Phenotypic selection on leaf water use efficiency and related ecophysiological traits for natural populations of desert sunflowers

Rapid phenotypic differentiation in the iconic Japanese knotweed s.l. invading novel habitats

Understanding the mechanisms that underlie plant invasions is critical for management and conservation of biodiversity. At the same time, invasive species also provide a unique opportunity to study rapid adaptation to complex environmental conditions. Using four replicate reciprocal transplant experiments across three habitats, we described patterns of phenotypic response and assessed the degree of local adaptation in knotweed populations. We found plants from beach habitats were generally smaller than plants from marsh and roadside habitats when grown in their home habitat. In the marsh habitat, marsh plants were generally larger than beach plants, but not different from roadside plants. There were no differences among plants grown in the roadside habitat. We found mixed evidence for local adaptation: plants from the marsh habitat had greater biomass in their "home" sites, while plants from beaches and roadsides had greater survival in their "home" sites compared to other plants. In sum, we found phenotypic differentiation and some support for the hypothesis of rapid local adaptation of plants from beach, marsh and roadside habitats. Identifying whether these patterns of differentiation result from genetic or heritable non-genetic mechanisms will require further work.

Water use efficiency in China is impacted by climate change and land use and land cover

A crucial physiological indicator known as water use efficiency (WUE) (Foley et al.) assesses the trade-off between water loss and carbon uptake. The carbon and water coupling mechanisms, energy balance, and hydrological cycle processes in the ecosystem are impacted by climate change, vegetation dynamics, and land use change. In this study, we employed Sen trend analysis, the Mann-Kendall test, the land-use transfer matrix, and multiple linear regression analysis to investigate the regional and temporal dynamics of WUE and its reaction to climate change and land-use transfer changes in China. According to the findings, the annual average WUE in China was 0.998 gC/mm·m2 from 2000 to 2017. Of the nine major river basins, the Continental Basin had the lowest WUE (0.529 gC/mm·m2), and the Southwest River Basin had the highest WUE (0.691 gC/mm·m2), while the Pearl River Basin and the Southeast River Basin had the highest WUEs (1.184 gC/mm·m2). The Haihe River Basin and the Yellow River Basin were the key regions with elevated WUE. Forest had the greatest WUE (1.134 gC/mm·m2; out of the nine major river basins), followed by shrub (1.109 gC/mm·m2). Vegetation dynamics changes had a higher impact on WUE than climate change and land use changes, when the contributions of climate change, vegetation dynamics changes, and land use changes to WUE were separated. The largest climatic factor influencing variations in WUE was VPD (28.04% ± 3.98%), whereas among the vegetation dynamics factors, NDVI (33.75% ± 6.90%) and LAI (22.21% ± 2.11%) contributed the most. The transition from high to low vegetation cover led to a relative decrease in WUE, and vice versa, according to data on land use change in China from 2000 to 2017. Land use change made a positive impact to WUE change. The findings of this study may be helpful in China for choosing a suitable regional plant cover and managing local water resources sustainably.

Characterization of water use efficiency changes in Tibetan Plateau grasslands based on eco-geographic zoning

Water use efficiency (WUE) is an effective indicator to study the coupling of terrestrial carbon and water cycles. The Tibetan Plateau (TP) is the most important ecological security barrier in China, and it is important to understand the characteristics of WUE and the change mechanism to study the carbon and water cycles of plateau ecosystems and the rational use of water resources. This study analyzes the spatial and temporal characteristics of WUE on the TP and the influence of climate factors on WUE based on the gross primary productivity (GPP) and evapotranspiration (ET) data from GLASS. The results show that from 1985 to 2018, the WUE of the TP is on the rise under the combined effect of GPP and ET; the regions with higher mean WUE values are the southeastern and eastern parts of the plateau, and the low value areas are the central and northwestern parts of the plateau. Compared with precipitation, WUE is influenced by temperature over a larger area. The correlations between precipitation and temperature and WUE in different eco-geographic regions are complex, and there is a threshold effect on the correlation between WUE and temperature and precipitation. Temperature is the main driver of WUE changes in HIIA and HIB1 regions, while precipitation has a greater impact on WUE changes in HIIC2, HIIC2, HIC2, HIID3, and HIIC regions. Precipitation, temperature, and elevation are the main factors explaining the variation of WUE in the TP. According to the risk detector, it can be determined that grassland vegetation in warm and humid steep areas of low and medium elevations is more able to maintain efficient use of water. Meanwhile, grasslands located in the shade of northern slopes have weaker transpiration, which is conducive to vegetation accumulation of growth water, and thus can ensure higher WUE. The related study can provide a reference for the response of vegetation WUE to global changes in key climatic regions.

Natural variation identifies new effectors of water-use efficiency in Arabidopsis

Water-use efficiency (WUE) is the ratio of biomass produced per unit of water consumed; thus, it can be altered by genetic factors that affect either side of the ratio. In the present study, we exploited natural variation for WUE to discover loci affecting either biomass accumulation or water use as factors affecting WUE. Genome-wide association studies (GWAS) using integrated WUE measured through carbon isotope discrimination (δ¹³C) of Arabidopsis thaliana accessions identified genomic regions associated with WUE. Reverse genetic analysis of 70 candidate genes selected based on the GWAS results and transcriptome data identified 25 genes affecting WUE as measured by gravimetric and δ¹³C analyses. Mutants of four genes had higher WUE than wild type, while mutants of the other 21 genes had lower WUE. The differences in WUE were caused by either altered biomass or water consumption (or both). Stomatal density (SD) was not a primary cause of altered WUE in these mutants. Leaf surface temperatures indicated that transpiration differed for mutants of 16 genes, but generally biomass accumulation had a greater effect on WUE. The genes we identified are involved in diverse cellular processes, including hormone and calcium signaling, meristematic activity, photosynthesis, flowering time, leaf/vasculature development, and cell wall composition; however, none of them had been previously linked to WUE. Thus, our study successfully identified effectors of WUE that can be used to understand the genetic basis of WUE and improve crop productivity.

Higher leaf nitrogen content is linked to tighter stomatal regulation of transpiration and more efficient water use across dryland trees

The least-cost economic theory of photosynthesis shows that water and nitrogen are mutually substitutable resources to achieve a given carbon gain. However, vegetation in the Sahel has to cope with the dual challenge imposed by drought and nutrient-poor soils. We addressed how variation in leaf nitrogen per area (N_area ) modulates leaf oxygen and carbon isotopic composition (δ¹⁸ O, δ¹³ C), as proxies of stomatal conductance and water-use efficiency, across 34 Sahelian woody species. Dryland species exhibited diverging leaf δ¹⁸ O and δ¹³ C values, indicating large interspecific variation in time-integrated stomatal conductance and water-use efficiency. Structural equation modeling revealed that leaf N_area is a pivotal trait linked to multiple water-use traits. Leaf N_area was positively linked to both δ¹⁸ O and δ¹³ C, suggesting higher carboxylation capacity and tighter stomatal regulation of transpiration in N-rich species, which allows them to achieve higher water-use efficiency and more conservative water use. These adaptations represent a key physiological advantage of N-rich species, such as legumes, that could contribute to their dominance across many dryland regions. This is the first report of a robust mechanistic link between leaf N_area and δ¹⁸ O in dryland vegetation that is consistent with core principles of plant physiology.

Spatiotemporal Variability in Water-Use Efficiency in Tianshan Mountains (Xinjiang, China) and the Influencing Factors

Water-use efficiency (WUE) is a crucial physiological index in carbon–water interactions and is defined as the ratio of vegetation productivity to water loss. The variation in climatic variables and drought have the most significant effects on WUE and evapotranspiration (ET). Nevertheless, how WUE varies with climate factors and drought processes in the Tianshan Mountains (TMS) is still poorly understood. In the present work, we analyzed the spatiotemporal variations in WUE, and investigated the correlations between WUE, climate factors, and drought, in the study area. The results showed that, in the TMS during 2000–2020, annual net primary productivity (NPP) ranged from 147.9 to 189.4 gC·m−2, annual ET was in the range of 212.5–285.8 mm, and annual WUE ranged from 0.66 to 0.78 gC·kg−1·H2O. Both NPP and ET exhibited an increasing trend with some fluctuation, whereas WUE showed the opposite tendency during the study period. The obtained results demonstrated that the decrease in WUE was primarily because of the increase in ET. There were obvious differences in WUE, under different land-use types, caused by NPP and ET. However, the interannual variation in WUE showed small fluctuations and the dynamic process of WUE in each land-use type showed good consistency. Temperature and wind speed had a positive influence on WUE in the middle and eastern regions of the TMS. Precipitation also played a mainly positive role in enhancing WUE, especially on the northern slope of the TMS. There was strong spatial heterogeneity of the correlation coefficient (0.68, p < 0.05) between WUE and the temperature vegetation drought index (TVDI). Moreover, the slopes of WUE and TVDI showed good consistency in terms of spatial distribution, suggesting that drought had a significant impact on ecosystem WUE. This work will enhance the understanding of WUE variation, and provide scientific evidence for water resource management and sustainable utilization in the study area.

Remote Sensing of Ecosystem Water Use Efficiency in Different Ecozones of the North China Plain

Water use efficiency (WUE), as an environmental factor of metabolism in different ecosystem functional areas, is a key indicator of the ecosystem carbon-water cycle. WUE is defined as the ratio of carbon absorbed by ecosystems to water evaporated. Exploring the spatiotemporal variation in carbon and water cycles in different ecological zones of the North China Plain and their driving factors is important for the ecological management and sustainable development of the different ecological zones in the North China Plain. Based on remote sensing data products, this paper studies the spatiotemporal variations of WUE and their driving factors in different ecological functional areas of the North China Plain from 2001 to 2017. This study found that: (1) The spatial distribution of WUE and gross primary production (GPP) in the North China Plain is similar, with the multiyear average of WUE at 0.74 g C m−2 y−1. The variation trend of WUE is mainly affected by the variation trend of GPP (44.38% of the area of the North China Plain). (2) The change trend of WUE mainly showed a mild decrease and a mild increase, accounting for 73.22% of the area of the North China Plain; the area with medium-low fluctuation of WUE accounted for the largest proportion, accounting for 59.90% of the area of the North China Plain. In addition, the multiyear average values of WUE in the ecological functional area are Qin Ling Mountains deciduous forests > Central China loess plateau mixed forests > Mongolian-Manchurian grassland > Ordos Plateau steppe > Changjiang Plain evergreen forests > Huang He Plain mixed forests > Bohai Sea saline meadow, in the order from high to low. (3) The influence of precipitation on WUE was higher than that of temperature. The area of WUE that increased with the increase of precipitation accounted for 23.74% of the area of the North China Plain and was mainly distributed in the Qin Ling Mountains deciduous forests, Changjiang Plain evergreen forests, and Huang He Plain mixed forests’ ecological functional areas. The results of the study can provide a reference and theoretical basis for the conservation and management of carbon and water cycles in the functional areas of North China’s ecosystems.

The importance of ecophysiological traits in response of Festuca rubra to changing climate

Knowledge of the ability of plants to respond to climate change via phenotypic plasticity or genetic adaptation in ecophysiological traits and of the link of these traits to fitness is still limited. We studied the clonal grass Festuca rubra from 11 localities representing factorially crossed gradients of temperature and precipitation and cultivated them in growth chambers simulating temperature and moisture regime in the four extreme localities. We measured net photosynthetic rate, F_v /F_m , specific leaf area, osmotic potential and stomatal density and length and tested their relationship to proxies of fitness. We found strong phenotypic plasticity in photosynthetic traits and genetic differentiation in stomatal traits. The effects of temperature and moisture interacted (either as conditions of origin or growth chambers), as were effects of growth and origin. The relationships between the ecophysiological and fitness-related traits were significant but weak. Phenotypic plasticity and genetic differentiation of the species indicate the potential ability of F. rubra to adapt to novel climatic conditions. The most important challenge for the plants seems to be increasing moisture exposing plants to hypoxia. However, the plants have the potential to respond to increased moisture by changes in stomatal size and density and adjustments of osmotic potential. Changes in ecophysiological traits translate into variation in plant fitness, but the selection on the traits is relatively weak and depends on actual conditions. Despite the selection, the plants do not show strong local adaptation and local adaptation is thus likely not restricting species ability to adjust to novel conditions.

Ecosystem Water Use Efficiency in the Three-North Region of China Based on Long-Term Satellite Data

Water use efficiency (WUE), given by the ratio between organic matter production and water consumption, could be considered as a very important ecological indicator for assessing vegetation system growth conditions by combining organic matter production and water consumption. It is especially important for regional vegetation sustainable management by creating enough organic matter with restricted water supply. Furthermore, proper analysis of WUE is vital for the evaluation and future plans of ecological restoration projects in ecologically fragile regions such as the Three-North region of China. In this study, ecosystem WUE across the Three-North region of China from 2001 to 2017 was obtained, and the variation trends and major influencing factors were also analyzed. The results demonstrated that (1) the average WUE across the Three-North region of China is 0.7376 g∙C∙m−2∙mm−1 with an annual increase of 0.002 g∙C∙m−2∙mm−1∙y−1; (2) the spatiotemporal variation trends of WUE are similar to those of gross primary production (GPP); and (3) in the southeastern parts of the Three-North region, the vegetation conditions are better with sustainable improvements, while in Xinjiang Province, the sustainable degradation areas are widely spread. The results of this research reveal large spatial heterogeneity of WUE, with high WUE mainly in the southeastern region with sufficient precipitation and afforestation programs. For those areas far away from this region, WUE is not satisfactory, suggesting that, for a sustainable vegetation growth, it is important to consider the water supply to maintain suitable vegetation cover. Furthermore, the results of this research are important for future ecological restoration and sustainable management of environment.

Growth and fecundity of colonizing hybrid Raphanus populations are environmentally dependent

PREMISE

Hybrid gene pools harbor more genetic variation than progenitor populations. Thus, we expect hybrid populations to exhibit more dynamic evolutionary responses to environmental variation. We ask how environmental variation experienced by adapted and transplanted populations influence the success of late-generation hybrid populations during invasion.

METHODS

For four generations, 20 wild (Raphanus raphanistrum) and 20 hybrid radish (R. sativus × R. raphanistrum) plant populations evolved under experimentally manipulated moisture conditions (dry, wet, control-sheltered, or control-unsheltered plots; i.e., evolutionary environment) in old fields near Toronto, Canada. We planted advanced-generation wild and hybrid radishes in sheltered plots and exposed them to either an evolutionary or a novel watering environment. To determine how soil moisture would influence invasion success, we compared the phenotype and fecundity of plants grown in these various environments.

RESULTS

Hybridization produced larger plants. In wet environments, hybrid seedlings emerged more frequently and expressed higher photosynthetic activity. Low-moisture, novel conditions delayed and reduced seedling emergence frequency. Hybrid plants and those that evolved under relatively wet environments exhibited higher aboveground biomass. Hybrid plants from control-sheltered plots colonizing novel moisture environments were more fecund than comparable wild plants.

CONCLUSIONS

Dry environments are less likely than other evolutionary environments to contribute colonists. However, relatively wet locations support the evolution of relatively fecund plants, especially crop-wild hybrid populations. Thus, our results provide a strong mechanistic explanation for variation in the relative success of crop-wild hybrids among study locations and a new standard for studies that assess the risk of crop-wild hybridization events.

Selection patterns on early-life phenotypic traits in Pinus sylvestris are associated with precipitation and temperature along a climatic gradient in Europe

Understanding the dynamics of selection is key to predicting the response of tree species to new environmental conditions in the current context of climate change. However, selection patterns acting on early recruitment stages and their climatic drivers remain largely unknown in most tree species, despite being a critical period of their life cycle. We measured phenotypic selection on Pinus sylvestris seed mass, emergence time and early growth rate over 2 yr in four common garden experiments established along the latitudinal gradient of the species in Europe. Significant phenotypic plasticity and among-population genetic variation were found for all measured phenotypic traits. Heat and drought negatively affected fitness in the southern sites, but heavy rainfalls also decreased early survival in middle latitudes. Climate-driven directional selection was found for higher seed mass and earlier emergence time, while the form of selection on seedling growth rates differed among sites and populations. Evidence of adaptive and maladaptive phenotypic plasticity was found for emergence time and early growth rate, respectively. Seed mass, emergence time and early growth rate have an adaptive role in the early stages of P. sylvestris and climate strongly influences the patterns of selection on these fitness-related traits.

Evolutionary divergence of potential drought adaptations between two subspecies of an annual plant: Are trait combinations facilitated, independent, or constrained?

PREMISE

Whether drought-adaptation mechanisms tend to evolve together, evolve independently, or evolve constrained by genetic architecture is incompletely resolved, particularly for water-relations traits besides gas exchange. We addressed this issue in two subspecies of Clarkia xantiana (Onagraceae), California winter annuals that separated approximately 65,000 years ago and are adapted, partly by differences in flowering time, to native ranges differing in precipitation.

METHODS

In these subspecies and in recombinant inbred lines (RILs) from a cross between them, we scored traits related to drought adaptation (timing of seed germination and of flowering, succulence, pressure-volume curve variables) in common environments.

RESULTS

The subspecies native to more arid environments (parviflora) exhibited slower seed germination in saturated conditions, earlier flowering, and greater succulence, likely indicating superior drought avoidance, drought escape, and dehydration resistance via water storage. The other subspecies (xantiana) had lower osmotic potential at full turgor and lower water potential at turgor loss, implying superior dehydration tolerance. Genetic correlations among RILs suggest facilitated evolution of some trait combinations and independence of others. Where genetic correlations exist, subspecies differences fell along them, with the exception of differences in succulence and turgor loss point. In that case, subspecies difference overcame genetic correlations, possibly reflecting strong selection and/or antagonistic genetic correlations with other traits.

CONCLUSIONS

Clarkia xantiana subspecies' differ in multiple mechanisms of drought adaptation. Genetic architecture generally does not seem to have constrained the evolution of these mechanisms, and it may have facilitated the evolution of some of trait combinations.

Frameworks on Patterns of Grasslands’ Sensitivity to Forecast Extreme Drought

Climate models have predicted the future occurrence of extreme drought (ED). The management, conservation, or restoration of grasslands following ED requires a robust prior knowledge of the patterns and mechanisms of sensitivity—declining rate of ecosystem functions due to ED. Yet, the global-scale pattern of grasslands’ sensitivity to any ED event remains unresolved. Here, frameworks were built to predict the sensitivity patterns of above-ground net primary productivity (ANPP) spanning the global precipitation gradient under ED. The frameworks particularly present three sensitivity patterns that could manipulate (weaken, strengthen, or erode) the orthodox positive precipitation–productivity relationship which exists under non-drought (ambient) condition. First, the slope of the relationship could become steeper via higher sensitivity at xeric sites than mesic and hydric ones. Second, if the sensitivity emerges highest in hydric, followed by mesic, then xeric, a weakened slope, flat line, or negative slope would emerge. Lastly, if the sensitivity emerges unexpectedly similar across the precipitation gradient, the slope of the relationship would remain similar to that of the ambient condition. Overall, the frameworks provide background knowledge on possible differences or similarities in responses of grasslands to forecast ED, and could stimulate increase in conduct of experiments to unravel the impacts of ED on grasslands. More importantly, the frameworks indicate the need for reconciliation of conflicting hypotheses of grasslands’ sensitivity to ED through global-scale experiments.

The Role of Population and Half-Sib Family on Driving Suitable Functional Traits for Quercus suber L. Forest Restoration

Research Highlights: Seedlings of different Quercus suber L. populations and half-sib families differ in their response to multiple stressors, which may have consequences on the future distribution of this Mediterranean species. Background and Objectives: Global change will likely increase the frequency and severity of drought in drylands. Plant species’ distributions will largely depend on their ability to respond to the combined effect of drought and other environmental stressors. Genetic diversity in morpho-functional traits are key components of this response. Yet, information on the response to multiple stresses is scarce for many iconic species. The present study assessed the variability in the response of populations and half-sib families of a Mediterranean acidophilous tree, cork oak, to drought and changes in soil conditions. Materials and Methods: We sampled acorns of half-sib families from two cork oak populations genetically introgressed with the alkaline-tolerant species Quercus ilex L., and from a non-introgressed cork oak population located in its core habitat. We germinated the acorns and subjected seedlings to contrasted levels of water availability and additions of calcium and magnesium carbonate, and assessed their morpho-physiological response. Results: Response to drought and soil chemistry composition differed between populations and families. For some traits, introgressed populations responded similarly to drought than the non-introgressed population. Conversely, the response to soil chemistry was not clearly related to introgression. When considering half-sib families within populations, the population effect diminished, which revealed the importance of intra-population variation. However, relevant traits for water scarcity adaptations, such as specific leaf area and root:shoot ratio, remained significantly different at the population level, which highlights the relevance of these traits for management. Conclusions: Our study shows that the adaptive management and restoration of cork oak forests should consider not only geographic provenances, but also half-sib lines within populations.

Trait means predict performance under water limitation better than plasticity for seedlings of Poaceae species on the eastern Tibetan Plateau

Water availability may be altered by changes in precipitation under global climate change in alpine areas. Trait means and plasticity are important for plants in response to a changing environment. In an examination of alpine plant responses to changed water availability, and for determination of how trait means and plasticity predict the performance (e.g., biomass) of these species, seeds of ten Poaceae species from the eastern Tibetan Plateau were sown and grown in a manipulated environment during a growing season in which rainfall was removed and other climate conditions remained unchanged. Growth and leaf traits of these species were measured. We found significant effects of moderate water stress on the seedling biomass of these species; however, the responses of these species to changed water condition were strongly dependent on species identity. For example, the biomass of some species significantly decreased under moderate drought, whereas that of others were either significantly increased or unaffected. This pattern was also observed for growth and leaf traits. Overall, the alpine Poaceae species showed low plasticity of traits in response to water availability relative to reports from other areas. Notably, the results show that trait means were better correlated with the productivity than with the plasticity of traits; thus, we argue that the trait means were better predictors of performance than plasticity for alpine Poaceae species. Poaceae species in alpine areas are important for forage production and for water catchment health worldwide, and these species may face water shortage because of current and future climate change. Understanding the response of alpine Poaceae species to water availability would facilitate our ability to predict the impacts of climate change on the alpine vegetation.

Determining the scale at which variation in a single gene changes population yields

Plant trait diversity is known to influence population yield, but the scale at which this happens remains unknown: divergent individuals might change yields of immediate neighbors (neighbor scale) or of plants across a population (population scale). We use Nicotiana attenuata plants silenced in mitogen-activated protein kinase 4 (irMPK4) - with low water-use efficiency (WUE) - to study the scale at which water-use traits alter intraspecific population yields. In the field and glasshouse, we observed overyielding in populations with low percentages of irMPK4 plants, unrelated to water-use phenotypes. Paired-plant experiments excluded the occurrence of overyielding effects at the neighbor scale. Experimentally altering field arbuscular mycorrhizal fungal associations by silencing the Sym-pathway gene NaCCaMK did not affect reproductive overyielding, implicating an effect independent of belowground AMF interactions. Additionally, micro-grafting experiments revealed dependence on shoot-expressed MPK4 for N. attenuata to vary its yield per neighbor presence. We find that variation in a single gene, MPK4, is responsible for population overyielding through a mechanism, independent of irMPK4's WUE phenotype, at the aboveground, population scale.

Growth and physiological responses of three poplar clones grown on soils artificially contaminated with heavy metals, diesel fuel, and herbicides

We tested the growth and physiological responses of three poplar clones [Populus deltoides Bartr. ex Marsh. 'Bora', 'PE 19/66'; Populus × euramericana (Dode) Guinier 'Pannonia'] grown for 3 years on soils artificially contaminated with heavy metals, diesel fuel, and herbicides at the Experimental Estate of the Institute of Lowland Forestry and Environment (ILFE), University of Novi Sad, Serbia. Within three field blocks, clonal whole-plots were divided into seven subplots containing a non-contaminated control and six artificially-contaminated soil treatments: (1) 10.6 kg Cd ha^-1, (2) 247 kg Cu ha^-1, (3) 183.3 kg Ni ha^-1, (4) 6,667 L diesel fuel ha^-1, (5) 236 g Oxyfluorfen ha^-1, and (6) 1,320 g Pendimethalin ha^-1. Significant clone × treatment interactions governed growth and physiology throughout the study (p < 0.05), and the influence of inorganics versus organics varied with tree age. Heavy metals had a more substantial influence on growth and physiology as the trees matured, while diesel and herbicide treatments were most pronounced during the first growing season (p < 0.0001). Clones 'Bora' and 'PE 19/66' exhibited greater biomass than 'Pannonia', with trees growing in the control soils exhibiting 13.8 and 19.6 times greater biomass than 'Pannonia', respectively.

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.

Parallel functional differentiation of an invasive annual plant on two continents

Rapid local adaptation frequently occurs during the spread of invading species. It remains unclear, however, how consistent, and therefore potentially predictable, such patterns of local adaptation are. One approach to this question is to measure patterns of local differentiation in functional traits and plasticity levels in invasive species in multiple regions. Finding consistent patterns of local differentiation in replicate regions suggests that these patterns are adaptive. Further, this outcome indicates that the invading species likely responds predictably to selection along environmental gradients, even though standing genetic variation is likely to have been reduced during introduction. We studied local differentiation in the invasive annual plant Erodium cicutarium in two invaded regions, California and Chile. We collected seeds from across strong gradients in precipitation and temperature in Mediterranean-climate parts of the two regions (10 populations per region). We grew seeds from maternal families from these populations through two generations and exposed the second generation to contrasting levels of water and nutrient availability. We measured growth, flowering time and leaf functional traits across these treatments to obtain trait means and plasticity measures. We found strong differentiation among populations in all traits. Plants from drier environments flowered earlier, were less plastic in flowering time and reached greater size in all treatments. Correlations among traits within regions suggested a coordinated evolutionary response along environmental gradients associated with growing season length. There was little divergence in traits and trait intercorrelations between regions, but strongly parallel divergence in traits within regions. Similar, statistically consistent patterns of local trait differentiation across two regions suggest that local adaptation to environmental gradients has aided the spread of this invasive species, and that the formation of ecotypes in newly invaded environments has been relatively consistent and predictable.

Evolutionary potential varies across populations and traits in the neotropical oak Quercus oleoides

Heritable variation in polygenic (quantitative) traits is critical for adaptive evolution and is especially important in this era of rapid climate change. In this study, we examined the levels of quantitative genetic variation of populations of the tropical tree Quercus oleoides Cham. and Schlect. for a suite of traits related to resource use and drought resistance. We tested whether quantitative genetic variation differed across traits, populations and watering treatments. We also tested potential evolutionary factors that might have shaped such a pattern: selection by climate and genetic drift. We measured 15 functional traits on 1322 1-year-old seedlings of 84 maternal half-sib families originating from five populations growing under two watering treatments in a greenhouse. We estimated the additive genetic variance, coefficient of additive genetic variation and narrow-sense heritability for each combination of traits, populations and treatments. In addition, we genotyped a total of 119 individuals (with at least 20 individuals per population) using nuclear microsatellites to estimate genetic diversity and population genetic structure. Our results showed that gas exchange traits and growth exhibited strikingly high quantitative genetic variation compared with traits related to leaf morphology, anatomy and photochemistry. Quantitative genetic variation differed between populations even at geographical scales as small as a few kilometers. Climate was associated with quantitative genetic variation, but only weakly. Genetic structure and diversity in neutral markers did not relate to coefficient of additive genetic variation. Our study demonstrates that quantitative genetic variation is not homogeneous across traits and populations of Q. oleoides. More importantly, our findings suggest that predictions about potential responses of species to climate change need to consider population-specific evolutionary characteristics.

Physiological and Proteomic Analysis of the Rice Mutant cpm2 Suggests a Negative Regulatory Role of Jasmonic Acid in Drought Tolerance

It is widely known that numerous adaptive responses of drought-stressed plants are stimulated by chemical messengers known as phytohormones. Jasmonic acid (JA) is one such phytohormone. But there are very few reports revealing its direct implication in drought related responses or its cross-talk with other phytohormones. In this study, we compared the morpho-physiological traits and the root proteome of a wild type (WT) rice plant with its JA biosynthesis mutant coleoptile photomorphogenesis 2 (cpm2), disrupted in the allene oxide cyclase (AOC) gene, for insights into the role of JA under drought. The mutant had higher stomatal conductance, higher water use efficiency and higher shoot ABA levels under severe drought as compared to the WT. Notably, roots of cpm2 were better developed compared to the WT under both, control and drought stress conditions. Root proteome was analyzed using the Tandem Mass Tag strategy to better understand this difference at the molecular level. Expectedly, AOC was unique but notably highly abundant under drought in the WT. Identification of other differentially abundant proteins (DAPs) suggested increased energy metabolism (i.e., increased mobilization of resources) and reactive oxygen species scavenging in cpm2 under drought. Additionally, various proteins involved in secondary metabolism, cell growth and cell wall synthesis were also more abundant in cpm2 roots. Proteome-guided transcript, metabolite, and histological analyses provided further insights into the favorable adaptations and responses, most likely orchestrated by the lack of JA, in the cpm2 roots. Our results in cpm2 are discussed in the light of JA crosstalk to other phytohormones. These results together pave the path for understanding the precise role of JA during drought stress in rice.

The more things change, the more they stay the same? When is trait variability important for stability of ecosystem function in a changing environment

The importance of intraspecific trait variability for community dynamics and ecosystem functioning has been underappreciated. There are theoretical reasons for predicting that species that differ in intraspecific trait variability will also differ in their effects on ecosystem functioning, particularly in variable environments. We discuss whether species with greater trait variability are likely to exhibit greater temporal stability in their population dynamics, and under which conditions this might lead to stability in ecosystem functioning. Resolving this requires us to consider several questions. First, are species with high levels of variation for one trait equally variable in others? In particular, is variability in response and effects traits typically correlated? Second, what is the relative contribution of local adaptation and phenotypic plasticity to trait variability? If local adaptation dominates, then stability in function requires one of two conditions: (i) individuals of appropriate phenotypes present in the environment at high enough frequencies to allow for populations to respond rapidly to the changing environment, and (ii) high levels of dispersal and gene flow. While we currently lack sufficient information on the causes and distribution of variability in functional traits, filling in these key data gaps should increase our ability to predict how changing biodiversity will alter ecosystem functioning.

Population-Level Differentiation in Growth Rates and Leaf Traits in Seedlings of the Neotropical Live Oak Quercus oleoides Grown under Natural and Manipulated Precipitation Regimes

Widely distributed species are normally subjected to spatial heterogeneity in environmental conditions. In sessile organisms like plants, adaptive evolution and phenotypic plasticity of key functional traits are the main mechanisms through which species can respond to environmental heterogeneity and climate change. While extended research has been carried out in temperate species in this regard, there is still limited knowledge as to how species from seasonally-dry tropical climates respond to spatial and temporal variation in environmental conditions. In fact, studies of intraspecific genetically-based differences in functional traits are still largely unknown and studies in these ecosystems have largely focused on in situ comparisons where environmental and genetic effects cannot be differentiated. In this study, we tested for ecotypic differentiation and phenotypic plasticity in leaf economics spectrum (LES) traits, water use efficiency and growth rates under natural and manipulated precipitation regimes in a common garden experiment where seedlings of eight populations of the neotropical live oak Quercus oleoides were established. We also examined the extent to which intraspecific trait variation was associated with plant performance under different water availability. Similar to interspecific patterns among seasonally-dry tropical tree species, live oak populations with long and severe dry seasons had higher leaf nitrogen content and growth rates than mesic populations, which is consistent with a "fast" resource-acquisition strategy aimed to maximize carbon uptake during the wet season. Specific leaf area (SLA) was the best predictor of plant performance, but contrary to expectations, it was negatively associated with relative and absolute growth rates. This observation was partially explained by the negative association between SLA and area-based photosynthetic rates, which is contrary to LES expectations but similar to other recent intraspecific studies on evergreen oaks. Overall, our study shows strong intraspecific differences in functional traits in a tropical oak, Quercus oleoides, and suggests that precipitation regime has played an important role in driving adaptive divergence in this widespread species.

Interactive effects of water limitation and elevated temperature on the physiology, development and fitness of diverse accessions of Brachypodium distachyon

An enduring question in plant physiology and evolution is how single genotypes of plants optimize performance in diverse, often highly variable, environments. We grew 35 natural accessions of the grass Brachypodium distachyon in four environments in the glasshouse, contrasting soil water deficit, elevated temperature and their interaction. We modeled treatment, genotype and interactive effects on leaf-level and whole-plant traits, including fecundity. We also assessed the relationship between glasshouse-measured traits and parameters related to climate at the place of origin. We found abundant genetic variation in both constitutive and induced traits related to plant-water relations. Most traits showed strong interaction between temperature and water availability, and we observed genotype-by-environment interaction for several traits. Notably, leaf free proline abundance showed a strong effect of genotype × temperature × water. We found strong associations between phenology, biomass and water use efficiency (WUE) with parameters describing climate of origin. Plants respond to multiple stressors in ways not directly predictable from single stressors, underscoring the complex and trait-specific mechanisms of environmental response. Climate-trait correlations support a role for WUE and phenology in local adaptation to climate in B. distachyon.

Quantitative trait loci associated with natural diversity in water-use efficiency and response to soil drying in Brachypodium distachyon

All plants must optimize their growth with finite resources. Water use efficiency (WUE) measures the relationship between biomass acquisition and transpired water. In the present study, we performed two experiments to understand the genetic basis of WUE and other parameters of plant-water interaction under control and water-limited conditions. Our study used two inbred natural accessions of Brachypodium distachyon, a model grass species with close phylogenetic affinity to temperate forage and cereal crops. First, we identify the soil water content which causes a reduction in leaf relative water content and an increase in WUE. Second, we present results from a large phenotyping experiment utilizing a recombinant inbred line mapping population derived from these same two natural accessions. We identify QTLs associated with environmentally-insensitive genetic variation in WUE, including a pair of epistatically interacting loci. We also identify QTLs associated with constitutive differences in biomass and a QTL describing an environmentally-sensitive difference in leaf carbon content. Finally, we present a new linkage map for this mapping population based on new SNP markers as well as updated genomic positions for previously described markers. Our studies provide an initial characterization of plant-water relations in B. distachyon and identify candidate genomic regions involved in WUE.

Genetics of water use physiology in locally adapted Arabidopsis thaliana

Identifying the genetic basis of adaptation to climate has long been a goal in evolutionary biology and has applications in agriculture. Adaptation to drought represents one important aspect of local adaptation, and drought is the major factor limiting agricultural yield. We examined local adaptation between Sweden and Italy Arabidopsis thaliana ecotypes, which show contrasting levels of water availability in their local environments. To identify quantitative trait loci (QTL) controlling water use physiology traits and adaptive trait QTL (genomic regions where trait QTL and fitness QTL colocalize), we performed QTL mapping on 374F9 recombinant inbred lines in well-watered and terminal drought conditions. We found 72 QTL (32 in well-watered, 31 in drought, 9 for plasticity) across five water use physiology traits: δ(13)C, rosette area, dry rosette weight, leaf water content and percent leaf nitrogen. Some of these genomic regions colocalize with fitness QTL and with other physiology QTL in defined hotspots. In addition, we found evidence of both constitutive and inducible water use physiology QTL. Finally, we identified highly divergent candidate genes, in silico. Our results suggest that many genes with minor effects may influence adaptation through water use physiology and that pleiotropic water use physiology QTL have fitness consequences.

Up-regulation of abscisic acid signaling pathway facilitates aphid xylem absorption and osmoregulation under drought stress

The activation of the abscisic acid (ABA) signaling pathway reduces water loss from plants challenged by drought stress. The effect of drought-induced ABA signaling on the defense and nutrition allocation of plants is largely unknown. We postulated that these changes can affect herbivorous insects. We studied the effects of drought on different feeding stages of pea aphids in the wild-type A17 of Medicago truncatula and ABA signaling pathway mutant sta-1. We examined the impact of drought on plant water status, induced plant defense signaling via the abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) pathways, and on the host nutritional quality in terms of leaf free amino acid content. During the penetration phase of aphid feeding, drought decreased epidermis/mesophyll resistance but increased mesophyll/phloem resistance of A17 but not sta-1 plants. Quantification of transcripts associated with ABA, JA and SA signaling indicated that the drought-induced up-regulation of ABA signaling decreased the SA-dependent defense but increased the JA-dependent defense in A17 plants. During the phloem-feeding phase, drought had little effect on the amino acid concentrations and the associated aphid phloem-feeding parameters in both plant genotypes. In the xylem absorption stage, drought decreased xylem absorption time of aphids in both genotypes because of decreased water potential. Nevertheless, the activation of the ABA signaling pathway increased water-use efficiency of A17 plants by decreasing the stomatal aperture and transpiration rate. In contrast, the water potential of sta-1 plants (unable to close stomata) was too low to support xylem absorption activity of aphids; the aphids on sta-1 plants had the highest hemolymph osmolarity and lowest abundance under drought conditions. Taken together this study illustrates the significance of cross-talk between biotic-abiotic signaling pathways in plant-aphid interaction, and reveals the mechanisms leading to alter aphid fecundity in water stresses plants.

Maximal stomatal conductance to water and plasticity in stomatal traits differ between native and invasive introduced lineages of Phragmites australis in North America

The fitness costs of reproduction by clonal growth can include a limited ability to adapt to environmental and temporal heterogeneity. Paradoxically, some facultatively clonal species are not only able to survive, but colonize, thrive and expand in heterogeneous environments. This is likely due to the capacity for acclimation (sensu stricto) that compensates for the fitness costs and complements the ecological advantages of clonality. Introduced Phragmites australis demonstrates great phenotypic plasticity in response to temperature, nutrient availability, geographic gradient, water depths, habitat fertility, atmospheric CO2, interspecific competition and intraspecific competition for light. However, no in situ comparative subspecies studies have explored the difference in plasticity between the non-invasive native lineage and the highly invasive introduced lineage. Clonality of the native and introduced lineages makes it possible to control for genetic variation, making P. australis a unique system for the comparative study of plasticity. Using previously identified clonal genotypes, we investigated differences in their phenotypic plasticity through measurements of the lengths and densities of stomata on both the abaxial (lower) and adaxial (upper) surfaces of leaves, and synthesized these measurements to estimate impacts on maximum stomatal conductance to water (gwmax). Results demonstrated that at three marsh sites, invasive lineages have consistently greater gwmax than their native congeners, as a result of greater stomatal densities and smaller stomata. Our analysis also suggests that phenotypic plasticity, determined as within-genotype variation in gwmax, of the invasive lineage is similar to, or exceeds, that shown by the native lineage.

Transcriptome-wide characterization of candidate genes for improving the water use efficiency of energy crops grown on semiarid land

Understanding the genetic basis of water use efficiency (WUE) and its roles in plant adaptation to a drought environment is essential for the production of second-generation energy crops in water-deficit marginal land. In this study, RNA-Seq and WUE measurements were performed for 78 individuals of Miscanthus lutarioriparius grown in two common gardens, one located in warm and wet Central China near the native habitats of the species and the other located in the semiarid Loess Plateau, the domestication site of the energy crop. The field measurements showed that WUE of M. lutarioriparius in the semiarid location was significantly higher than that in the wet location. A matrix correlation analysis was conducted between gene expression levels and WUE to identify candidate genes involved in the improvement of WUE from the native to the domestication site. A total of 48 candidate genes were identified and assigned to functional categories, including photosynthesis, stomatal regulation, protein metabolism, and abiotic stress responses. Of these genes, nearly 73% were up-regulated in the semiarid site. It was also found that the relatively high expression variation of the WUE-related genes was affected to a larger extent by environment than by genetic variation. The study demonstrates that transcriptome-wide correlation between physiological phenotypes and expression levels offers an effective means for identifying candidate genes involved in the adaptation to environmental changes.

Natural variation, differentiation, and genetic trade-offs of ecophysiological traits in response to water limitation in Brachypodium distachyon and its descendent allotetraploid B. hybridum (Poaceae)

Differences in tolerance to water stress may underlie ecological divergence of closely related ploidy lineages. However, the mechanistic basis of physiological variation governing ecogeographical cytotype segregation is not well understood. Here, using Brachypodium distachyon and its derived allotetraploid B. hybridum as model, we test the hypothesis that, for heteroploid annuals, ecological divergence of polyploids in drier environments is based on trait differentiation enabling drought escape. We demonstrate that under water limitation allotetraploids maintain higher photosynthesis and stomatal conductance and show earlier flowering than diploids, concordant with a drought-escape strategy to cope with water stress. Increased heterozygosity and greater genetic variability and plasticity of polyploids could confer a superior adaptive capability. Consistent with these predictions, we document (1) greater standing within-population genetic variation in water-use efficiency (WUE) and flowering time in allotetraploids, and (2) the existence of (nonlinear) environmental clines in physiology across allotetraploid populations. Increased gas exchange and diminished WUE occurred at the driest end of the gradient, consistent with a drought-escape strategy. Finally, we found that allotetraploids showed weaker genetic correlations than diploids congruous with the expectation of relaxed pleiotropic constraints in polyploids. Our results suggest evolutionary divergence of ecophysiological traits in each ploidy lineage.

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.

Combining quantitative trait loci analysis with physiological models to predict genotype-specific transpiration rates

Transpiration is controlled by evaporative demand and stomatal conductance (gs ), and there can be substantial genetic variation in gs . A key parameter in empirical models of transpiration is minimum stomatal conductance (g0 ), a trait that can be measured and has a large effect on gs and transpiration. In Arabidopsis thaliana, g0 exhibits both environmental and genetic variation, and quantitative trait loci (QTL) have been mapped. We used this information to create a genetically parameterized empirical model to predict transpiration of genotypes. For the parental lines, this worked well. However, in a recombinant inbred population, the predictions proved less accurate. When based only upon their genotype at a single g0 QTL, genotypes were less distinct than our model predicted. Follow-up experiments indicated that both genotype by environment interaction and a polygenic inheritance complicate the application of genetic effects into physiological models. The use of ecophysiological or 'crop' models for predicting transpiration of novel genetic lines will benefit from incorporating further knowledge of the genetic control and degree of independence of core traits/parameters underlying gs variation.

Direct and indirect selection on flowering time, water-use efficiency (WUE, δ (13)C), and WUE plasticity to drought in Arabidopsis thaliana

Flowering time and water-use efficiency (WUE) are two ecological traits that are important for plant drought response. To understand the evolutionary significance of natural genetic variation in flowering time, WUE, and WUE plasticity to drought in Arabidopsis thaliana, we addressed the following questions: (1) How are ecophysiological traits genetically correlated within and between different soil moisture environments? (2) Does terminal drought select for early flowering and drought escape? (3) Is WUE plasticity to drought adaptive and/or costly? We measured a suite of ecophysiological and reproductive traits on 234 spring flowering accessions of A. thaliana grown in well-watered and season-ending soil drying treatments, and quantified patterns of genetic variation, correlation, and selection within each treatment. WUE and flowering time were consistently positively genetically correlated. WUE was correlated with WUE plasticity, but the direction changed between treatments. Selection generally favored early flowering and low WUE, with drought favoring earlier flowering significantly more than well-watered conditions. Selection for lower WUE was marginally stronger under drought. There were no net fitness costs of WUE plasticity. WUE plasticity (per se) was globally neutral, but locally favored under drought. Strong genetic correlation between WUE and flowering time may facilitate the evolution of drought escape, or constrain independent evolution of these traits. Terminal drought favored drought escape in these spring flowering accessions of A. thaliana. WUE plasticity may be favored over completely fixed development in environments with periodic drought.

Selection on crop-derived traits and QTL in sunflower (Helianthus annuus) crop-wild hybrids under water stress

Locally relevant conditions, such as water stress in irrigated agricultural regions, should be considered when assessing the risk of crop allele introgression into wild populations following hybridization. Although research in cultivars has suggested that domestication traits may reduce fecundity under water stress as compared to wild-like phenotypes, this has not been investigated in crop-wild hybrids. In this study, we examine phenotypic selection acting on, as well as the genetic architecture of vegetative, reproductive, and physiological characteristics in an experimental population of sunflower crop-wild hybrids grown under wild-like low water conditions. Crop-derived petiole length and head diameter were favored in low and control water environments. The direction of selection differed between environments for leaf size and leaf pressure potential. Interestingly, the additive effect of the crop-derived allele was in the direction favored by selection for approximately half the QTL detected in the low water environment. Selection favoring crop-derived traits and alleles in the low water environment suggests that a subset of these alleles would be likely to spread into wild populations under water stress. Furthermore, differences in selection between environments support the view that risk assessments should be conducted under multiple locally relevant conditions.

Adaptive differentiation of traits related to resource use in a desert annual along a resource gradient

• Plant resource-use traits are generally hypothesized to be adaptively differentiated for populations distributed along resource gradients. Although nutrient limitations are expected to select for resource-conservative strategies, water limitations may select for either resource-conservative or -acquisitive strategies. We test whether population differentiation reflects local adaptation for traits associated with resource-use strategies in a desert annual (Helianthus anomalus) distributed along a gradient of positively covarying water and nutrient availability. • We compared quantitative trait variation (Q(ST)) with neutral genetic differentiation (F(ST)), in a common garden glasshouse study, for leaf economics spectrum (LES) and related traits: photosynthesis (A(mass), A(area)), leaf nitrogen (N(mass), N(area)), leaf lifetime (LL), leaf mass per area (LMA), leaf water content (LWC), water-use efficiency (WUE, estimated as δ(13)C) and days to first flower (DFF). • Q(ST)-F(ST) differences support adaptive differentiation for Amass , N(mass), N(area), LWC and DFF. The trait combinations associated with drier and lower fertility sites represent correlated trait evolution consistent with the more resource-acquisitive end of the LES. There was no evidence for adaptive differentiation for A(area), LMA and WUE. • These results demonstrate that hot dry environments can selectively favor correlated evolution of traits contributing to a resource-acquisitive and earlier reproduction 'escape' strategy, despite lower fertility.

Differences in gas exchange contribute to habitat differentiation in Iberian columbines from contrasting light and water environments

During photosynthesis, respiration and transpiration, gas exchange occurs via the stomata and so plants face a trade-off between maximising photosynthesis while minimising transpiration (expressed as water use efficiency, WUE). The ability to cope with this trade-off and regulate photosynthetic rate and stomatal conductance may be related to niche differentiation between closely related species. The present study explored this as a possible mechanism for habitat differentiation in Iberian columbines. The roles of irradiance and water stress were assessed to determine niche differentiation among Iberian columbines via distinct gas exchange processes. Photosynthesis-irradiance curves (P-I curves) were obtained for four taxa, and common garden experiments were conducted to examine plant responses to water and irradiance stress, by measuring instantaneous gas exchange and plant performance. Gas exchange was also measured in ten individuals using two to four field populations per taxon. The taxa had different P-I curves and gas exchange in the field. At the species level, water stress and irradiance explained habitat differentiation. Within each species, a combination of irradiance and water stress explained the between-subspecies habitat differentiation. Despite differences in stomatal conductance and CO2 assimilation, taxa did not have different WUE under field conditions, which suggests that the environment equally modifies photosynthesis and transpiration. The P-I curves, gas exchange in the field and plant responses to experimental water and irradiance stresses support the hypothesis that habitat differentiation is associated with differences among taxa in tolerance to abiotic stress mediated by distinct gas exchange responses.

Variation in MPK12 affects water use efficiency in Arabidopsis and reveals a pleiotropic link between guard cell size and ABA response

Plant water relations are critical for determining the distribution, persistence, and fitness of plant species. Studying the genetic basis of ecologically relevant traits, however, can be complicated by their complex genetic, physiological, and developmental basis and their interaction with the environment. Water use efficiency (WUE), the ratio of photosynthetic carbon assimilation to stomatal conductance to water, is a dynamic trait with tremendous ecological and agricultural importance whose genetic control is poorly understood. In the present study, we use a quantitative trait locus-mapping approach to locate, fine-map, clone, confirm, and characterize an allelic substitution that drives differences in WUE among natural accessions of Arabidopsis thaliana. We show that a single amino acid substitution in an abscisic acid-responsive kinase, AtMPK12, causes reduction in WUE, and we confirm its functional role using transgenics. We further demonstrate that natural alleles at AtMPK12 differ in their response to cellular and environmental cues, with the allele from the Cape Verde Islands (CVI) being less responsive to hormonal inhibition of stomatal opening and more responsive to short-term changes in vapor pressure deficit. We also show that the CVI allele results in constitutively larger stomata. Together, these differences cause higher stomatal conductance and lower WUE compared with the common allele. These physiological changes resulted in reduced whole-plant transpiration efficiency and reduced fitness under water-limited compared with well-watered conditions. Our work demonstrates how detailed analysis of naturally segregating functional variation can uncover the molecular and physiological basis of a key trait associated with plant performance in ecological and agricultural settings.

Genotypic variation in traits linked to climate and aboveground productivity in a widespread C₄ grass: evidence for a functional trait syndrome

Examining intraspecific variation in growth and function in relation to climate may provide insight into physiological evolution and adaptation, and is important for predicting species responses to climate change. Under common garden conditions, we grew nine genotypes of the C₄ species Panicum virgatum originating from different temperature and precipitation environments. We hypothesized that genotype productivity, morphology and physiological traits would be correlated with climate of origin, and a suite of adaptive traits would show high broad-sense heritability (H(2)). Genotype productivity and flowering time increased and decreased, respectively, with home-climate temperature, and home-climate temperature was correlated with genotypic differences in a syndrome of morphological and physiological traits. Genotype leaf and tiller size, leaf lamina thickness, leaf mass per area (LMA) and C : N ratios increased with home-climate temperature, whereas leaf nitrogen per unit mass (Nm ) and chlorophyll (Chl) decreased with home-climate temperature. Trait variation was largely explained by genotypic differences (H(2) = 0.33-0.85). Our results provide new insight into the role of climate in driving functional trait coordination, local adaptation and genetic divergence within species. These results emphasize the importance of considering intraspecific variation in future climate change scenarios.

Pleiotropy of FRIGIDA enhances the potential for multivariate adaptation

An evolutionary response to selection requires genetic variation; however, even if it exists, then the genetic details of the variation can constrain adaptation. In the simplest case, unlinked loci and uncorrelated phenotypes respond directly to multivariate selection and permit unrestricted paths to adaptive peaks. By contrast, 'antagonistic' pleiotropic loci may constrain adaptation by affecting variation of many traits and limiting the direction of trait correlations to vectors that are not favoured by selection. However, certain pleiotropic configurations may improve the conditions for adaptive evolution. Here, we present evidence that the Arabidopsis thaliana gene FRI (FRIGIDA) exhibits 'adaptive' pleiotropy, producing trait correlations along an axis that results in two adaptive strategies. Derived, low expression FRI alleles confer a 'drought escape' strategy owing to fast growth, low water use efficiency and early flowering. By contrast, a dehydration avoidance strategy is conferred by the ancestral phenotype of late flowering, slow growth and efficient water use during photosynthesis. The dehydration avoidant phenotype was recovered when genotypes with null FRI alleles were transformed with functional alleles. Our findings indicate that the well-documented effects of FRI on phenology result from differences in physiology, not only a simple developmental switch.

Phylogenetic influences on leaf trait integration in Pelargonium (Geraniaceae): convergence, divergence, and historical adaptation to a rapidly changing climate

PREMISE OF THE STUDY

Trait integration may improve prediction of species and lineage responses to future climate change more than individual traits alone, particularly when analyses incorporate effects of phylogenetic relationships. The South African genus Pelargonium contains divergent major clades that have radiated along the same seasonal aridity gradient, presenting the opportunity to ask whether patterns of evolution in mean leaf trait values are achieved through the same set of coordinated changes among traits in each clade.

METHODS

Seven leaf traits were measured on field-collected leaves from one-third of the species (98) of the genus. Trait relationships were examined using phylogenetic regression within major clades. Disparity analysis determined whether the course of trait evolution paralleled historical climate change events.

KEY RESULTS

Divergence in mean trait values between sister clades A1 and A2 was consistent with expectations for leaves differing in longevity, despite strong similarity between clades in trait interactions. No traits in either clade exhibited significant relationships with multivariate climate axes, with one exception. Species in clades C and A2 included in this study occupied similar environments. These clades had similar values of individual trait means, except for δ(13)C, but they exhibited distinctive patterns of trait integration.

CONCLUSIONS

Differing present-day patterns of trait integration are consistent with interpretations of adaptive responses to the prevailing climate at the time of each clade's origin. These differing patterns of integration are likely to exert strong effects on clade-level responses to future climate change in the winter rainfall region of South Africa.

Genetic analyses of nickel tolerance in a North American serpentine endemic plant, Caulanthus amplexicaulis var. barbarae (Brassicaceae)

PREMISE OF THE STUDY

The evolution of metal tolerance in plants is an important model for studies of adaptation to environment, population genetics, and speciation. Here, we investigated nickel tolerance in the North American serpentine endemic Caulanthus amplexicaulis var. barbarae in comparison with its nonserpentine sister taxon C. amplexicaulis var. amplexicaulis. We hypothesized that the serpentine endemic would have a heritable growth advantage on nickel-containing substrates.

METHODS

We employed an artificial growth assay to quantify biomass accumulation. Study plants were crossed to create an F(2:3) population that was used to determine the heritability of nickel tolerance and to map quantitative trait loci (QTL). Nickel accumulation in both laboratory populations and native specimens was examined using energy-dispersive x-ray fluorescence (EDXRF).

KEY RESULTS

The serpentine endemic had a dramatic growth advantage at concentrations of nickel >30 µmol/L. Caulanthus amplexicaulis var. barbarae and its nonserpentine sister taxon both accumulated nickel to substantial levels. Nickel tolerance was highly heritable (h(2) = 0.59) and not associated with accumulation. The QTL analyses identified two major loci for nickel tolerance, on linkage group 2 (LG2) and linkage group 9 (LG9).

CONCLUSIONS

In our study, nickel tolerance was determined by two major loci with large effects. At both loci, alleles from the serpentine parent conferred positive effects on nickel tolerance, suggesting that they are adaptive in the natural serpentine environment. The mechanism of nickel tolerance in the serpentine plant was not exclusion of nickel. Nickel tolerance may have an inducible component in C. amplexicaulis var. barbarae.