsive to short- or long-term water limitations. Additionally, δ¹³ C was not correlated with gas-exchange estimates of WUE_i under short- and long-term water limitations, but showed a significant negative relationship with WUE_plant . The observed association between the δ¹³ C and WUE_plant suggests an intrinsic link of δ¹³ C with WUE_i in this C₄ plant, and can potentially be used as a screening tool for WUE_plant in sorghum.

Identification of quantitative trait loci for carbon isotope ratio (δ13C) in a recombinant inbred population of soybean

KEY MESSAGE

QTL analysis identified 16 QTLs, grouped in eight loci on seven soybean chromosomes that were associated with carbon isotope ratio (δ13C) in a biparental recombinant inbred population. Drought is a major limitation to soybean yield, and the frequency of drought stress is likely to increase under future climatic scenarios. Water use efficiency (WUE) is associated with drought tolerance, and carbon isotope ratio (δ13C) is positively correlated with WUE. In this study, 196 F6-derived recombinant inbred lines from a cross of PI 416997 (high WUE) × PI 567201D (low WUE) were evaluated in four environments to identify genomic regions associated with δ13C. There were positive correlations of δ13C values between different environments (0.67 ≤ r ≤ 0.78). Genotype, environment, and genotype × environment interactions had significant effects on δ13C. Narrow sense heritability of δ13C was 90% when estimated across environments. There was a total of 16 QTLs on seven chromosomes with individual QTLs explaining between 2.5 and 29.9% of the phenotypic variation and with additive effects ranging from 0.07 to 0.22‰. These 16 QTLs likely identified eight loci based on their overlapping confidence intervals. Of these eight loci, two loci on chromosome 20 (Gm20) were detected in at least three environments and were considered as stable QTLs. Additive QTLs on Gm20 showed epistatic interactions with 10 QTLs present across nine chromosomes. Five QTLs were identified across environments and showed significant QTL × environment interactions. These findings demonstrate that additive QTLs and QTL × QTL interactions play significant roles in genetic control of the δ13C trait. Markers flanking identified QTLs may facilitate marker-assisted selection to accumulate desirable QTLs to improve WUE and drought tolerance in soybean.

Dynamic responses of gas exchange and photochemistry to heat interference during drought in wheat and sorghum

Drought and heat stress significantly affect crop growth and productivity worldwide. It is unknown how heat interference during drought affects physiological processes dynamically in crops. Here we focussed on gas exchange and photochemistry in wheat and sorghum in response to simulated heat interference via +15°C of temperature during ~2 week drought and re-watering. Results showed that drought decreased net photosynthesis (Anet), stomatal conductance (gs), maximum velocity of ribulose-1, 5-bisphosphate carboxylase/oxygenase carboxylation (Vcmax) and electron transport rate (J) in both wheat and sorghum. Heat interference did not further reduce Anet or gs. Drought increased non-photochemical quenching (Φnpq), whereas heat interference decreased Φnpq. The δ13C of leaf, stem and roots was higher in drought-treated wheat but lower in drought-treated sorghum. The results suggest that (1) even under drought conditions wheat and sorghum increased or maintained gs for transpirational cooling to alleviate negative effects by heat interference; (2) non-photochemical quenching responded differently to drought and heat stress; (3) wheat and sorghum responded in opposing patterns in δ13C. These findings point to the importance of stomatal regulation under heat crossed with drought stress and could provide useful information on development of better strategies to secure crop production for future climate change.

Cold acclimation of mesophyll conductance, bundle-sheath conductance and leakiness in Miscanthus × giganteus

The cold acclimations of mesophyll conductance (g_m ), bundle-sheath conductance (g_bs ) and the CO₂ concentrating mechanism (CCM) of C₄ plants have not been well studied. Here, we estimated the temperature response of g_m , g_bs and leakiness (ϕ), the amount of concentrated CO₂ that escapes the bundle-sheath cells, for the chilling-tolerant C₄ plant Miscanthus × giganteus grown at 14 and 25°C. To estimate these parameters, we combined the C₄ -enzyme-limited photosynthesis model and the Δ¹³ C discrimination model. These combined models were parameterised using in vitro activities of carbonic anhydrase (CA), pyruvate, phosphate dikinase (PPDK), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and phosphoenolpyruvate carboxylase (PEPc). Cold-grown Miscanthus plants increased in vitro activities of RuBisCO and PPDK but decreased PEPc activity compared with warm-grown plants. Mesophyll conductance and g_bs responded strongly to measurement temperatures but did not differ between plants from the two growth temperatures. Furthermore, modelling showed that ϕ increased with measurement temperatures for both cold-grown and warm-grown plants, but was only marginally larger in cold-grown compared with warm-grown plants. Our results in Miscanthus support that g_m and g_bs are unresponsive to growth temperature and that the CCM is able to acclimate to cold through increased activity of PPDK and RuBisCO.

A theory of plant function helps to explain leaf-trait and productivity responses to elevation

Several publications have examined leaf-trait and carbon-cycling shifts along an Amazon-Andes transect spanning 3.5 km in elevation and 16°C in mean annual temperature. Photosynthetic capacity was previously shown to increase as temperature declines with increasing elevation, counteracting enzyme-kinetic effects. Primary production declines, nonetheless, due to decreasing light availability. We aimed to predict leaf-trait and production gradients from first principles, using published data to test an emerging theory whereby photosynthetic traits and primary production depend on optimal acclimation and/or adaptation to environment. We re-analysed published data for 210 species at 25 sites, fitting linear relationships to elevation for both predicted and observed photosynthetic traits and primary production. Declining leaf-internal/ambient CO₂ ratio (χ) and increasing carboxylation (V_cmax ) and electron-transport (J_max ) capacities with increasing elevation were predicted. Increases in leaf nitrogen content with elevation were explained by increasing V_cmax and leaf mass-per-area. Leaf and soil phosphorus covaried, but after controlling for elevation, no nutrient metric accounted for any additional variance in photosynthetic traits. Primary production was predicted to decline with elevation. This analysis unifies leaf and ecosystem observations in a common theoretical framework. The insensitivity of primary production to temperature is shown to emerge as a consequence of the optimisation of photosynthetic traits.

Why are tropical conifers disadvantaged in fertile soils? Comparison of Podocarpus guatemalensis with an angiosperm pioneer, Ficus insipida

Conifers are, for the most part, competitively excluded from tropical rainforests by angiosperms. Where they do occur, conifers often occupy sites that are relatively infertile. To gain insight into the physiological mechanisms by which angiosperms outcompete conifers in more productive sites, we grew seedlings of a tropical conifer (Podocarpus guatemalensis Standley) and an angiosperm pioneer (Ficus insipida Willd.) with and without added nutrients, supplied in the form of a slow-release fertilizer. At the conclusion of the experiment, the dry mass of P. guatemalensis seedlings in fertilized soil was approximately twofold larger than that of seedlings in unfertilized soil; on the other hand, the dry mass of F. insipida seedlings in fertilized soil was ~20-fold larger than seedlings in unfertilized soil. The higher relative growth rate of F. insipida was associated with a larger leaf area ratio and a higher photosynthetic rate per unit leaf area. Higher overall photosynthetic rates in F. insipida were associated with an approximately fivefold larger stomatal conductance than in P. guatemalensis. We surmise that a higher whole-plant hydraulic conductance in the vessel bearing angiosperm F. insipida enabled higher leaf area ratio and higher stomatal conductance per unit leaf area than in the tracheid bearing P. guatemalensis, which enabled F. insipida to capitalize on increased photosynthetic capacity driven by higher nitrogen availability in fertilized soil.

Low Salinity Improves Photosynthetic Performance in Panicum antidotale Under Drought Stress

Salinity and drought are two often simultaneously occurring abiotic stresses that limit the production of food crops worldwide. This study aimed to distinguish between the separate and combined impacts of drought and salinity on the plant response. Panicum antidotale was cultivated in a greenhouse under the following growth conditions: control, 100 mM NaCl (100) and 300 mM NaCl (300) salinity, drought (D; 30% irrigation), and two combinations of salinity and drought (100 + D and 300 + D). The growth response was as follows: 0 ≈ 100 > 100 + D > > D ≈ 300 ≈ 300 + D. Growth correlated directly with photosynthesis. The net photosynthesis, stomatal conductance, intercellular CO2, transpiration, ribulose 1,5-bisphosphate carboxylase (Rubisco), ribulose 1,5-bisphosphate (RuBP) regeneration, and triose phosphate utilization protein (e.g., phosphoenolpyruvate carboxylase) were highest in the control and declined most at 300 + D, while 100 + D performed significantly better as compared to drought. Maximum and actual photosystem II (PSII) efficiencies, along with photochemical quenching during light harvesting, resemble the plant growth and contemporary CO2/H2O gas exchange parameters in the given treatments. Plant improves water use efficiency under salt and drought treatments, which reflects the high water conservation ability of Panicum. Our findings indicate that the combination of low salinity with drought was able to minimize the deleterious effects of drought alone on growth, chlorophyll content, cell integrity, photosynthesis, leaf water potential, and water deficit. This synergetic effect demonstrates the positive role of Na+ and Cl- in carbon assimilation and osmotic adjustment. In contrast, the combination of high salinity and drought enforced the negative response of plants in comparison to single stress, demonstrating the antagonistic impact of water availability and ion toxicity.

Isotopic biomonitoring of anthropic carbon emissions in a megalopolis

Atmospheric pollution has become a serious threat for human health and the environment. However, the deployment, operation and maintenance of monitoring networks can represent a high cost for local governments. In certain locations, the use of naturally occurring plants for monitoring pollution can be a useful supplement of existing monitoring networks, and even provide information when other types of monitoring are lacking. In this work, we (i) determined the tissue carbon content and the δ13C values for the epiphytic CAM bromeliad Tillandsia recurvata and the relationship of both parameters with the existing CO concentrations in the Valley of Mexico basin and (ii) mapped the spatial distribution of such elemental and isotopic composition for this plant within the basin, in order to assess its potential as an atmospheric biomonitor of carbon monoxide, a pollutant with important repercussions on public health. The CO concentrations in the basin ranged from 0.41 ppm at rural locations to 0.81 ppm at urban sites. The carbon content of T. recurvata, which averaged 42.9 ± 0.34% (dry weight), was not influenced by the surrounding CO concentration. In contrast, the δ13C depended on the sites where the plants were collected. For example, the values were -13.2‰ in rural areas and as low as -17.5‰ in an urban site. Indeed, the isotopic values had a positive linear relationship with the atmospheric CO concentrations. Given the close relationship observed between the isotopic composition of T. recurvata with the CO concentrations in the Valley of Mexico, the δ13C values can be useful for the detection of atmospheric carbonaceous emissions.

Thirst or Malnutrition: The Impacts of Invasive Insect Agrilus mali on the Physiological Status of Wild Apple Trees

Malus sieversii (Ledeb.) M. Roem is a tertiary relict tree species and a rare and valuable resource for germplasm conservation. Since 1995, its wild forest has been severely destroyed by a devastating wood-boring beetle Agrilus mali Matsumura (Coleoptera: Buprestidae) in Xinjiang Uygur Autonomous Region, China. Where it invaded, this beetle infested more than 95% of the forests, and 80% of wild apple trees were reported dead in the hotspots. The physiological damage by A. mali infestation and their causality to tree death remain unclear. In this study, we attempted to explain the wild apple dieback from plant physiological perspectives, based on the hypothesis that the more damage M. sieversii suffered from the infestation of A. mali, the less water and fewer nutrients it could utilize. The study was conducted on trees with different extents of damage in wild apple forests over a large scale during 2016 and 2017. The stable carbon isotope ratio in leaves was analyzed to indicate tree water stress status. Total N, total P, total K, Ca2+ and Mg2+ were analyzed to reflect plant mineral nutrient status. The extent of damage was significantly associated with the leaf stable carbon isotope ratio in the drier year of 2016, but not significantly in 2017 with heavy rainfall in spring. The mineral nutrient contents of leaves were not significantly different among the four damage rankings in either year. The water stress experienced by M. sieversii was aggravated by the damage caused by A. mali, especially in a drought year, and indicates that the long-term water deficit caused by A. mali infestation may be the key factor leading to the decline of wild apple forests. The finding suggests the necessity of aerial irrigation for sustainable integrated pest management in wild apple trees.

Functional Divergence Drives Invasibility of Plant Communities at the Edges of a Resource Availability Gradient

Invasive Alien Species (IAS) are a serious threat to biodiversity, severely affecting natural habitats and species assemblages. However, no consistent empirical evidence emerged on which functional traits or trait combination may foster community invasibility. Novel insights on the functional features promoting community invasibility may arise from the use of mechanistic traits, like those associated with drought resistance, which have been seldom included in trait-based studies. Here, we tested for the functional strategies of native and invasive assemblage (i.e., environmental filtering hypothesis vs. niche divergence), and we assessed how the functional space determined by native species could influence community invasibility at the edges of a resource availability gradient. Our results showed that invasive species pools need to have a certain degree of differentiation in order to persist in highly invaded communities, suggesting that functional niche divergence may foster community invasibility. In addition, resident native communities more susceptible to invasion are those which, on average, have higher resource acquisition capacity, and lower drought resistance coupled with an apparently reduced water-use efficiency. We advocate the use of a mechanistic perspective in future research to comprehensively understand invasion dynamics, providing also new insights on the factors underlying community invasibility in different ecosystems.

Carbon and nitrogen isotopic variability in foxtail millet (Setaria italica) with watering regime

RATIONALE

Carbonised plant remains are analysed for reconstruction of past climates and agricultural regimes. Several recent studies have used C4 plants to address related questions, and correlations between modern C4 plant δ13 C values and rainfall have been found. The millets were important food crops in prehistoric Eurasia, yet little is known about causes of isotopic variation within millet species. Previous research has shown there to be significant isotopic variation between millet accessions. Here we compare isotope ratios from plants grown under different watering regimes. This allows for a consideration of whether or not Setaria italica is a good proxy for environmental reconstruction.

METHODS

We compare stable isotope ratios of Setaria italica plants grown in a controlled environment chamber with different watering regimes. We compare the carbon isotope ratios of leaves and grains, and the nitrogen isotope ratios of grains, from 12 accessions of Setaria italica.

RESULTS

We find significant isotopic variability between watering regimes. Carbon isotope ratios are positively correlated with water availability, and on average vary by 1.9‰ and 1.7‰ for leaves and grains, respectively. Grain nitrogen isotope ratios also vary with watering regime; however, the highest isotope ratios are found with the 130-mL watering regime.

CONCLUSIONS

The carbon isotope ratios of Setaria italica are strongly correlated with water availability. However, the correlation is the opposite to that seen in studies of C3 plants. The difference in isotopic ratio due to watering regime is comparable with that seen between different accessions; thus distinguishing between changing varieties of Setaria italica and changing climate is problematic. In terms of grain nitrogen isotope ratios, the highest δ15 N values were not associated with the lowest watering regime. Again, δ15 N variation is comparable with that which would be expected from an aridity effect or a manuring effect, and thus distinguishing between these factors is probably problematic.

Camelid husbandry in the Atacama Desert? A stable isotope study of camelid bone collagen and textiles from the Lluta and Camarones Valleys, northern Chile

Management of camelids in the coastal valleys of the Andes has generated much debate in recent years. Zooarchaeological and isotopic studies have demonstrated that in the coastal valleys of northern and southern Peru there were locally maintained camelid herds. Because of the hyperarid conditions of the northern coast of Chile, this region has been assumed to be unsuitable for the raising of camelids. In this study we report stable carbon and nitrogen isotopic compositions of camelid bone collagen and textiles made from camelid fiber from Late Intermediate Period (LIP) and Late Horizon (LH) occupations in northern Chilean river valleys. The camelid bone collagen isotopic compositions are consistent with these animals originating in the highlands, although there is a significant difference in the camelids dating to the LIP and LH, possibly because of changes made to distribution and exchange networks by the Inca in the LH. There were no differences between the isotopic compositions of the camelid fibers sampled from textiles in the LIP and LH, suggesting that either the production of camelid fiber was unchanged by the Inca or the changes that were made do not present visible isotopic evidence. Several camelid fiber samples from both the LIP and LH present very high δ13C and δ15N values, comparable to human hair samples from one site (Huancarane) in the Camarones Valley. These data suggest that people in the northern valleys of Chile may have kept small numbers of animals specifically for fiber production. Overall, however, the vast majority of the textile samples have isotopic compositions that are consistent with an origin in the highlands. These data suggest that the hyperarid coastal river valleys of northern Chile did not support substantial camelid herds as has been interpreted for northern Peru.

Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

The conductance of carbon dioxide (CO₂ ) from the substomatal cavities to the initial sites of CO₂ fixation (g_m ) can significantly reduce the availability of CO₂ for photosynthesis. There have been many recent reviews on: (i) the importance of g_m for accurately modelling net rates of CO₂ assimilation, (ii) on how leaf biochemical and anatomical factors influence g_m , (iii) the technical limitation of estimating g_m , which cannot be directly measured, and (iv) how g_m responds to long- and short-term changes in growth and measurement environmental conditions. Therefore, this review will highlight these previous publications but will attempt not to repeat what has already been published. We will instead initially focus on the recent developments on the two-resistance model of g_m that describe the potential of photorespiratory and respiratory CO₂ released within the mitochondria to diffuse directly into both the chloroplast and the cytosol. Subsequently, we summarize recent developments in the three-dimensional (3-D) reaction-diffusion models and 3-D image analysis that are providing new insights into how the complex structure and organization of the leaf influences g_m . Finally, because most of the reviews and literature on g_m have traditionally focused on C₃ plants we review in the final sections some of the recent developments, current understanding and measurement techniques of g_m in C₄ and crassulacean acid metabolism (CAM) plants. These plants have both specialized leaf anatomy and either a spatially or temporally separated CO₂ concentrating mechanisms (C₄ and CAM, respectively) that influence how we interpret and estimate g_m compared with a C₃ plants.

Gas exchange and water-use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water-use efficiency and carbon isotope discrimination in C₃ plant canopies. I then present a case study of water-use efficiency in northern Australian tree species. In general, C₃ plants face a trade-off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO₂ assimilation rate, but a lower photosynthetic water-use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water-use efficiency. This may explain why community-level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water-use efficiency that take place during leaf expansion in C₃ plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water-use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO₂ can increase whole-plant water-use efficiency. Finally, I discuss the role of water-use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO₂ . In coming decades, increases in plant water-use efficiency caused by rising CO₂ are likely to partially mitigate impacts on plants of drought stress caused by global warming.

Plant functional traits differ in adaptability and are predicted to be differentially affected by climate change

Climate change is testing the resilience of forests worldwide pushing physiological tolerance to climatic extremes. Plant functional traits have been shown to be adapted to climate and have evolved patterns of trait correlations (similar patterns of distribution) and coordinations (mechanistic trade-off). We predicted that traits would differentiate between populations associated with climatic gradients, suggestive of adaptive variation, and correlated traits would adapt to future climate scenarios in similar ways.We measured genetically determined trait variation and described patterns of correlation for seven traits: photochemical reflectance index (PRI), normalized difference vegetation index (NDVI), leaf size (LS), specific leaf area (SLA), δ13C (integrated water-use efficiency, WUE), nitrogen concentration (NCONC), and wood density (WD). All measures were conducted in an experimental plantation on 960 trees sourced from 12 populations of a key forest canopy species in southwestern Australia.Significant differences were found between populations for all traits. Narrow-sense heritability was significant for five traits (0.15-0.21), indicating that natural selection can drive differentiation; however, SLA (0.08) and PRI (0.11) were not significantly heritable. Generalized additive models predicted trait values across the landscape for current and future climatic conditions (>90% variance). The percent change differed markedly among traits between current and future predictions (differing as little as 1.5% (δ13C) or as much as 30% (PRI)). Some trait correlations were predicted to break down in the future (SLA:NCONC, δ13C:PRI, and NCONC:WD).Synthesis: Our results suggest that traits have contrasting genotypic patterns and will be subjected to different climate selection pressures, which may lower the working optimum for functional traits. Further, traits are independently associated with different climate factors, indicating that some trait correlations may be disrupted in the future. Genetic constraints and trait correlations may limit the ability for functional traits to adapt to climate change.

Phenotypic plasticity of natural Populus trichocarpa populations in response to temporally environmental change in a common garden

BACKGROUND

Natural selection on fitness-related traits can be temporally heterogeneous among populations. As climate changes, understanding population-level responses is of scientific and practical importance. We examined 18 phenotypic traits associated with phenology, biomass, and ecophysiology in 403 individuals of natural Populus trichocarpa populations, growing in a common garden.

RESULTS

Compared with tree origin settings, propagules likely underwent drought exposures in the common garden due to significantly low rainfall during the years of measurement. All study traits showed population differentiation reflecting adaptive responses due to local genetic adaptation. Phenology and biomass traits were strongly under selection and showed plastic responses between years, co-varying with latitude. While phenological events (e.g., bud set and growth period) and biomass were under positive directional selection, post-bud set period, particularly from final bud set to the onset of leaf drop, was selected against. With one exception to water-use efficiency, ecophysiology traits were under negative directional selection. Moreover, extended phenological events jointly evolved with source niches under increased temperature and decreased rainfall exposures. High biomass coevolved with climatic niches of high temperature; low rainfall promoted high photosynthetic rates evolution.

CONCLUSIONS

This work underpins that P. trichocarpa is likely to experience increased fitness (height gain) by evolving toward extended bud set and growth period, abbreviated post-bud set period, and increased drought resistance, potentially constituting a powerful mechanism for long-lived tree species in surviving unpredictably environmental extremes (e.g., drought).

The impact of rising CO2 and acclimation on the response of US forests to global warming

The response of forests to climate change depends in part on whether the photosynthetic benefit from increased atmospheric CO2 (∆Ca = future minus historic CO2) compensates for increased physiological stresses from higher temperature (∆T). We predicted the outcome of these competing responses by using optimization theory and a mechanistic model of tree water transport and photosynthesis. We simulated current and future productivity, stress, and mortality in mature monospecific stands with soil, species, and climate sampled from 20 continental US locations. We modeled stands with and without acclimation to ∆Ca and ∆T, where acclimated forests adjusted leaf area, photosynthetic capacity, and stand density to maximize productivity while avoiding stress. Without acclimation, the ∆Ca-driven boost in net primary productivity (NPP) was compromised by ∆T-driven stress and mortality associated with vascular failure. With acclimation, the ∆Ca-driven boost in NPP and stand biomass (C storage) was accentuated for cooler futures but negated for warmer futures by a ∆T-driven reduction in NPP and biomass. Thus, hotter futures reduced forest biomass through either mortality or acclimation. Forest outcomes depended on whether projected climatic ∆Ca/∆T ratios were above or below physiological thresholds that neutralized the negative impacts of warming. Critically, if forests do not acclimate, the ∆Ca/∆T must be above ca 89 ppm⋅°C-1 to avoid chronic stress, a threshold met by 55% of climate projections. If forests do acclimate, the ∆Ca/∆T must rise above ca 67 ppm⋅°C-1 for NPP and biomass to increase, a lower threshold met by 71% of projections.

An inter-island comparison of Darwin's finches reveals the impact of habitat, host phylogeny, and island on the gut microbiome

Darwin's finch species in the Galapagos Archipelago are an iconic adaptive radiation that offer a natural experiment to test for the various factors that influence gut microbiome composition. The island of Floreana has the longest history of human settlement within the archipelago and offers an opportunity to compare island and habitat effects on Darwin's finch microbiomes. In this study, we compare gut microbiomes in Darwin's finch species on Floreana Island to test for effects of host phylogeny, habitat (lowlands, highlands), and island (Floreana, Santa Cruz). We used 16S rRNA Illumina sequencing of fecal samples to assess the gut microbiome composition of Darwin's finches, complemented by analyses of stable isotope values and foraging data to provide ecological context to the patterns observed. Overall bacterial composition of the gut microbiome demonstrated co-phylogeny with Floreana hosts, recapitulated the effect of habitat and diet, and showed differences across islands. The finch phylogeny uniquely explained more variation in the microbiome than did foraging data. Finally, there were interaction effects for island × habitat, whereby the same Darwin's finch species sampled on two islands differed in microbiome for highland samples (highland finches also had different diets across islands) but not lowland samples (lowland finches across islands had comparable diet). Together, these results corroborate the influence of phylogeny, age, diet, and sampling location on microbiome composition and emphasize the necessity for comprehensive sampling given the multiple factors that influence the gut microbiome in Darwin's finches, and by extension, in animals broadly.

Rising CO2 drives divergence in water use efficiency of evergreen and deciduous plants

Intrinsic water use efficiency (iWUE), defined as the ratio of photosynthesis to stomatal conductance, is a key variable in plant physiology and ecology. Yet, how rising atmospheric CO₂ concentration affects iWUE at broad species and ecosystem scales is poorly understood. In a field-based study of 244 woody angiosperm species across eight biomes over the past 25 years of increasing atmospheric CO₂ (~45 ppm), we show that iWUE in evergreen species has increased more rapidly than in deciduous species. Specifically, the difference in iWUE gain between evergreen and deciduous taxa diverges along a mean annual temperature gradient from tropical to boreal forests and follows similar observed trends in leaf functional traits such as leaf mass per area. Synthesis of multiple lines of evidence supports our findings. This study provides timely insights into the impact of Anthropocene climate change on forest ecosystems and will aid the development of next-generation trait-based vegetation models.

Giant cacti: isotopic recorders of climate variation in warm deserts of the Americas

The plant family Cactaceae is considered among the most threatened groups of organisms on the planet. The threatened status of the cacti family has created a renewed interest in the highly evolved physiological and morphological traits that underpin their persistence in some of the harshest subtropical environments in the Americas. Among the most important anatomical features of cacti is the modification of leaves into spines, and previous work has shown that the stable isotope chemistry of cacti spines records potential variations in stem water balance, stress, and Crassulacean acid metabolism (CAM). We review the opportunities, challenges, and pitfalls in measuring δ 13C, δ 2H, and δ 18O ratios captured in spine tissues that potentially reflect temporal and spatial patterns of stomatal conductance, internal to atmospheric CO2 partial pressures, and subsequent patterns of photosynthetic gas exchange. We then evaluate the challenges in stable isotope analysis in spine tissues related to variation in CAM expression, stem water compartmentalization, and spine whole-tissue composition among other factors. Finally, we describe how the analysis of all three isotopes can be used in combination to provide potentially robust analysis of photosynthetic function in cacti, and other succulent-stemmed taxa across broad spatio-temporal environmental gradients.

Transgenic maize phosphoenolpyruvate carboxylase alters leaf-atmosphere CO2 and 13CO2 exchanges in Oryza sativa

The engineering process of C4 photosynthesis into C3 plants requires an increased activity of phosphoenolpyruvate carboxylase (PEPC) in the cytosol of leaf mesophyll cells. The literature varies on the physiological effect of transgenic maize (Zea mays) PEPC (ZmPEPC) leaf expression in Oryza sativa (rice). Therefore, to address this issue, leaf-atmosphere CO2 and 13CO2 exchanges were measured, both in the light (at atmospheric O2 partial pressure of 1.84 kPa and at different CO2 levels) and in the dark, in transgenic rice expressing ZmPEPC and wild-type (WT) plants. The in vitro PEPC activity was 25 times higher in the PEPC overexpressing (PEPC-OE) plants (~20% of maize) compared to the negligible activity in WT. In the PEPC-OE plants, the estimated fraction of carboxylation by PEPC (β) was ~6% and leaf net biochemical discrimination against 13CO2[Formula: see text] was ~ 2‰ lower than in WT. However, there were no differences in leaf net CO2 assimilation rates (A) between genotypes, while the leaf dark respiration rates (Rd) over three hours after light-dark transition were enhanced (~ 30%) and with a higher 13C composition [Formula: see text] in the PEPC-OE plants compared to WT. These data indicate that ZmPEPC in the PEPC-OE rice plants contributes to leaf carbon metabolism in both the light and in the dark. However, there are some factors, potentially posttranslational regulation and PEP availability, which reduce ZmPEPC activity in vivo.

Geochemical Evidence for the Control of Fire by Middle Palaeolithic Hominins

The use of fire played an important role in the social and technological development of the genus Homo. Most archaeologists agree that this was a multi-stage process, beginning with the exploitation of natural fires and ending with the ability to create fire from scratch. Some have argued that in the Middle Palaeolithic (MP) hominin fire use was limited by the availability of fire in the landscape. Here, we present a record of the abundance of polycyclic aromatic hydrocarbons (PAHs), organic compounds that are produced during the combustion of organic material, from Lusakert Cave, a MP site in Armenia. We find no correlation between the abundance of light PAHs (3-4 rings), which are a major component of wildfire PAH emissions and are shown to disperse widely during fire events, and heavy PAHs (5-6 rings), which are a major component of particulate emissions of burned wood. Instead, we find heavy PAHs correlate with MP artifact density at the site. Given that hPAH abundance correlates with occupation intensity rather than lPAH abundance, we argue that MP hominins were able to control fire and utilize it regardless of the variability of fires in the environment. Together with other studies on MP fire use, these results suggest that the ability of hominins to manipulate fire independent of exploitation of wildfires was spatially variable in the MP and may have developed multiple times in the genus Homo.

Optimization of photosynthesis and stomatal conductance in the date palm Phoenix dactylifera during acclimation to heat and drought

We studied acclimation of leaf gas exchange to differing seasonal climate and soil water availability in slow-growing date palm (Phoenix dactylifera) seedlings. We used an extended Arrhenius equation to describe instantaneous temperature responses of leaf net photosynthesis (A) and stomatal conductance (G), and derived physiological parameters suitable for characterization of acclimation (T_opt , A_opt and T_equ ). Optimum temperature of A (T_opt ) ranged between 20-33°C in winter and 28-45°C in summer. Growth temperature (T_growth ) explained c. 50% of the variation in T_opt , which additionally depended on leaf water status at the time of measurement. During water stress, light-saturated rates of A at T_opt (i.e. A_opt ) were reduced to 30-80% of control levels, albeit not limited by CO₂ supply per se. Equilibrium temperature (T_equ ), around which A/G and substomatal [CO₂ ] are constant, remained tightly coupled with T_opt . Our results suggest that acclimatory shifts in T_opt and A_opt reflect a balance between maximization of photosynthesis and minimization of the risk of metabolic perturbations caused by imbalances in cellular [CO₂ ]. This novel perspective on acclimation of leaf gas exchange is compatible with optimization theory, and might help to elucidate other acclimation and growth strategies in species adapted to differing climates.

Identification of microRNAS differentially regulated by water deficit in relation to mycorrhizal treatment in wheat

Arbuscular mycorrhizal fungi (AMF) are soil microrganisms that establish symbiosis with plants positively influencing their resistance to abiotic stresses. The aim of this work was to identify wheat miRNAs differentially regulated by water deficit conditions in presence or absence of AMF treatment. Small RNA libraries were constructed for both leaf and root tissues considering four conditions: control (irrigated) or water deficit in presence/absence of mycorrhizal (AMF) treatment. A total of 12 miRNAs were significantly regulated by water deficit in leaves: five in absence and seven in presence of AMF treatment. In roots, three miRNAs were water deficit-modulated in absence of mycorrhizal treatment while six were regulated in presence of it. The most represented miRNA family was miR167 that was regulated by water deficit in both leaf and root tissues. Interestingly, miR827-5p was differentially regulated in leaves in the absence of mycorrhizal treatment while it was water deficit-modulated in roots irrespective of AMF treatment. In roots, water deficit repressed miR827-5p, miR394, miR6187, miR167e-3p, and miR9666b-3p affecting transcription, RNA synthesis, protein synthesis, and protein modifications. In leaves, mycorrhizae modulated miR5384-3p and miR156e-3p affecting trafficking and cell redox homeostasis. DNA replication and transcription regulation should be targeted by the repression of miR1432-5p and miR166h-3p. This work provided interesting insights into the post-transcriptional mechanisms of wheat responses to water deficit in relation to mycorrhizal symbiosis.

A leaf-level biochemical model simulating the introduction of C2 and C4 photosynthesis in C3 rice: gains, losses and metabolite fluxes

This work aims at developing an adequate theoretical basis for comparing assimilation of the ancestral C₃ pathway with CO₂ concentrating mechanisms (CCM) that have evolved to reduce photorespiratory yield losses. We present a novel model for C₃ , C₂ , C₂  + C₄ and C₄ photosynthesis simulating assimilatory metabolism, energetics and metabolite traffic at the leaf level. It integrates a mechanistic description of light reactions to simulate ATP and NADPH production, and a variable engagement of cyclic electron flow. The analytical solutions are compact and thus suitable for larger scale simulations. Inputs were derived with a comprehensive gas-exchange experiment. We show trade-offs in the operation of C₄ that are in line with ecophysiological data. C₄ has the potential to increase assimilation over C₃ at high temperatures and light intensities, but this benefit is reversed under low temperatures and light. We apply the model to simulate the introduction of progressively complex levels of CCM into C₃ rice, which feeds > 3.5 billion people. Increasing assimilation will require considerable modifications such as expressing the NAD(P)H Dehydrogenase-like complex and upregulating cyclic electron flow, enlarging the bundle sheath, and expressing suitable transporters to allow adequate metabolite traffic. The simpler C₂ rice may be a desirable alternative.

Robust Response of Terrestrial Plants to Rising CO2

Human-caused CO₂ emissions over the past century have caused the climate of the Earth to warm and have directly impacted on the functioning of terrestrial plants. We examine the global response of terrestrial gross primary production (GPP) to the historic change in atmospheric CO₂. The GPP of the terrestrial biosphere has increased steadily, keeping pace remarkably in proportion to the rise in atmospheric CO₂. Water-use efficiency, namely the ratio of CO₂ uptake by photosynthesis to water loss by transpiration, has increased as a direct leaf-level effect of rising CO₂. This has allowed an increase in global leaf area, which has conspired with stimulation of photosynthesis per unit leaf area to produce a maximal response of the terrestrial biosphere to rising atmospheric CO₂ and contemporary climate change.

Diurnal variation in mesophyll conductance and its influence on modelled water-use efficiency in a mature boreal Pinus sylvestris stand

Mesophyll conductance (gm) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although gm is similar in magnitude to stomatal conductance (gs), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO2 analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic 13C discrimination (Δ13C) on mature Pinus sylvestris trees. This allowed the calculation of gm, gs, net photosynthesis (Anet), and WUE. These measurements highlighted the asynchronous diurnal behaviour of gm and gs. While gs declined from around 10:00, Anet declined first after 12:00, and gm remained near its maximum until 16:00. We suggest that high gm played a role in supporting an extended Anet peak despite stomatal closure. Comparing three models to estimate WUE from ∆13C, we found that a simple model, assuming constant net fractionation during carboxylation (27‰), predicted WUE well, but only for about 75% of the day. A more comprehensive model, accounting explicitly for gm and the effects of daytime respiration and photorespiration, gave reliable estimates of WUE, even in the early morning hours when WUE was more variable. Considering constant, finite gm or gm/gs yielded similar WUE estimates on the diurnal scale, while assuming infinite gm led to overestimation of WUE. These results highlight the potential of high-resolution gm measurements to improve modelling of Anet and WUE and demonstrate that such gm data can be acquired, even under field conditions.

Critical review: incorporating the arrangement of mitochondria and chloroplasts into models of photosynthesis and carbon isotope discrimination

The arrangement of mitochondria and chloroplasts, together with the relative resistances of cell wall and chloroplast, determine the path of diffusion out of the leaf for (photo)respired CO₂. Traditional photosynthesis models have assumed a tight arrangement of chloroplasts packed together against the cell wall with mitochondria located behind the chloroplasts, deep inside the cytosol. Accordingly, all (photo)respired CO₂ must cross the chloroplast before diffusing out of the leaf. Different arrangements have recently been considered, where all or part of the (photo)respired CO₂ diffuses through the cytosol without ever entering the chloroplast. Assumptions about the path for the (photo)respiratory flux are particularly relevant for the calculation of mesophyll conductance (g_m). If (photo)respired CO₂ can diffuse elsewhere besides the chloroplast, apparent g_m is no longer a mere physical resistance but a flux-weighted variable sensitive to the ratio of (photo)respiration to net CO₂ assimilation. We discuss existing photosynthesis models in conjunction with their treatment of the (photo)respiratory flux and present new equations applicable to the generalized case where (photo)respired CO₂ can diffuse both into the chloroplast and through the cytosol. Additionally, we present a new generalized Δ¹³C model that incorporates this dual diffusion pathway. We assess how assumptions about the fate of (photo)respired CO₂ affect the interpretation of photosynthetic data and the challenges it poses for the application of different models.

Global patterns of intraspecific leaf trait responses to elevation

Elevational gradients are often used to quantify how traits of plant species respond to abiotic and biotic environmental variations. Yet, such analyses are frequently restricted spatially and applied along single slopes or mountain ranges. Since we know little on the response of intraspecific leaf traits to elevation across the globe, we here perform a global meta-analysis of leaf traits in 109 plant species located in 4 continents and reported in 71 studies published between 1983 and 2018. We quantified the intraspecific change in seven morpho-ecophysiological leaf traits along global elevational gradients: specific leaf area (SLA), leaf mass per area (LMA), leaf area (LA), nitrogen concentration per unit of area (Narea), nitrogen concentration per unit mass (Nmass), phosphorous concentration per unit mass (Pmass) and carbon isotope composition (δ¹³ C). We found LMA, Narea, Nmass and δ¹³ C to significantly increase and SLA to decrease with increasing elevation. Conversely, LA and Pmass showed no significant pattern with elevation worldwide. We found significantly larger increase in Narea, Nmass, Pmass and δ¹³ C with elevation in warmer regions. Larger responses to increasing elevation were apparent for SLA of herbaceous compared to woody species, but not for the other traits. Finally, we also detected evidences of covariation across morphological and physiological traits within the same elevational gradient. In sum, we demonstrate that there are common cross-species patterns of intraspecific leaf trait variation across elevational gradients worldwide. Irrespective of whether such variation is genetically determined via local adaptation or attributed to phenotypic plasticity, the leaf trait patterns quantified here suggest that plant species are adapted to live on a range of temperature conditions. Since the distribution of mountain biota is predominantly shifting upslope in response to changes in environmental conditions, our results are important to further our understanding of how plants species of mountain ecosystems adapt to global environmental change.

Partitioning of mesophyll conductance for CO2 into intercellular and cellular components using carbon isotope composition of cuticles from opposite leaf sides

We suggest a new technique for estimating the relative drawdown of CO₂ concentration (c) in the intercellular air space (IAS) across hypostomatous leaves (expressed as the ratio c_d/c_b, where the indexes d and b denote the adaxial and abaxial edges, respectively, of IAS), based on the carbon isotope composition (δ¹³C) of leaf cuticular membranes (CMs), cuticular waxes (WXs) or epicuticular waxes (EWXs) isolated from opposite leaf sides. The relative drawdown in the intracellular liquid phase (i.e., the ratio c_c/c_bd, where c_c and c_bd stand for mean CO₂ concentrations in chloroplasts and in the IAS), the fraction of intercellular resistance in the total mesophyll resistance (r_IAS/r_m), leaf thickness, and leaf mass per area (LMA) were also assessed. We show in a conceptual model that the upper (adaxial) side of a hypostomatous leaf should be enriched in ¹³C compared to the lower (abaxial) side. CM, WX, and/or EWX isolated from 40 hypostomatous C₃ species were ¹³C depleted relative to bulk leaf tissue by 2.01-2.85‰. The difference in δ¹³C between the abaxial and adaxial leaf sides (δ¹³C_AB - ¹³C_AD, Δ_b-d), ranged from - 2.22 to + 0.71‰ (- 0.09 ± 0.54‰, mean ± SD) in CM and from - 7.95 to 0.89‰ (- 1.17 ± 1.40‰) in WX. In contrast, two tested amphistomatous species showed no significant Δ_b-d difference in WX. Δ_b-d correlated negatively with LMA and leaf thickness of hypostomatous leaves, which indicates that the mesophyll air space imposes a non-negligible resistance to CO₂ diffusion. δ¹³C of EWX and 30-C aldehyde in WX reveal a stronger CO₂ drawdown than bulk WX or CM. Mean values of c_d/c_b and c_c/c_bd were 0.90 ± 0.12 and 0.66 ± 0.11, respectively, across 14 investigated species in which wax was isolated and analyzed. The diffusion resistance of IAS contributed 20 ± 14% to total mesophyll resistance and reflects species-specific and environmentally-induced differences in leaf functional anatomy.

Differences in carbon isotope leaf-to-phloem fractionation and mixing patterns along a vertical gradient in mature European beech and Douglas fir

While photosynthetic isotope discrimination is well understood, the postphotosynthetic and transport-related fractionation mechanisms that influence phloem and subsequently tree ring δ¹³ C are less investigated and may vary among species. We studied the seasonal and diel courses of leaf-to-phloem δ¹³ C differences of water-soluble organic matter (WSOM) in vertical crown gradients and followed the assimilate transport via the branches to the trunk phloem at breast height in European beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii). δ¹³ C of individual sugars and cyclitols from a subsample was determined by compound-specific isotope analysis. In beech, leaf-to-phloem δ¹³ C differences in WSOM increased with height and were partly caused by biochemical isotope fractionation between leaf compounds. ¹³ C-Enrichment of phloem sugars relative to leaf sucrose implies an additional isotope fractionation mechanism related to leaf assimilate export. In Douglas fir, leaf-to-phloem δ¹³ C differences were much smaller and isotopically invariant pinitol strongly influenced leaf and phloem WSOM. Trunk phloem WSOM at breast height reflected canopy-integrated δ¹³ C in beech but not in Douglas fir. Our results demonstrate that leaf-to-phloem isotope fractionation and δ¹³ C mixing patterns along vertical gradients can differ between tree species. These effects have to be considered for functional interpretations of trunk phloem and tree ring δ¹³ C.

Compound-specific carbon isotope patterns in needles of conifer tree species from the Swiss National Park under recent climate change

Elevated CO₂ along with rising temperature and water deficits can lead to changes in tree physiology and leaf biochemistry. These changes can increase heat- and drought-induced tree mortality. We aim to reveal the impacts of climatic drivers on individual compounds at the leaf level among European larch (Larix decidua) and mountain pine (Pinus mugo) trees, which are widely distributed at high elevations. We investigated seasonal carbon isotope composition (δ¹³C) and concentration patterns of carbohydrates and organic acids in needles of these two different species from a case study in the Swiss National Park (SNP). We found that average and minimum air temperatures were the main climatic drivers of seasonal variation of δ¹³C in sucrose and glucose as well as in concentrations of carbohydrates and citric acid/citrate in needles of both tree species. The impact of seasonal climatic drivers on larch and mountain pine trees at the needle level is in line with our earlier study in this region for long-term changes at the tree-ring level. We conclude that the species-specific changes in δ¹³C and concentrations of carbohydrates and organic acids are sensitive indicators of changes in the metabolic pathways occurring as a result of climatic changes.

Accelerated flowering time reduces lifetime water use without penalizing reproductive performance in Arabidopsis

Natural selection driven by water availability has resulted in considerable variation for traits associated with drought tolerance and leaf-level water-use efficiency (WUE). In Arabidopsis, little is known about the variation of whole-plant water use (PWU) and whole-plant WUE (transpiration efficiency). To investigate the genetic basis of PWU, we developed a novel proxy trait by combining flowering time and rosette water use to estimate lifetime PWU. We validated its usefulness for large-scale screening of mapping populations in a subset of ecotypes. This parameter subsequently facilitated the screening of water use and drought tolerance traits in a recombinant inbred line population derived from two Arabidopsis accessions with distinct water-use strategies, namely, C24 (low PWU) and Col-0 (high PWU). Subsequent quantitative trait loci mapping and validation through near-isogenic lines identified two causal quantitative trait loci, which showed that a combination of weak and nonfunctional alleles of the FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) genes substantially reduced plant water use due to their control of flowering time. Crucially, we observed that reducing flowering time and consequently water use did not penalize reproductive performance, as such water productivity (seed produced per unit of water transpired) improved. Natural polymorphisms of FRI and FLC have previously been elucidated as key determinants of natural variation in intrinsic WUE (δ13 C). However, in the genetic backgrounds tested here, drought tolerance traits, stomatal conductance, δ13 C. and rosette water use were independent of allelic variation at FRI and FLC, suggesting that flowering is critical in determining lifetime PWU but not always leaf-level traits.

The ideal percentage of K substitution by Na in Eucalyptus seedlings: Evidences from leaf carbon isotopic composition, leaf gas exchanges and plant growth

Potassium (K) is the most required macronutrient by Eucalyptus, while sodium (Na) can partially substitute some physiological functions of K and have a positive response on plant growth in K-depleted tropical soils. However, the right percentage of K substitution by Na is not yet known for Eucalyptus seedlings, since a few experiments have only compared treatments receiving K or Na. This study evaluated five levels of Na supply (0, 0.45, 0.90, 1.35 and 1.80 mM) as substitution for K in Eucalyptus seedlings grown in nutrient solution. Plants growth, biomass, K-nutritional status, leaf gas exchange, leaf carbon isotopic composition (δ¹³C ‰), leaf water potential (Ψ_w), leaf area (LA), stomatal density (SD) and water use efficiency (WUE) were measured. The highest total biomass yield was achieved by the Na estimated rate of 0.25 mM, corresponding to a leaf K: Na ratio of 3.41, and having the lowest δ¹³C values. Conversely, the highest Na rate (1.8 mM) induced K deficiency symptoms, lower growth, reduced total dry matter yield, leaf gas exchange, LA, SD and a higher δ¹³C, which presented a trend to an inverse correlation with CO₂ assimilation rate (A), WUE and shoot dry matter. Collectively, our results conclude that substitution of 25% of K by Na (0.45 mM of Na) provided significant gains in nutritional status and positive plant physiological responses by increasing WUE, stomatal diffusion, and by augmenting CO₂ uptake efficiency. This nutritional management can therefore be an alternative option to optimize yields and resource use efficiencies in Eucalyptus cultivation.

Tree-ring isotopes adjacent to Lake Superior reveal cold winter anomalies for the Great Lakes region of North America

Tree-ring carbon isotope discrimination (Δ¹³C) and oxygen isotopes (δ¹⁸O) collected from white pine (Pinus strobus) trees adjacent to Lake Superior show potential to produce the first winter-specific paleoclimate reconstruction with inter-annual resolution for this region. Isotopic signatures from 1976 to 2015 were strongly linked to antecedent winter minimum temperatures (T_min), Lake Superior peak ice cover, and regional to continental-scale atmospheric winter pressure variability including the North American Dipole. The immense thermal inertia of Lake Superior underlies the unique connection between winter conditions and tree-ring Δ¹³C and δ¹⁸O signals from the following growing season in trees located near the lake. By combining these signals, we demonstrate feasibility to reconstruct variability in T_min, ice cover, and continental-scale atmospheric circulation patterns (r ≥ 0.65, P < 0.001).

C4-like photosynthesis and the effects of leaf senescence on C4-like physiology in Sesuvium sesuvioides (Aizoaceae)

Sesuvium sesuvioides (Sesuvioideae, Aizoaceae) is a perennial, salt-tolerant herb distributed in flats, depressions, or disturbed habitats of southern Africa and the Cape Verdes. Based on carbon isotope values, it is considered a C4 species, despite a relatively high ratio of mesophyll to bundle sheath cells (2.7:1) in the portulacelloid leaf anatomy. Using leaf anatomy, immunocytochemistry, gas exchange measurements, and enzyme activity assays, we sought to identify the biochemical subtype of C4 photosynthesis used by S. sesuvioides and to explore the anatomical, physiological, and biochemical traits of young, mature, and senescing leaves, with the aim to elucidate the plasticity and possible limitations of the photosynthetic efficiency in this species. Assays indicated that S. sesuvioides employs the NADP-malic enzyme as the major decarboxylating enzyme. The activity of C4 enzymes, however, declined as leaves aged, and the proportion of water storage tissue increased while air space decreased. These changes suggest a functional shift from photosynthesis to water storage in older leaves. Interestingly, S. sesuvioides demonstrated CO2 compensation points ranging between C4 and C3-C4 intermediate values, and immunocytochemistry revealed labeling of the Rubisco large subunit in mesophyll cells. We hypothesize that S. sesuvioides represents a young C4 lineage with C4-like photosynthesis in which C3 and C4 cycles are running simultaneously in the mesophyll.

Isotopic variance among plant lipid homologues correlates with biodiversity patterns of their source communities

Plant diversity is important to human welfare worldwide, and this importance is exemplified in subtropical and tropical [(sub)tropical] African savannahs where regional biodiversity enhances the sustaining provision of basic ecosystem services available to millions of residents. Yet, there is a critical lack of knowledge about how savannahs respond to climate change. Here, we report the relationships between savannah vegetation structure, species richness, and bioclimatic variables as recorded by plant biochemical fossils, called biomarkers. Our analyses reveal that the stable carbon isotope composition (δ¹³C) of discrete sedimentary plant biomarkers reflects vegetation structure, but the isotopic range among plant biomarkers–which we call LEaf Wax Isotopic Spread (LEWIS)–reflects species richness. Analyses of individual biomarker δ¹³C values and LEWIS for downcore sediments recovered from southeast Africa reveal that the region’s species richness mirrored trends in atmospheric carbon dioxide concentration (pCO₂) throughout the last 25,000 years. This suggests that increasing pCO₂ levels during post-industrialization may prompt future declines in regional biodiversity (1–10 species per unit CO₂ p.p.m.v.) through imminent habitat loss or extinction.

Leaf stable carbon isotope composition reflects transpiration efficiency in Zea mays

The increasing demand for food production and predicted climate change scenarios highlight the need for improvements in crop sustainability. The efficient use of water will become increasingly important for rain-fed agricultural crops even in fertile regions that have historically received ample precipitation. Improvements in water-use efficiency in Zea mays have been limited, and warrant a renewed effort aided by molecular breeding approaches. Progress has been constrained by the difficulty of measuring water-use in a field environment. The stable carbon isotope composition (δ¹³ C) of the leaf has been proposed as an integrated signature of carbon fixation with a link to stomatal conductance. However, additional factors affecting leaf δ¹³ C exist, and a limited number of studies have explored this trait in Z. mays. Here we present an extensive characterization of leaf δ¹³ C in Z. mays. Significant variation in leaf δ¹³ C exists across diverse lines of Z. mays, which we show to be heritable across several environments. Furthermore, we examine temporal and spatial variation in leaf δ¹³ C to determine the optimum sampling time to maximize the use of leaf δ¹³ C as a trait. Finally, our results demonstrate the relationship between transpiration and leaf δ¹³ C in the field and the greenhouse. Decreasing transpiration and soil moisture are associated with decreasing leaf δ¹³ C. Taken together these results outline a strategy for using leaf δ¹³ C and reveal its usefulness as a measure of transpiration efficiency under well-watered conditions rather than a predictor of performance under drought.

The validity of optimal leaf traits modelled on environmental conditions

The ratio of leaf intercellular to ambient CO₂ (χ) is modulated by stomatal conductance (g_s ). These quantities link carbon (C) assimilation with transpiration, and along with photosynthetic capacities (V_cmax and J_max ) are required to model terrestrial C uptake. We use optimization criteria based on the growth environment to generate predicted values of photosynthetic and water-use efficiency traits and test these against a unique dataset. Leaf gas-exchange parameters and carbon isotope discrimination were analysed in relation to local climate across a continental network of study sites. Sun-exposed leaves of 50 species at seven sites were measured in contrasting seasons. Values of χ predicted from growth temperature and vapour pressure deficit were closely correlated to ratios derived from C isotope (δ¹³ C) measurements. Correlations were stronger in the growing season. Predicted values of photosynthetic traits, including carboxylation capacity (V_cmax ), derived from δ¹³ C, growth temperature and solar radiation, showed meaningful agreement with inferred values derived from gas-exchange measurements. Between-site differences in water-use efficiency were, however, only weakly linked to the plant's growth environment and did not show seasonal variation. These results support the general hypothesis that many key parameters required by Earth system models are adaptive and predictable from plants' growth environments.

Quantifying leaf-trait covariation and its controls across climates and biomes

Plant functional ecology requires the quantification of trait variation and its controls. Field measurements on 483 species at 48 sites across China were used to analyse variation in leaf traits, and assess their predictability. Principal components analysis (PCA) was used to characterize trait variation, redundancy analysis (RDA) to reveal climate effects, and RDA with variance partitioning to estimate separate and overlapping effects of site, climate, life-form and family membership. Four orthogonal dimensions of total trait variation were identified: leaf area (LA), internal-to-ambient CO₂ ratio (χ), leaf economics spectrum traits (specific leaf area (SLA) versus leaf dry matter content (LDMC) and nitrogen per area (N_area )), and photosynthetic capacities (V_cmax , J_max at 25°C). LA and χ covaried with moisture index. Site, climate, life form and family together explained 70% of trait variance. Families accounted for 17%, and climate and families together 29%. LDMC and SLA showed the largest family effects. Independent life-form effects were small. Climate influences trait variation in part by selection for different life forms and families. Trait values derived from climate data via RDA showed substantial predictive power for trait values in the available global data sets. Systematic trait data collection across all climates and biomes is still necessary.

The effect of saline-alkaline and water stresses on water use efficiency and standing biomass of Phragmites australis and Bolboschoenus planiculmis

Salt marsh plants in the West Songnen Plain, northern China, are threatened by increasing soil salinity and alkalinity since the late 20th century. To explore how these wetland ecosystems respond to such environmental changes, we examined the effect of saline-alkaline stresses and water stress (flooding/drought) on water use efficiency (WUE, assessed with stable carbon isotopes) and standing biomass of Phragmites australis and Bolboschoenus planiculmis under both greenhouse and field conditions. In the field, sodium bicarbonate (NaHCO₃) was the main saline-alkaline component, and the soil total ion content was negatively related to water level. Higher soil ion content decreased standing biomass of P. australis and B. planiculmis in the field and greenhouse, and increased WUE in the greenhouse. With higher water level, standing biomass of P. australis increased, while that of B. planiculmis decreased in both the field and greenhouse. Alkaline stress exerted the greatest negative influence on growth of P. australis, but only under high ion content. Low alkaline ion content promoted growth of B. planiculmis. Soil ion content exerted the strongest influence on foliar δ¹³C (and thus WUE) and standing biomass of both species compared to water level and stress type. Our findings suggest that under high ion contents, P. australis is more tolerant to flooding stress while B. planiculmis is more tolerant to drought stress. Moreover, P. australis has a high ability to modulate and increase WUE to resist its adverse environment. Our study will contribute to a better understanding of the influence of climate change and increasingly serious human disturbances on the distribution and productivity of these two important wetland species.

Stable carbon and nitrogen isotopes in kukui (Aleurites moluccanus) endocarp along rainfall and elevation gradients: Archaeological implications

Stable carbon and nitrogen isotopes are often used to make inferences of past environments and social patterns. We analyze δ ¹³C and δ ¹⁵N values in contemporary kukui (Aleurites moluccanus) endocarp to examine the effects of site environment. Results from across environmental transects on Hawai‘i Island show strong patterns for both stable isotopes. For δ ¹³C a robust linear relationship with elevation is exhibited, strengthened by the inclusion of rainfall. This relationship breaks down at a minimum threshold of annual rainfall, possible relating to physiological responses to drought. For δ ¹⁵N, the only significant relationship observed pertains to substrate age. The endocarp from kukui is one of the most readily identified plant remains in the Pacific archaeological records and is often targeted for radiocarbon dating. We discuss the potential implications of our results regarding ancient climate, inferred diets, and habitat composition.