Heterotic growth of hybrids of Arabidopsis thaliana is enhanced by elevated atmospheric CO2.

PREMISE: With the global atmospheric CO2 concentration on the rise, developing crops that can thrive in elevated CO2 has become paramount. We investigated the potential of hybridization as a strategy for creating crops with improved growth in predicted elevated atmospheric CO2. METHODS: We grew parent accessions and their F1 hybrids of Arabidopsis thaliana in ambient and elevated atmospheric CO2 and analyzed numerous growth traits to assess their productivity and underlying mechanisms. RESULTS: The heterotic increase in total dry mass, relative growth rate and leaf net assimilation rate was significantly greater in elevated CO2 than in ambient CO2. The CO2 response of net assimilation rate was positively correlated with the CO2 response of leaf nitrogen productivity and with that of leaf traits such as leaf size and thickness, suggesting that hybridization-induced changes in leaf traits greatly affected the improved performance in elevated CO2. CONCLUSIONS: Vegetative growth of hybrids seems to be enhanced in elevated CO2 due to improved photosynthetic nitrogen-use efficiency compared with parents. The results suggest that hybrid crops should be well-suited for future conditions, but hybrid weeds may also be more competitive.

Does selection occur at the intermediate zone of two insufficiently isolated populations? A whole-genome analysis along an altitudinal gradient.

Adaptive divergence occurs even between insufficiently isolated populations when there is a great difference in environments between their habitats. Individuals present in an intermediate zone of the two divergent populations are expected to have an admixed genetic structure due to gene flow. A selective pressure that acts on the genetically admixed individuals may limit the gene flow and maintain the adaptive divergence. Here, we addressed a question whether selection occurs in the genetically admixed individuals between two divergent populations. Arabidopsis halleri is a perennial montane plant, which has clear phenotypic dimorphisms between highland and lowland habitats in Mt. Ibuki, central Japan. We obtained the whole-genome sequences of Arabidopsis halleri plants along an altitudinal gradient of 359-1,317 m with a high spatial resolution (mean altitudinal interval of 20 m). We found a zone where the highland and lowland genes were mixing (intermediate subpopulation). In the intermediate subpopulation, we identified 5 and 13 genome regions, which included 3 and 8 genes, that had a high frequency of alleles that are accumulated in highland and lowland subpopulations, respectively. In addition, we also found that the frequency of highland alleles of these selected genome regions was smaller in the lowland subpopulation compared with that of the non-selected regions. These results suggest that the selection in the intermediate subpopulation might limit the gene flow and contribute to the adaptive divergence between altitudes. We also identified 7 genome regions that had low heterozygote frequencies in the intermediate subpopulation. We conclude that different types of selection in addition to gene flow occur at the intermediate altitude and shape the genetic structure across altitudes.

Enhanced growth rate under elevated CO2 conditions was observed for transgenic lines of genes identified by intraspecific variation analyses in Arabidopsis thaliana.

KEY MESSAGE: Using the whole genome and growth data of Arabidopsis thaliana ecotypes, we identified two genes associated with enhancement of the growth rate in response to elevated CO2 conditions. Improving plant growth under elevated CO2 conditions may contribute to enhanced agricultural yield under future global climate change. In this study, we examined the genes implicated in the enhancement of growth rates under elevated CO2 conditions by analyzing the growth rates of Arabidopsis thaliana ecotypes originating from various latitudes and altitudes throughout the world. We also performed a genome-wide association study and a transcriptome study to identify single nucleic polymorphisms that were correlated with the relative growth rate (RGR) under elevated CO2 conditions or with CO2 response of RGR. We then selected 43 candidate genes and generated their overexpression and/or RNA interference (RNAi) transgenic mutants for screening. After screening, we have found that RNAi lines of AT3G4000 and AT5G50900 showed significantly higher growth rates under the elevated CO2 condition. As per our findings, we conclude that natural variation includes genetic variation associated with the enhancement of plant productivity under elevated CO2 conditions.

Estimating leaf photosynthesis of C3 plants grown under different environments from pigment index, photochemical reflectance index, and chlorophyll fluorescence.

Photosynthetic rates vary depending on growth conditions, even within species. Remote sensing techniques have a great potential to predict the photosynthetic rates of leaves with different characteristics. Here, we demonstrate that the photosynthetic rates of leaves acclimated to different light and nutrient conditions can be estimated based on the chlorophyll fluorescence (ChlF), the photochemical reflectance index (PRI), and a chlorophyll index. Chenopodium album plants were grown under different light and nutrient conditions. PRI, ChlF parameters, and CO2/H2O gas exchange rates of leaves were simultaneously determined under the various light and CO2 conditions. PRI was used to assess non-photochemical quenching (NPQ), but the relationship between NPQ and PRI was weakened when the data on leaves grown under different conditions were pooled, because PRI in darkness ([Formula: see text]) changed with the leaf pigment composition. Among 15 pigment indices, we found that [Formula: see text], a reflectance index related to the leaf chlorophyll content, had the best correlation with [Formula: see text] ([Formula: see text]) across the studied leaves, and the correction of PRI by [Formula: see text] improved the predictability of NPQ ([Formula: see text]). Using the steady-state ChlF, the NPQ estimated from PRI and [Formula: see text], and the stomatal conductance coefficient, we calculated the CO2 assimilation rates, which were strongly correlated with the actual rates (RMSE = 4.85 [Formula: see text]mol m[Formula: see text] s[Formula: see text]), irrespective of growth conditions. Our approach has the potential to contribute to a more accurate estimation of photosynthetic rates in remote sensing. However, further studies on species variations and connecting with radiative transfer models are needed to demonstrate this at the canopy scale.

Dividing the pie: A quantitative review on plant density responses.

Plant population density is an important variable in agronomy and forestry and offers an experimental way to better understand plant-plant competition. We made a meta-analysis of responses of even-aged mono-specific stands to population density by quantifying for 3 stand and 33 individual plant variables in 334 experiments how much both plant biomass and phenotypic traits change with a doubling in density. Increasing density increases standing crop per area, but decreases the mean size of its individuals, mostly through reduced tillering and branching. Among the phenotypic traits, stem diameter is negatively affected, but plant height remains remarkably similar, partly due to an increased stem length-to-mass ratio and partly by increased allocation to stems. The reduction in biomass is caused by a lower photosynthetic rate, mainly due to shading of part of the foliage. Total seed mass per plant is also strongly reduced, marginally by lower mass per seed, but mainly because of lower seed numbers. Plants generally have fewer shoot-born roots, but their overall rooting depth seems hardly affected. The phenotypic plasticity responses to high densities correlate strongly with those to low light, and less with those to low nutrients, suggesting that at high density, shading affects plants more than nutrient depletion.

Leaf density and chemical composition explain variation in leaf mass area with spectral composition among 11 widespread forbs in a common garden.

Leaf mass per area (LMA) is a key leaf functional trait correlated with plant strategies dictating morphology, physiology, and biochemistry. Although sunlight is generally accepted as a dominant factor driving LMA, the contribution of each spectral region of sunlight in shaping LMA is poorly understood. In the present study, we grew 11 widespread forb species in a common garden and dissected the traits underpinning differences in LMA, such as its morphological components (leaf density [LD] and leaf thickness [LT]), macroelement, and metabolite composition under five spectral-attenuation treatments: (1) transmitting c. 95% of the whole solar spectrum (> 280 nm), (2) attenuating ultraviolet-B radiation (UV-B), (3) attenuating both UV-A and UV-B radiation, (4) attenuating UV radiation and blue light, (5) attenuating UV radiation, blue, and green light. We found that LMA, LD, and chemical traits varied significantly across species depending on spectral treatments. LMA was significantly increased by UV-B radiation and green light, while LD was increased by UV-A but decreased by blue light. LMA positively correlated with LD across treatments but was only weakly related to LT, suggesting that LD was a better determinate of LMA for this specific treatment. Regarding leaf elemental and metabolite composition, carbon, nitrogen, and total phenolics were all positively correlated with LMA, whereas lignin, non-structural carbohydrates, and soluble sugars had negative relationships with LMA. These trends imply a tradeoff between biomass allocation to structural and metabolically functional components. In conclusion, sunlight can spectrally drive LMA mainly through modifying functional and structural support.

Linking remote sensing parameters to CO2 assimilation rates at a leaf scale.

Solar-induced chlorophyll fluorescence (SIF) and photochemical reflectance index (PRI) are expected to be useful for remote sensing of photosynthetic activity at various spatial scales. This review discusses how chlorophyll fluorescence and PRI are related to the CO2 assimilation rate at a leaf scale. Light energy absorbed by photosystem II chlorophylls is allocated to photochemistry, fluorescence, and heat dissipation evaluated as non-photochemical quenching (NPQ). PRI is correlated with NPQ because it reflects the composition of xanthophylls, which are involved in heat dissipation. Assuming that NPQ is uniquely related to the photochemical efficiency (quantum yield of photochemistry), photochemical efficiencies can be assessed from either chlorophyll fluorescence or PRI. However, this assumption may not be held under some conditions such as low temperatures and photoinhibitory environments. Even in such cases, photosynthesis may be estimated more accurately if both chlorophyll fluorescence and PRI are determined simultaneously. To convert from photochemical efficiency to CO2 assimilation, environmental responses in stomatal conductance also need to be considered. Models linking chlorophyll fluorescence and PRI with CO2 assimilation rates will contribute to understanding and future prediction of the global carbon cycle.

Corrected photochemical reflectance index (PRI) is an effective tool for detecting environmental stresses in agricultural crops under light conditions.

High-throughput detection of plant environmental stresses is required for minimizing the reduction in crop yield. Environmental stresses in plants have primarily been validated by the measurements of photosynthesis with gas exchange and chlorophyll fluorescence, which involve complicated procedures. Remote sensing technologies that monitor leaf reflectance in intact plants enable real-time visualization of plant responses to environmental fluctuations. The photochemical reflectance index (PRI), one of the vegetation indices of spectral leaf reflectance, is related to changes in xanthophyll pigment composition. Xanthophyll dynamics are strongly correlated with plant stress because they contribute to the thermal dissipation of excess energy. However, an accurate assessment of plant stress based on PRI requires correction by baseline PRI (PRIo) in the dark, which is difficult to obtain in the field. In this study, we propose a method to correct the PRI using NPQT, which can be measured under light. By this method, we evaluated responses of excess light energy stress under drought in wild watermelon (Citrullus lanatus L.), a xerophyte. Demonstration on the farm, the stress behaviors were observed in maize (Zea mays L.). Furthermore, the stress status of plants and their recovery following re-watering were captured as visual information. These results suggest that the PRI is an excellent indicator of environmental stress and recovery in plants and could be used as a high-throughput stress detection tool in agriculture.

Temperature-related cline in the root mass fraction in East Asian wild radish along the Japanese archipelago.

Wild plants with a wide distribution, including those exposed to a wide variety of environmental conditions, may have variations in key functional traits relevant for agricultural applications. The East Asian wild radish (Raphanus sativus var. raphanistroides) is an appropriate model plant because it is widely distributed and has outstanding sink capacity as well as two cultivars within the species. Multiple common garden trials with 14 populations and three testing sites were conducted across the Japanese archipelago to quantify variations in yield and allocation. Significant inter-population variations and interaction effects with testing sites were detected for the root and shoot mass and the root mass fraction (RMF). While the rank order of the population changed drastically among sites and the variance components of genetic effects were small in yield traits (2.4%-4.7%), RMF displayed a large genetic variance (23.2%) and was consistently higher in the northern populations at all sites. Analyses revealed that the mean temperature of growing season of the seed origin was the most prominent factor explaining variation in RMF, irrespective of the sites. We concluded that the trait of resource allocation had a temperature-related cline and plants in cooler climates could invest more resources into their roots.

Acclimation and adaptation components of the temperature dependence of plant photosynthesis at the global scale.

The temperature response of photosynthesis is one of the key factors determining predicted responses to warming in global vegetation models (GVMs). The response may vary geographically, owing to genetic adaptation to climate, and temporally, as a result of acclimation to changes in ambient temperature. Our goal was to develop a robust quantitative global model representing acclimation and adaptation of photosynthetic temperature responses. We quantified and modelled key mechanisms responsible for photosynthetic temperature acclimation and adaptation using a global dataset of photosynthetic CO2 response curves, including data from 141 C3 species from tropical rainforest to Arctic tundra. We separated temperature acclimation and adaptation processes by considering seasonal and common-garden datasets, respectively. The observed global variation in the temperature optimum of photosynthesis was primarily explained by biochemical limitations to photosynthesis, rather than stomatal conductance or respiration. We found acclimation to growth temperature to be a stronger driver of this variation than adaptation to temperature at climate of origin. We developed a summary model to represent photosynthetic temperature responses and showed that it predicted the observed global variation in optimal temperatures with high accuracy. This novel algorithm should enable improved prediction of the function of global ecosystems in a warming climate.

Modeling leaf CO2 assimilation and Photosystem II photochemistry from chlorophyll fluorescence and the photochemical reflectance index.

We present a simple model to assess the quantum yield of photochemistry (ΦP ) and CO2 assimilation rate from two parameters that are detectable by remote sensing: chlorophyll (chl) fluorescence and the photochemical reflectance index (PRI). ΦP is expressed as a simple function of the chl fluorescence yield (ΦF ) and nonphotochemical quenching (NPQ): ΦP  = 1-bΦF (1 + NPQ). Because NPQ is known to be related with PRI, ΦP can be remotely assessed from solar-induced fluorescence and the PRI. The CO2 assimilation rate can be assessed from the estimated ΦP value with either the maximum carboxylation rate (Vcmax ), the intercellular CO2 concentration (Ci ), or parameters of the stomatal conductance model. The model was applied to experimental data obtained for Chenopodium album leaves under various environmental conditions and was able to successfully predict ΦF values and the CO2 assimilation rate. The present model will improve the accuracy of assessments of gas exchange rates and primary productivity by remote sensing.

Plasticity of functional traits and optimality of biomass allocation in elevational ecotypes of Arabidopsis halleri grown at different soil nutrient availabilities.

In mountainous areas, plant distribution is constrained by various environmental stresses. Plasticity and constancy in plant functional traits may relate to optimal strategies at respective habitats and to ecotypic differentiation along elevation. Although plant biomass allocation has been extensively studied in relation to adaptation to soil nutrient availability along elevation, its optimality is still poorly understood. We examined soil nutrient availability in the field and conducted growth analysis for two elevational ecotypes of Arabidopsis halleri grown under different nutrient availabilities. We determined plasticity in morphological and physiological traits and evaluated optimal biomass allocation using an optimality model. Our field investigation indicated that soil nitrogen (N) availability increased rather than decreased with increasing elevation. Our growth analysis revealed that lowland ecotype was more plastic in morphological variables and N concentrations, whereas the highland ecotype was more plastic in other physiological variables such as the net assimilation rate (NAR). The leaf mass ratio (LMR) in the lowland ecotype was moderately plastic at the whole range of N availabilities, whereas LMR in the highland ecotype was very plastic at higher N availabilities only. The optimality model indicated that the LMR of the lowland ecotype was nearly optimal throughout the range of studied N availabilities, whereas that of the highland ecotype was suboptimal at low N availability. These results suggest that highland ecotype is adapted only to high N availability, whereas the lowland ecotype is adapted to a relatively wide range of N availabilities as a result of natural selection in their respective habitats. We conclude that an adaptive differentiation has occurred between the two ecotypes and plasticity in the biomass allocation is directly related to its optimization in changing environments.

Physiological validation of photochemical reflectance index (PRI) as a photosynthetic parameter using Arabidopsis thaliana mutants.

Non-photochemical quenching (NPQ) is the most important photoprotective system in higher plants. NPQ can be divided into several steps according to the timescale of relaxation of chlorophyll fluorescence after reaching a steady state (i.e., the fast phase, qE; middle phase, qZ or qT; and slow phase, qI). The dissipation of excess energy as heat during the xanthophyll cycle, a large component of NPQ, is detectable during the fast to middle phase (sec to min). Although thermal dissipation is primarily investigated using indirect methods such as chlorophyll a fluorescence measurements, such analyses require dark adaptation or the application of a saturating pulse during measurement, making it difficult to continuously monitor this process. Here, we designed an unconventional technique for real-time monitoring of changes in thylakoid lumen pH (as reflected by changes in xanthophyll pigment content) based on the photochemical reflectance index (PRI), which we estimated by measuring light-driven leaf reflectance at 531 nm. We analyzed two Arabidopsis thaliana mutants, npq1 (unable to convert violaxanthin to zeaxanthin due to inhibited violaxanthin de-epoxidase [VDE] activity) and npq4 (lacking PsbS protein), to uncover the regulator of the PRI. The PRI was variable in wild-type and npq4 plants, but not in npq1, indicating that the PRI is related to xanthophyll cycle-dependent thermal energy quenching (qZ) rather than the linear electron transport rate or NPQ. In situ lumen pH substitution using a pH-controlled buffer solution caused a shift in PRI. These results suggest that the PRI reflects only xanthophyll cycle conversion and is therefore a useful parameter for monitoring thylakoid lumen pH (reflecting VDE activity) in vivo.

The role of biomass allocation between lamina and petioles in a game of light competition in a dense stand of an annual plant.

Background and Aims: Models of plant three-dimensional (3-D) architecture have been used to find optimal morphological characteristics for light capture or carbon assimilation of a solitary plant. However, optimality theory is not necessarily useful to predict the advantageous strategy of an individual in dense stands, where light capture of an individual is influenced not only by its architecture but also by the architecture of its neighbours. Here, we analysed optimal and evolutionarily stable biomass allocation between the lamina and petiole (evolutionarily stable strategy; ESS) under various neighbour conditions using a 3-D simulation model based on the game theory. Methods: We obtained 3-D information of every leaf of actual Xanthium canadense plants grown in a dense stand using a ruler and a protractor. We calculated light capture and carbon assimilation of an individual plant when it stands alone and when it is surrounded by neighbours in the stand. We considered three trade-offs in petiole length and lamina area: biomass allocation, biomechanical constraints and photosynthesis. Optimal and evolutionarily stable biomass allocation between petiole and lamina were calculated under various neighbour conditions. Key Results: Optimal petiole length varied depending on the presence of neighbours and on the architecture of neighbours. The evolutionarily stable petiole length of plants in the stand tended to be longer than the optimal length of solitary plants. The mean of evolutionarily stable petiole length in the stand was similar to the real one. Trade-offs of biomechanical constraint and photosynthesis had minor effects on optimal and evolutionarily stable petiole length. Conclusion: Actual plants realize evolutionarily stable architecture in dense stands. Interestingly, there were multiple evolutionarily stable petiole lengths even in one stand, suggesting that plants with different architectures can coexist across plant communities.

Dependence of functional traits related to growth rates and their CO2 response on multiple habitat climate factors across Arabidopsis thaliana populations.

The values of many plant traits are often different even within a species as a result of local adaptation. Here, we studied how multiple climate variables influence trait values in Arabidopsis thaliana grown under common conditions. We examined 9 climate variables and 29 traits related to vegetative growth rate in 44 global A. thaliana accessions grown at ambient or elevated CO2 concentration ([CO2]) and applied a multiple regression analysis. We found that genetic variations in the traits related to growth rates were associated with various climate variables. At ambient [CO2], plant size was positively correlated with precipitation in the original habitat. This may be a result of larger biomass investment in roots at the initial stage in plants adapting to a lower precipitation. Stomatal conductance and photosynthetic nitrogen use efficiency were negatively correlated with vapor pressure deficit, probably as a result of the trade-off between photosynthetic water- and nitrogen-use efficiency. These results suggest that precipitation and air humidity influence belowground and aboveground traits, respectively. Elevated [CO2] altered climate dependences in some of the studied traits. The CO2 response of relative growth rate was negatively correlated with altitude, indicating that plants inhabiting a higher altitude have less plasticity to changing [CO2]. These results are useful not only for understanding evolutionary process but also to predict the plant species that are favored under future global change.

A Genome Scan for Genes Underlying Microgeographic-Scale Local Adaptation in a Wild Arabidopsis Species.

Adaptive divergence at the microgeographic scale has been generally disregarded because high gene flow is expected to disrupt local adaptation. Yet, growing number of studies reporting adaptive divergence at a small spatial scale highlight the importance of this process in evolutionary biology. To investigate the genetic basis of microgeographic local adaptation, we conducted a genome-wide scan among sets of continuously distributed populations of Arabidopsis halleri subsp. gemmifera that show altitudinal phenotypic divergence despite gene flow. Genomic comparisons were independently conducted in two distinct mountains where similar highland ecotypes are observed, presumably as a result of convergent evolution. Here, we established a de novo reference genome and employed an individual-based resequencing for a total of 56 individuals. Among 527,225 reliable SNP loci, we focused on those showing a unidirectional allele frequency shift across altitudes. Statistical tests on the screened genes showed that our microgeographic population genomic approach successfully retrieve genes with functional annotations that are in line with the known phenotypic and environmental differences between altitudes. Furthermore, comparison between the two distinct mountains enabled us to screen out those genes that are neutral or adaptive only in either mountain, and identify the genes involved in the convergent evolution. Our study demonstrates that the genomic comparison among a set of genetically connected populations, instead of the commonly-performed comparison between two isolated populations, can also offer an effective screening for the genetic basis of local adaptation.

Physiological and structural tradeoffs underlying the leaf economics spectrum.

The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO2 diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18-70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO2 diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.

Mutant selection in the self-incompatible plant radish (Raphanus sativus L. var. sativus) using two-step TILLING.

Radish (Raphanus sativus L. var. sativus), a widely cultivated root vegetable crop, possesses a large sink organ (the root), implying that photosynthetic activity in radish can be enhanced by altering both the source and sink capacity of the plant. However, since radish is a self-incompatible plant, improved mutation-breeding strategies are needed for this crop. TILLING (Targeting Induced Local Lesions IN Genomes) is a powerful method used for reverse genetics. In this study, we developed a new TILLING strategy involving a two-step mutant selection process for mutagenized radish plants: the first selection is performed to identify a BC1M1 line, that is, progenies of M1 plants crossed with wild-type, and the second step is performed to identify BC1M1 individuals with mutations. We focused on Rubisco as a target, since Rubisco is the most abundant plant protein and a key photosynthetic enzyme. We found that the radish genome contains six RBCS genes and one pseudogene encoding small Rubisco subunits. We screened 955 EMS-induced BC1M1 lines using our newly developed TILLING strategy and obtained six mutant lines for the six RsRBCS genes, encoding proteins with four different types of amino acid substitutions. Finally, we selected a homozygous mutant and subjected it to physiological measurements.

Plant-plant interactions mediate the plastic and genotypic response of Plantago asiatica to CO2: an experiment with plant populations from naturally high CO2 areas.

BACKGROUND AND AIMS: The rising atmospheric CO2 concentration ([CO2]) is a ubiquitous selective force that may strongly impact species distribution and vegetation functioning. Plant-plant interactions could mediate the trajectory of vegetation responses to elevated [CO2], because some plants may benefit more from [CO2] elevation than others. The relative contribution of plastic (within the plant's lifetime) and genotypic (over several generations) responses to elevated [CO2] on plant performance was investigated and how these patterns are modified by plant-plant interactions was analysed. METHODS: Plantago asiatica seeds originating from natural CO2 springs and from ambient [CO2] sites were grown in mono stands of each one of the two origins as well as mixtures of both origins. In total, 1944 plants were grown in [CO2]-controlled walk-in climate rooms, under a [CO2] of 270, 450 and 750 ppm. A model was used for upscaling from leaf to whole-plant photosynthesis and for quantifying the influence of plastic and genotypic responses. KEY RESULTS: It was shown that changes in canopy photosynthesis, specific leaf area (SLA) and stomatal conductance in response to changes in growth [CO2] were mainly determined by plastic and not by genotypic responses. We further found that plants originating from high [CO2] habitats performed better in terms of whole-plant photosynthesis, biomass and leaf area, than those from ambient [CO2] habitats at elevated [CO2] only when both genotypes competed. Similarly, plants from ambient [CO2] habitats performed better at low [CO2], also only when both genotypes competed. No difference in performance was found in mono stands. CONCLUSION: The results indicate that natural selection under increasing [CO2] will be mainly driven by competitive interactions. This supports the notion that plant-plant interactions have an important influence on future vegetation functioning and species distribution. Furthermore, plant performance was mainly driven by plastic and not by genotypic responses to changes in atmospheric [CO2].

A meta-analysis of leaf nitrogen distribution within plant canopies.

BACKGROUND AND AIMS: Leaf nitrogen distribution in the plant canopy is an important determinant for canopy photosynthesis. Although the gradient of leaf nitrogen is formed along light gradients in the canopy, its quantitative variations among species and environmental responses remain unknown. Here, we conducted a global meta-analysis of leaf nitrogen distribution in plant canopies. METHODS: We collected data on the nitrogen distribution and environmental variables from 393 plant canopies (100, 241 and 52 canopies for wheat, other herbaceous and woody species, respectively). KEY RESULTS: The trends were clearly different between wheat and other species; the photosynthetic nitrogen distribution coefficient (Kb) was mainly determined by leaf area index (LAI) in wheat, whereas it was correlated with the light extinction coefficient (KL) and LAI in other species. Some other variables were also found to influence Kb We present the best equations for Kb as a function of environmental variables and canopy characteristics. As a more simple function, Kb = 0·5KL can be used for canopies of species other than wheat. Sensitivity analyses using a terrestrial carbon flux model showed that gross primary production tended to be more sensitive to the Kb value especially when nitrogen content of the uppermost leaf was fixed. CONCLUSION: Our results reveal that nitrogen distribution is mainly driven by the vertical light gradient but other factors such as LAI also have significant effects. Our equations contribute to an improvement in the projection of plant productivity and cycling of carbon and nitrogen in terrestrial ecosystems.