Accounting for mesophyll conductance substantially improves 13 C-based estimates of intrinsic water-use efficiency

Stable isotope analysis in tree rings of conifer species relevant to fire history study

Smoke and particulate matter released from forest fires, affecting the photosynthetic rate and stomatal conductance, may change the isotope composition in tree rings. Therefore, analysis of tree-ring isotopes could be a promising approach to monitor fires. We hypothesized that forest fires could influence the abundance of carbon (δ13C), oxygen (δ18O) and nitrogen (δ15N) isotopes and the radial growth of tree rings of conifers through their impact on the physiological processes. We collected wood cores from four coniferous species in northern and southern China. The isotope composition of these samples was analysed to shed light on the correlation between fire occurrence and tree-ring isotopes. We found that fires led to an increase in δ13C but a decrease in δ15N in the whole wood, while significant increases of above 0.5‰ in δ13C and a decrease of 0.2 to 0.5‰ in δ18O in the α-cellulose were observed. Meteorological factors including precipitation and relative humidity influenced the isotope abundance. Besides, forest fires inhibited the radial growth of conifer trees, particularly Cryptomeria fortunei. Our results suggest that variations in δ13C and δ18O abundance in tree rings play an essential role as an indicator of forest fire occurrence, providing additional insights into the study of fire history.This article is part of the theme issue 'Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks'.

Shining a new light on the classical concepts of carbon-isotope dendrochronology

Retrospective information about plant ecophysiology and the climate system are key inputs in Earth system and vegetation models. Dendrochronology provides such information with large spatiotemporal coverage, and carbon-isotope analysis across tree-ring series is among the most advanced dendrochronological tools. For the past 70 years, this analysis was performed on whole molecules and, to this day, 13C discrimination during carbon assimilation is invoked to explain isotope variation and associated climate signals. However, recently it was reported that tree-ring glucose exhibits multiple isotope signals at the intramolecular level (see Wieloch et al., 2025). Here, I estimated the signals' contribution to whole-molecule isotope variation and found that downstream processes in leaf and stem metabolism each introduce more variation than carbon assimilation. Moreover, downstream processes introduce most of the climate information. These findings are inconsistent with the classical concepts/practices of carbon-isotope dendrochronology. More importantly, intramolecular tree-ring isotope analysis promises novel insights into forest metabolism and the climate of the past.

Unraveling the drivers of optimal stomatal behavior in global C3 plants: A carbon isotope perspective

Understanding the drivers of stomatal behavior is critical for modeling terrestrial carbon cycle and water balance. The unified stomatal optimization (USO) model provides a mechanistic linkage between stomatal conductance (gs) and photosynthesis (A), with its slope parameter (g1) inversely related to intrinsic water use efficiency (iWUE), providing a key proxy to characterize the differences in iWUE and stomatal behavior. While many studies have identified multiple environmental factors influencing g1, the potential role of evolutionary history in shaping g1 remains incompletely understood. Leaf organic matter 13C discriminations (Δ13C) can be applied to estimate g1 over timescales from days to whole growing season. However, most applications assume that mesophyll conductance (gm)-a critical parameter in the Δ13C model-is infinite, due to limited information. Here, we incorporated new insight of gm to allow for more realistic parameterization of this variable, and subsequently to enable improved estimation of g1 based on a global bulk leaf Δ13C dataset comprising 2215 observations of 1521 species that span major biomes. Our analysis revealed a significant phylogenetic signal in g1 values, which differed among phylogenetic groups. Through a Bayesian phylogenetic linear mixed model, we found that species and phylogeny together explained 36.63 % of g1 variance, a contribution comparable to that of the environmental factors (44.59 %). Our findings uncovered for the first time that environmental factors, species-level and phylogenetic effects jointly shape g1 variability, thereby contributing to a more comprehensive understanding of optimal stomatal behavior in the context of global environmental change.

Water use efficiency in tropical plants based on a set of newly created leaf photosynthesis-related parameters

Water use efficiency (WUE) quantifies the amount of water expended per unit of dry leaf matter accumulated, reflecting the trade-offs between water consumption and carbon uptake. It is also a critical parameter for understanding plant responses to environmental changes. This study introduced an innovative set of WUE-related parameters, including maximum water use efficiency (WUEmax) and associated coefficients of water potential, loss, strategic usage, and total usage (WPC, WLC, WSC, and WTC, respectively) in providing a comprehensive evaluation of water use strategies in 48 common tropical plant species during the natural light fluctuations. These parameters exhibited significant differences among plant types, with sun-adapted and shade-tolerant plants (both C3) showing mean of WUEmax values of 3.81 ± 0.63 and 5.42 ± 1.61 μmol mmol-1, respectively, whereas C4 plants demonstrated a greater WUEmax of 7.04 ± 1.77 μmol mmol-1. Compared to C3 plants, particularly shade-tolerant types, C4 plants exhibited significantly higher WPC and WTC (p < 0.05). Furthermore, shade-tolerant plants displayed lower WLC and higher WSC than sun-adapted plants, suggestive of their specialized adaptations to variations in light intensity. The sensitivity of stomatal and mesophyll conductance (i.e., gs and gm) to incident light (Iinc) and/or intercellular CO2 concentration (Ci) helped clarify the source of variation in WUE-related parameters. In sun-adapted plants, gs was sensitive to changes in both Iinc and Ci. In terms of gm, shade-tolerant plants exhibited the lowest overall sensitivity to Iinc. Increasing atmospheric CO2 concentrations from 400 to 450 ppm caused WUE-related parameters to vary, with this response differing among plant types. These insights emphasize the significance of plant adaptation strategies in tropical rainforest ecosystems.

Reconciling water-use efficiency estimates from carbon isotope discrimination of leaf biomass and tree rings: nonphotosynthetic fractionation matters

Carbon isotope discrimination (∆) in leaf biomass (∆BL) and tree rings (∆TR) provides important proxies for plant responses to climate change, specifically in terms of intrinsic water-use efficiency (iWUE). However, the nonphotosynthetic 12C/13C fractionation in plant tissues has rarely been quantified and its influence on iWUE estimation remains uncertain. We derived a comprehensive, ∆ based iWUE model (iWUEcom) which includes nonphotosynthetic fractionations (d) and characterized tissue-specific d-values based on global compilations of data of ∆BL, ∆TR and real-time ∆ in leaf photosynthesis (∆online). iWUEcom was further validated with independent datasets. ∆BL was larger than ∆online by 2.53‰, while ∆BL and ∆TR showed a mean offset of 2.76‰, indicating that ∆TR is quantitatively very similar to ∆online. Applying the tissue-specific d-values (dBL = 2.5‰, dTR = 0‰), iWUE estimated from ∆BL aligned well with those estimated from ∆TR or gas exchange. ∆BL and ∆TR showed a consistent iWUE trend with an average CO2 sensitivity of 0.15 ppm ppm-1 during 1975-2015. Accounting for nonphotosynthetic fractionations improves the estimation of iWUE based on isotope records in leaf biomass and tree rings, which is ultimate for inferring changes in carbon and water cycles under historical and future climate.

Reduced mesophyll conductance under chronic O3 exposure in poplar reflects thicker cell walls and increased subcellular diffusion pathway lengths according to the anatomical model

Ozone (O3) is one of the most harmful and widespread air pollutants, affecting crop yield and plant health worldwide. There is evidence that O3 reduces the major limiting factor of photosynthesis, namely CO2 mesophyll conductance (gm), but there is little quantitative information of O3-caused changes in key leaf anatomical traits and their impact on gm. We exposed two O3-responsive clones of the economically important tree species Populus × canadensis Moench to 120 ppb O3 for 21 days. An anatomical diffusion model within the leaf was used to analyse the entire CO2 diffusion pathway from substomatal cavities to carboxylation sites and determine the importance of each structural and subcellular component as a limiting factor. gm decreased substantially under O3 and was found to be the most important limitation of photosynthesis. This decrease was mostly driven by an increased cell wall thickness and length of subcellular diffusion pathway caused by altered interchloroplast spacing and chloroplast positioning. By contrast, the prominent leaf integrative trait leaf dry mass per area was neither affected nor related to gm under O3. The observed relationship between gm and anatomy, however, was clone-dependent, suggesting that mechanisms regulating gm may differ considerably between closely related plant lines. Our results confirm the need for further studies on factors constraining gm under stress conditions.

Leaf stomatal configuration and photosynthetic traits jointly affect leaf water use efficiency in forests along climate gradients

Water use efficiency (WUE) represents the trade-off between carbon assimilation and water loss in plants. It remains unclear how leaf stomatal and photosynthetic traits regulate the spatial variation of leaf WUE in different natural forest ecosystems. We investigated 43 broad-leaf tree species spanning from cold-temperate to tropical forests in China. We quantified leaf WUE using leaf δ13C and measured stomatal traits, photosynthetic traits as well as maximum stomatal conductance (G w max ) and maximum carboxylation capacity (V c max ). We found that leaves in cold-temperate forests displayed 'fast' carbon economics, characterized by higher leaf nitrogen, Chl, specific leaf area, andV c max , as an adaptation to the shorter growing season. However, these leaves exhibited 'slow' hydraulic traits, with larger but fewer stomata and similarG w max , resulting in higher leaf WUE. By contrast, leaves in tropical forests had smaller and denser stomata, enabling swift response to heterogeneous light conditions. However, this stomatal configuration increased potential water loss, and coupled with their low photosynthetic capacity, led to lower WUE. Our findings contribute to understanding how plant photosynthetic and stomatal traits regulate carbon-water trade-offs across climatic gradients, advancing our ability to predict the impacts of climate changes on forest carbon and water cycles.

Thinning followed by slash burning enhances growth and reduces vulnerability to drought for Pinus nigra

Increasingly frequent severe drought events are pushing Mediterranean forests to unprecedented responses. Lack of management leads to dense forests that are highly susceptible to drought stress, potentially resulting in extensive dieback and increased vulnerability to other disturbances. Forest treatments like thinning and slash burning reduce competition for resources and have the potential to enhance tree growth and vigor and minimize tree vulnerability to drought. Here, we used tree rings to study the growth and physiological response of black pine (Pinus nigra) to drought in northeastern Spain under different treatments, including two thinning intensities (light and heavy, with 10% and 40% basal area reduction, respectively) followed by two understory treatments (clearing alone and in combination with slash burning), resulting in a research design of four treatments plus an untreated control with three replicates. Specifically, we studied basal area increment (BAI), resilience indices, and intrinsic water use efficiency (iWUE) using carbon and oxygen isotope composition (δ13C and δ18O in tree-ring cellulose) before and after treatments. Our results showed that BAI and resistance to drought increased in the heavy-thin (burned and unburned) and light-thin burned units. Resilience increased in the burned units regardless of the thinning intensity, while recovery was not affected by treatment. Slash burning additionally increased BAI in the light-thin and resistance and resilience in the heavy-thin units compared with clearing alone. The stable isotope analysis revealed a minor effect of treatments on δ13C and δ18O. No change in iWUE among treatments was presumably linked to a proportional increase in both net CO2 assimilation and stomatal conductance, which particularly increased in the heavy-thin (burned and unburned) and light-thin burned units, indicating that these trees were the least affected by drought. This study shows that management approaches aimed at reducing wildfire hazard can also increase the vigor of dominant trees under drought stress. By reducing competition both from the overstory and the understory, thinning followed by clearing alone or in combination with slash burning promotes tree growth and vigor and increases its resistance and resilience to drought.

Effect of nitrogen reduction combined with biochar application on soda saline soil and soybean growth in black soil areas

The combination of biochar and nitrogen (N) fertilization in agricultural salt-affected soils is an effective strategy for amending the soil and promoting production. To investigate the effect of nitrogen reduction combined with biochar application on a soda saline soil and soybean growth in black soil areas, a pot experiment was set up with two biochar application levels, 0 (B0) and 4.5 t/hm2 (B1); two biochar application depths, 0-20 cm (H1) and 0-40 cm (H2); and two nitrogen application levels, conventional nitrogen application (N0) and nitrogen reduction of 15% (N1). The results showed that the application of biochar improved the saline soil status and significantly increased soybean yield under lower nitrogen application. Moreover, increasing the depth of biochar application enhanced the effectiveness of biochar in reducing saline soil barriers to crop growth, which promoted soybean growth. Increasing the depth of biochar application increased the K+ and Ca2+ contents, soil nitrogen content, N fertilizer agronomic efficiency, leaf total nitrogen, N use efficiency, AN, Tr, gs, SPAD, leaf water potential, water content and soybean yield and its components. However, the Na+ content, SAR, ESP, Na+/K+, Ci and water use efficiency decreased with increasing biochar depth. Among the treatments with low nitrogen input and biochar, B1H1N1 resulted in the greatest soil improvement in the 0-20 cm soil layer compared with B0N0; for example, K+ content increased by 61.87%, Na+ content decreased by 44.80%, SAR decreased by 46.68%, and nitrate nitrogen increased by 26.61%. However, in the 20-40 cm soil layer, B1H2N1 had the greatest effect on improving the soil physicochemical properties, K+ content increased by 62.54%, Na+ content decreased by 29.76%, SAR decreased by 32.85%, and nitrate nitrogen content increased by 30.77%. In addition, compared with B0N0, total leaf nitrogen increased in B1H2N1 by 25.07%, N use efficiency increased by 6.7%, N fertilizer agronomic efficiency increased by 32.79%, partial factor productivity of nitrogen increased by 28.37%, gs increased by 22.10%, leaf water potential increased by 27.33% and water content increased by 6.44%. In conclusion, B1H2N1 had the greatest effect on improving the condition of saline soil; it not only effectively regulated the distribution of salt in soda saline soil and provided a low-salt environment for crop growth but also activated deep soil resources. Therefore, among all treatments investigated in this study, B1H2N1 was considered most suitable for improving the condition of soda saline soil in black soil areas and enhancing the growth of soybean plants.

The response of mesophyll conductance to short-term CO2 variation is related to stomatal conductance

The response of mesophyll conductance (gm) to CO2 plays a key role in photosynthesis and ecosystem carbon cycles under climate change. Despite numerous studies, there is still debate about how gm responds to short-term CO2 variations. Here we used multiple methods and looked at the relationship between stomatal conductance to CO2 (gsc) and gm to address this aspect. We measured chlorophyll fluorescence parameters and online carbon isotope discrimination (Δ) at different CO2 mole fractions in sunflower (Helianthus annuus L.), cowpea (Vigna unguiculata L.), and wheat (Triticum aestivum L.) leaves. The variable J and Δ based methods showed that gm decreased with an increase in CO2 mole fraction, and so did stomatal conductance. There were linear relationships between gm and gsc across CO2 mole fractions. gm obtained from A-Ci curve fitting method was higher than that from the variable J method and was not representative of gm under the growth CO2 concentration. gm could be estimated by empirical models analogous to the Ball-Berry model and the USO model for stomatal conductance. Our results suggest that gm and gsc respond in a coordinated manner to short-term variations in CO2, providing new insight into the role of gm in photosynthesis modelling.

The interplay of short-term mesophyll and stomatal conductance responses under variable environmental conditions

Understanding the short-term responses of mesophyll conductance (gm) and stomatal conductance (gsc) to environmental changes remains a challenging yet central aspect of plant physiology. This review synthesises our current knowledge of these short-term responses, which underpin CO2 diffusion within leaves. Recent methodological advances in measuring gm using online isotopic discrimination and chlorophyll fluorescence have improved our confidence in detecting short-term gm responses, but results need to be carefully evaluated. Environmental factors like vapour pressure deficit and CO2 concentration indirectly impact gm through gsc changes, highlighting some of the complex interactions between the two parameters. Evidence suggests that short-term responses of gm are not, or at least not fully, mechanistically linked to changes in gsc, cautioning against using gsc as a reliable proxy for gm. The overarching challenge lies in unravelling the mechanistic basis of short-term gm responses, which will contribute to the development of accurate models bridging laboratory insights with broader ecological implications. Addressing these gaps in understanding is crucial for refining predictions of gm behaviour under changing environmental conditions.

Carbon budget at the individual-tree scale: dominant Eucalyptus trees partition less carbon belowground

Large trees in plantations generally produce more wood per unit of resource use than small trees. Two processes may account for this pattern: greater photosynthetic resource use efficiency or greater partitioning of carbon to wood production. We estimated gross primary production (GPP) at the individual scale by combining transpiration with photosynthetic water-use efficiency of Eucalyptus trees. Aboveground production fluxes were estimated using allometric equations and modeled respiration; total belowground carbon fluxes (TBCF) were estimated by subtracting aboveground fluxes from GPP. Partitioning was estimated by dividing component fluxes by GPP. Dominant trees produced almost three times as much wood as suppressed trees. They used 25 ± 10% (mean ± SD) of their photosynthates for wood production, whereas suppressed trees only used 12 ± 2%. By contrast, dominant trees used 27 ± 19% of their photosynthate belowground, whereas suppressed trees used 58 ± 5%. Intermediate trees lay between these extremes. Photosynthetic water-use efficiency of dominant trees was c. 13% greater than the efficiency of suppressed trees. Suppressed trees used more than twice as much of their photosynthate belowground and less than half as much aboveground compared with dominant trees. Differences in carbon partitioning were much greater than differences in GPP or photosynthetic water-use efficiency.

δ13C of bulk organic matter and cellulose reveal post-photosynthetic fractionation during ontogeny in C4 grass leaves

The 13C isotope composition (δ13C) of leaf dry matter is a useful tool for physiological and ecological studies. However, how post-photosynthetic fractionation associated with respiration and carbon export influences δ13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ13C of mature leaves of Cleistogenes squarrosa, a perennial C4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With increasing leaf age class, the 12C/13C fractionation of leaf organic matter relative to the δ13C of atmosphere CO2 (ΔDM) increased while that of cellulose (Δcel) was almost constant. The divergence between ΔDM and Δcel increased with leaf age class, with a maximum value of 1.6‰, indicating the accumulation of post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent 12C/13C fractionation associated with carbon export of -0.5‰ to -1.0‰. Different ΔDM among leaves, pseudostems, daughter tillers, and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with ΔDM of old leaves, ΔDM of young leaves and Δcel are more reliable proxies for predicting physiological parameters due to the lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.

Temperature responses of leaf respiration in light and darkness are similar and modulated by leaf development

Our ability to predict temperature responses of leaf respiration in light and darkness (RL and RDk ) is essential to models of global carbon dynamics. While many models rely on constant thermal sensitivity (characterized by Q10 ), uncertainty remains as to whether Q10 of RL and RDk are actually similar. We measured short-term temperature responses of RL and RDk in immature and mature leaves of two evergreen tree species, Castanopsis carlesii and Ormosia henry in an open field. RL was estimated by the Kok method, the Yin method and a newly developed Kok-iterCc method. When estimated by the Yin and Kok-iterCc methods, RL and RDk had similar Q10 (c. 2.5). The Kok method overestimated both Q10 and the light inhibition of respiration. RL /RDk was not affected by leaf temperature. Acclimation of respiration in summer was associated with a decline in basal respiration but not in Q10 in both species, which was related to changes in leaf nitrogen content between seasons. Q10 of RL and RDk in mature leaves were 40% higher than in immature leaves. Our results suggest similar Q10 values can be used to model RL and RDk while leaf development-associated changes in Q10 require special consideration in future respiration models.

Woody species with higher hydraulic efficiency or lower photosynthetic capacity discriminate more against 13C at the global scale

Leaf carbon isotope composition (δ13C) provides an integrative record on the carbon and water balance of plants over long periods. Photosynthetic ability and hydraulic traits which are highly associated with stomatal behavior could affect leaf δ13C. Association between photosynthetic ability and leaf δ13C has been examined, however, how hydraulic traits influence leaf δ13C has not been fully understood. To fill this gap, we investigated the variations in leaf δ13C among 2591 woody species (547 shrub and 2044 tree species), and analyzed the link of leaf δ13C with leaf photosynthetic and xylem hydraulic traits. Our result showed that leaf δ13C was positively correlated to leaf photosynthetic ability and capacity. For hydraulic traits, leaf δ13C was negatively related to hydraulic conductivity (Ks), xylem pressure inducing 50 % loss of hydraulic conductivity (P50) and vessel diameter (Vdia). Associations of leaf δ13C with xylem hydraulic traits indicate woody species with stronger hydraulic safety discriminated less against 13C, while woody species with higher hydraulic efficiency had more negative leaf δ13C. Shrub species, which showed a lower Vdia and P50, had a significant less negative leaf δ13C than tree species. Furthermore, woody species inhabiting in dry regions discriminated less against 13C than those growing in humid regions. Moreover, leaf δ13C displayed a low phylogenetic signal based on Blomberg's K statistic. Overall, woody species with a higher leaf photosynthetic ability or stronger hydraulic safety system discriminated less against 13C and adopt the provident water use strategy.

Agrivoltaics mitigate drought effects in winter wheat

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

Stomatal, mesophyll and biochemical limitations to photosynthesis and their relationship with leaf structure over an elevation gradient in two conifers

Photosynthetic responses across complex elevational gradients provides insight into fundamental processes driving responses of plant growth and net primary production to environmental change. Gas exchange of needles and twig water potential were measured in two widespread coniferous tree species, Pinus contorta and Picea engelmannii, over an 800-m elevation gradient in southeastern Wyoming, USA. We hypothesized that limitations to photosynthesis imposed by mesophyll conductance (gm) would be greatest at the highest elevation sites due to higher leaf mass per area (LMA) and that estimations of maximum rate of carboxylation (Vcmax) without including gm would obscure elevational patterns of photosynthetic capacity. We found that gm decreased with elevation for P. contorta and remained constant for P. engelmannii, but in general, limitation to photosynthesis by gm was small. Indeed, estimations of Vcmax when including gm were equivalent to those estimated without including gm and no correlation was found between gm and LMA nor between gm and leaf N. Stomatal conductance (gs) and biochemical demand for CO2 were by far the most limiting processes to photosynthesis at all sites along the elevation gradient. Photosynthetic capacity (A) and gs were influenced strongly by differences in soil water availability across the elevation transect, while gm was less responsive to water availability. Based on our analysis, variation in gm plays only a minor role in driving patterns of photosynthesis in P. contorta and P. engelmannii across complex elevational gradients in dry, continental environments of the Rocky Mountains and accurate modeling of photosynthesis, growth and net primary production in these forests may not require detailed estimation of this trait value.

18 O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C3 grass-atmospheric drivers and physiological relations

The ¹⁸ O enrichment (Δ¹⁸ O) of leaf water affects the Δ¹⁸ O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ¹⁸ O of bulk leaf water (Δ¹⁸ O_LW ) and leaf sucrose (Δ¹⁸ O_Sucrose ). We grew Lolium perenne (a C₃ grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO₂ level (200, 400 or 800 μmol mol^-1 ) as factors, and determined Δ¹⁸ O_LW , Δ¹⁸ O_Sucrose and morphophysiological leaf parameters, including transpiration (E_leaf ), stomatal conductance (g_s ) and mesophyll conductance to CO₂ (g_m ). The Δ¹⁸ O of photosynthetic medium water (Δ¹⁸ O_SSW ) was estimated from Δ¹⁸ O_Sucrose and the equilibrium fractionation between water and carbonyl groups (ε_bio ). Δ¹⁸ O_SSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ¹⁸ O_e ) with adjustments that correlated with gas exchange parameters (g_s or total conductance to CO₂ ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ¹⁸ O_LW was a poor proxy for Δ¹⁸ O_Sucrose , mainly due to opposite Δ¹⁸ O responses of nonphotosynthetic tissue water (Δ¹⁸ O_non-SSW ) relative to Δ¹⁸ O_SSW , driven by atmospheric conditions.

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

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

Stable isotopes in tree rings record physiological trends in Larix gmelinii after fires

Fire is an important regulator of ecosystem dynamics in boreal forests, and especially has a complicated association with growth and physiological processes of fire-tolerant tree species. Stable isotope ratios in tree rings are used extensively in eco-physiological studies for evaluating the impact of past environmental (e.g., drought, air pollution) factors on tree growth and physiological processes. Yet, such studies based on carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings are rarely conducted on fire effect, especially not well explored for fire-tolerant trees. In this study, we investigated variations in basal area increment and isotopes of Larix gmelinii (Rupr.) Rupr. before and after three moderate fires (different fire years) at three sites across the Great Xing'an Mountains, Northeastern China. We found that the radial growth of L. gmelinii trees has significantly declined after the fires across study sites. Following the fires, a simultaneous increase in δ13C and δ18O has strengthened the link between the two isotopes. Further, fires have significantly enhanced the 13C-derived intrinsic water-use efficiency (iWUE) and largely altered the relationships between δ13C, δ18O, iWUE and climate (temperature and precipitation). A dual-isotope conceptual model revealed that an initial co-increase in δ13C and δ18O in the fire year can be mainly attributed to a reduction in stomatal conductance with a constant photosynthetic rate. However, this physiological response would shift to different patterns over post-fire time between sites, which might be partly related to spring temperature. This study is beneficial to better understand, in a physiological perspective, how fire-tolerant tree species adapt to a fire-prone environment. We also remind that the limitation of model assumptions and constraints may challenge model applicability and further inferred physiological response.

Estimating intraseasonal intrinsic water-use efficiency from high-resolution tree-ring δ13 C data in boreal Scots pine forests

Intrinsic water-use efficiency (iWUE), a key index for carbon and water balance, has been widely estimated from tree-ring δ¹³ C at annual resolution, but rarely at high-resolution intraseasonal scale. We estimated high-resolution iWUE from laser-ablation δ¹³ C analysis of tree-rings (iWUE_iso ) and compared it with iWUE derived from gas exchange (iWUE_gas ) and eddy covariance (iWUE_EC ) data for two Pinus sylvestris forests from 2002 to 2019. By carefully timing iWUE_iso via modeled tree-ring growth, iWUE_iso aligned well with iWUE_gas and iWUE_EC at intraseasonal scale. However, year-to-year patterns of iWUE_gas , iWUE_iso , and iWUE_EC were different, possibly due to distinct environmental drivers on iWUE across leaf, tree, and ecosystem scales. We quantified the modification of iWUE_iso by postphotosynthetic δ¹³ C enrichment from leaf sucrose to tree rings and by nonexplicit inclusion of mesophyll and photorespiration terms in photosynthetic discrimination model, which resulted in overestimation of iWUE_iso by up to 11% and 14%, respectively. We thus extended the application of tree-ring δ¹³ C for iWUE estimates to high-resolution intraseasonal scale. The comparison of iWUE_gas , iWUE_iso , and iWUE_EC provides important insights into physiological acclimation of trees across leaf, tree, and ecosystem scales under climate change and improves the upscaling of ecological models.

Effects of Geographical and Climatic Factors on the Intrinsic Water Use Efficiency of Tropical Plants: Evidence from Leaf 13C

Understanding the water use efficiency (WUE) and adaptation strategies of plants in high-temperature and rainy areas is essential under global climate change. The leaf carbon content (LCC) and intrinsic WUE of 424 plant samples (from 312 plant species) on Hainan Island were measured to examine their relationship with geographical and climatic factors in herbs, trees, vines and ferns. The LCC ranged from 306.30 to 559.20 mg g^-1, with an average of 418.85 mg g^-1, and decreased with increasing mean annual temperature (MAT). The range of intrinsic WUE was 8.61 to 123.39 μmol mol^-1 with an average value of 60.66 μmol mol^-1. The intrinsic WUE decreased with increasing altitude and relative humidity (RH) and wind speed (WS), but increased with increasing latitude, MAT and rainy season temperature (RST), indicating that geographical and climatic factors affect the intrinsic WUE. Stepwise regression suggested that in tropical regions with high temperature and humidity, the change in plant intrinsic WUE was mainly driven by WS. In addition, the main factors affecting the intrinsic WUE of different plant functional types of plants are unique, implying that plants of different plant functional types have distinctive adaptive strategies to environmental change. The present study may provide an insight in water management in tropical rainforest.

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

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

Impacts of Dutch elm disease-causing fungi on foliage photosynthetic characteristics and volatiles in Ulmus species with different pathogen resistance

Global warming affects the abiotic and biotic growth environment of plants, including the spread of fungal diseases such as Dutch elm disease (DED). Dutch elm disease-resistance of different Ulmus species varies, but how this is reflected in leaf-level physiological pathogen responses has not been investigated. We studied the impacts of mechanical injury alone and mechanical injury plus inoculation with the DED-causing pathogens Ophiostoma novo-ulmi subsp. novo-ulmi and O. novo-ulmi subsp. americana on Ulmus glabra, a more vulnerable species, and U. laevis, a more resistant species. Plant stress responses were evaluated for 12 days after stress application by monitoring leaf net CO2 assimilation rate (A), stomatal conductance (gs), ratio of ambient to intercellular CO2 concentration (Ca/Ci) and intrinsic water-use efficiency (A/gs), and by measuring biogenic volatile (VOC) release by plant leaves. In U. glabra and U. laevis, A was not affected by time, stressors or their interaction. Only in U. glabra, gs and Ca/Ci decreased in time, yet recovered by the end of the experiment. Although the emission compositions were affected in both species, the stress treatments enhanced VOC emission rates only in U. laevis. In this species, mechanical injury especially when combined with the pathogens increased the emission of lipoxygenase pathway volatiles and dimethylallyl diphosphate and geranyl diphosphate pathway volatiles. In conclusion, the more resistant species U. laevis had a more stable photosynthesis, but stronger pathogen-elicited volatile response, especially after inoculation by O. novo-ulmi subsp. novo-ulmi. Thus, stronger activation of defenses might underlay higher DED-resistance in this species.

Neglecting acclimation of photosynthesis under drought can cause significant errors in predicting leaf photosynthesis in wheat

Extreme climatic events, such as heat waves, cold snaps and drought spells, related to global climate change, have become more frequent and intense in recent years. Acclimation of plant physiological processes to changes in environmental conditions is a key component of plant adaptation to climate change. We assessed the temperature response of leaf photosynthetic parameters in wheat grown under contrasting water regimes and growth temperatures (T_growth ). Two independent experiments were conducted under controlled conditions. In Experiment 1, two wheat genotypes were subjected to well-watered or drought-stressed treatments; in Experiment 2, the two water regimes combined with high, medium and low T_growth were imposed on one genotype. Parameters of a biochemical C₃ -photosynthesis model were estimated at six leaf temperatures for each factor combination. Photosynthesis acclimated more to drought than to T_growth . Drought affected photosynthesis by lowering its optimum temperature (T_opt ) and the values at T_opt of light-saturated net photosynthesis, stomatal conductance, mesophyll conductance, the maximum rate of electron transport (J_max ) and the maximum rate of carboxylation by Rubisco (V_cmax ). T_opt for V_cmax was up to 40°C under well-watered conditions but 24-34°C under drought. The decrease in photosynthesis under drought varied among T_growth but was similar between genotypes. The temperature response of photosynthetic quantum yield under drought was partly attributed to photorespiration but more to alternative electron transport. All these changes in biochemical parameters could not be fully explained by the changed leaf nitrogen content. Further model analysis showed that both diffusional and biochemical parameters of photosynthesis and their thermal sensitivity acclimate little to T_growth , but acclimate considerably to drought and the combination of drought and T_growth . The commonly used modelling approaches, which typically consider the response of diffusional parameters, but ignore acclimation responses of biochemical parameters to drought and T_growth , strongly overestimate leaf photosynthesis under variable temperature and drought.

Estimating intra-seasonal photosynthetic discrimination and water use efficiency using δ13C of leaf sucrose in Scots pine

Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (δ13C) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow δ13C signals within trees. To study how the choice of organic material may bias the interpretation of δ13C records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time δ13C of different leaf carbon pools with δ13C of assimilates estimated by a chamber-Picarro system (δ13CA_Picarro), and a photosynthetic discrimination model (δ13CA_model). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of δ13CA_Picarro and δ13CA_model. Consequently, in comparison with the other carbon pools, sucrose δ13C was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from δ13C of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose δ13C to understand plant physiological responses in depth.

Relationships between stable isotope natural abundances (δ13C and δ15N) and water use efficiency in rice under alternate wetting and drying irrigation in soils with high clay contents

Natural abundance of the stable isotope (δ¹³C and δ¹⁵N) in plants is widely used to indicate water use efficiency (WUE). However, soil water and texture properties may affect this relationship, which remains largely elusive. Therefore, the purpose of this study was to evaluate δ¹³C as affected by different combinations of alternate wetting and drying irrigation (AWD) with varied soil clay contents in different organs and whole plant and assess the feasibility of using δ¹³C and δ¹⁵N as a physiological indicator of whole-plant water use efficiency (WUE_whole-plant). Three AWD regimes, I₁₀₀ (30 mm flooded when soil reached 100% saturation), I₉₀ (30 mm flooded when reached 90% saturation) and I₇₀ (30 mm flooded when reached 70% saturation) and three soil clay contents, 40% (S₄₀), 50% (S₅₀), and 60% (S₆₀), were included. Observed variations in WUE_whole-plant did not conform to theoretical expectations of the organs δ¹³C (δ¹³C_organs) of plant biomass based on pooled data from all treatments. However, a positive relationship between δ¹³C_leaf and WUE_ET (dry biomass/evapotranspiration) was observed under I₉₀ regime, whereas there were no significant relationships between δ¹³C_organs and WUE_ET under I₁₀₀ or I₇₀ regimes. Under I₁₀₀, weak relationships between δ¹³C_organs and WUE_ET could be explained by (i) variation in C allocation patterns under different clay content, and (ii) relatively higher rate of panicle water loss, which was independent of stomatal regulation and photosynthesis. Under I₇₀, weak relationships between δ¹³C_organs and WUE_ET could be ascribed to (i) bigger cracks induced by water-limited irrigation regime and high clay content soil, and (ii) damage caused by severe drought. In addition, a negative relationship was observed between WUE_whole-plant and shoot δ¹⁵N (δ¹⁵N_shoot) across the three irrigation treatments, indicating that WUE_whole-plant is tightly associated with N metabolism and N isotope discrimination in rice. Therefore, δ¹³C should be used cautiously as an indicator of rice WUE_whole-plant at different AWD regimes with high clay content, whereas δ¹⁵N could be considered an effective indicator of WUE_whole-plant.

Responses of tree growth, leaf area and physiology to pavement in Ginkgo biloba and Platanus orientalis

Trees growing on paved lands endure many environmental stresses in the urban environment. However, the morphological and physiological mechanisms underlying tree adaptation to pavement in the field are less known. In this study, we investigated 40 sites where Ginkgo biloba and Platanus orientalis grow on adjacent pairs of paved and vegetated plots in parks and roadsides in Beijing, China. Relative to the vegetated land, the mean increments in the diameter at breast height and height in the paved land were significantly decreased by 44.5% and 31.9% for G. biloba and 31.7% and 60.1% for P. orientalis, respectively. These decreases are related to both the decrease in assimilation products due to the reductions in leaf area, leaf total nitrogen content, and chlorophyll content and the increase in energy cost due to the synthesis of more soluble sugar and proline for mitigating stress. The increase in leaf soluble sugar content, proline content, and δ¹³C indicated that trees could adapt to the paved land through the regulation of osmotic balance and the enhancement of water-use efficiency. Piecewise structural equation models showed that trees growing on the paved land are stressed by compounding impacts of the leaf morphological and physiological changes. Therefore, it is critical to explore the complex response of plant morphological and physiological traits to the pavement-induced stress for improving tree health in urban greening.

Scaling from fluxes to organic matter: interpreting 13 C isotope ratios of plant material using flux models

This article is a Commentary on Leppä et al. (2022), 236: 2044–2060.

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

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

Overestimated gains in water-use efficiency by global forests

Increases in terrestrial water-use efficiency (WUE) have been reported in many studies, pointing to potential changes in physiological forcing of global carbon and hydrological cycles. However, gains in WUE are of uncertain magnitude over longer (i.e. >10 years) periods of time largely owing to difficulties in accounting for structural and physiological acclimation. ¹³ C signatures (i.e. δ¹³ C) of plant organic matter have long been used to estimate WUE at temporal scales ranging from days to centuries. Mesophyll conductance is a key uncertainty in estimated WUE owing to its influence on diffusion of CO₂ to sites of carboxylation. Here we apply new knowledge of mesophyll conductance to 464 δ¹³ C chronologies in tree-rings of 143 species spanning global biomes. Adjusted for mesophyll conductance, gains in WUE during the 20th century (0.15 ppm year^-1 ) were considerably smaller than those estimated from conventional modelling (0.26 ppm year^-1 ). Across the globe, mean sensitivity of WUE to atmospheric CO₂ was 0.15 ppm ppm^-1 . Ratios of internal-to-atmospheric CO₂ (on a mole fraction basis; c_i /c_a ) in leaves were mostly constant over time but differed among biomes and plant taxa-highlighting the significance of both plant structure and physiology. Together with synchronized responses in stomatal and mesophyll conductance, our results suggest that ratios of chloroplastic-to-atmospheric CO₂ (c_c /c_a ) are constrained over time. We conclude that forest WUE may have not increased as much as previously suggested and that projections of future climate forcing via CO₂ fertilization may need to be adjusted accordingly.

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.

Drought exerts a greater influence than growth temperature on the temperature response of leaf day respiration in wheat (Triticum aestivum)

We assessed how the temperature response of leaf day respiration (R_d ) in wheat responded to contrasting water regimes and growth temperatures. In Experiment 1, well-watered and drought-stressed conditions were imposed on two genotypes; in Experiment 2, the two water regimes combined with high (HT), medium (MT) and low (LT) growth temperatures were imposed on one of the genotypes. R_d was estimated from simultaneous gas exchange and chlorophyll fluorescence measurements at six leaf temperatures (T_leaf ) for each treatment, using the Yin method for nonphotorespiratory conditions and the nonrectangular hyperbolic fitting method for photorespiratory conditions. The two genotypes responded similarly to growth and measurement conditions. Estimates of R_d for nonphotorespiratory conditions were generally higher than those for photorespiratory conditions, but their responses to T_leaf were similar. Under well-watered conditions, R_d and its sensitivity to T_leaf slightly acclimated to LT, but did not acclimate to HT. Temperature sensitivities of R_d were considerably suppressed by drought, and the suppression varied among growth temperatures. Thus, it is necessary to quantify interactions between drought and growth temperature for reliably modelling R_d under climate change. Our study also demonstrated that the Kok method, one of the currently popular methods for estimating R_d , underestimated R_d significantly.

Grain carbon isotope composition is a marker for allocation and harvest index in wheat

The natural ¹³ C abundance (δ¹³ C) in plant leaves has been used for decades with great success in agronomy to monitor water-use efficiency and select modern cultivars adapted to dry conditions. However, in wheat, it is also important to find genotypes with high carbon allocation to spikes and grains, and thus with a high harvest index (HI) and/or low carbon losses via respiration. Finding isotope-based markers of carbon partitioning to grains would be extremely useful since isotope analyses are inexpensive and can be performed routinely at high throughput. Here, we took the advantage of a set of field trials made of more than 600 plots with several wheat cultivars and measured agronomic parameters as well as δ¹³ C values in leaves and grains. We find a linear relationship between the apparent isotope discrimination between leaves and grain (denoted as Δδ_corr ), and the respiration use efficiency-to-HI ratio. It means that overall, efficient carbon allocation to grains is associated with a small isotopic difference between leaves and grains. This effect is explained by postphotosynthetic isotope fractionations, and we show that this can be modelled by equations describing the carbon isotope composition in grains along the wheat growth cycle. Our results show that ¹³ C natural abundance in grains could be useful to find genotypes with better carbon allocation properties and assist current wheat breeding technologies.

Leafamine®, a Free Amino Acid-Rich Biostimulant, Promotes Growth Performance of Deficit-Irrigated Lettuce

Water deficit causes substantial yield losses that climate change is going to make even more problematic. Sustainable agricultural practices are increasingly developed to improve plant tolerance to abiotic stresses. One innovative solution amongst others is the integration of plant biostimulants in agriculture. In this work, we investigate for the first time the effects of the biostimulant -Leafamine®-a protein hydrolysate on greenhouse lettuce (Lactuca sativa L.) grown under well-watered and water-deficit conditions. We examined the physiological and metabolomic water deficit responses of lettuce treated with Leafamine® (0.585 g/pot) or not. Root application of Leafamine® increased the shoot fresh biomass of both well-watered (+40%) and deficit-irrigated (+20%) lettuce plants because the projected leaf area increased. Our results also indicate that Leafamine® application could adjust the nitrogen metabolism by enhancing the total nitrogen content, amino acid (proline) contents and the total protein level in lettuce leaves, irrespective of the water condition. Osmolytes such as soluble sugars and polyols, also increased in Leafamine®-treated lettuce. Our findings suggest that the protective effect of Leafamine is a widespread change in plant metabolism and could involve ABA, putrescine and raffinose.

Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (A_net ) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (g_m ) and its effect on the estimation of carboxylation capacity ( VCmax ). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over A_net . The seasonality of A_net was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited A_net in the spring, whereas daily temperatures and the gradual reduction of light availability limited A_net in the autumn, despite relatively high VCmax . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between g_m and A_net , but not between g_m and stomatal conductance.

Endophytic fungi and drought tolerance: ecophysiological adjustment in shoot and root of an annual mesophytic host grass

Epichloid endophytic fungi, vertically transmitted symbionts of grasses, can increase plant tolerance to biotic and abiotic stress. Our aim was to identify ecophysiological mechanisms by which the endophyte Epichloë occultans confers drought tolerance to the annual grass Lolium multiflorum Lam. Endophyte-associated or endophyte-free plants were either well-watered or subjected to water deficit. We evaluated plant biomass, root length and nitrogen concentration, and we assessed intrinsic water use efficiency (iWUE) and its components net photosynthesis and stomatal conductance, by carbon and oxygen isotope analysis of shoot tissues. Endophyte-free plants produced more biomass than endophyte-associated ones at field capacity, while water deficit strongly reduced endophyte-free plants biomass. As a result, both types of plants produced similar biomass under water restriction. Based on oxygen isotope composition of plant cellulose, stomatal conductance decreased with water deficit in both endophyte-associated and endophyte-free plants. Meanwhile, carbon isotope composition indicated that iWUE increased with water deficit only in endophyte-associated plants. Thus, the isotope data indicated that net photosynthesis decreased more strongly in endophyte-free plants under water deficit. Additionally, endophyte presence reduced root length but increased its hydraulic conductivity. In conclusion, endophytic fungi confer drought tolerance to the host grass by adjusting shoot and root physiology.

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

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

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

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

CO2, nitrogen deposition and a discontinuous climate response drive water use efficiency in global forests

Reduced stomatal conductance is a common plant response to rising atmospheric CO₂ and increases water use efficiency (W). At the leaf-scale, W depends on water and nitrogen availability in addition to atmospheric CO₂. In hydroclimate models W is a key driver of rainfall, droughts, and streamflow extremes. We used global climate data to derive Aridity Indices (AI) for forests over the period 1965-2015 and synthesised those with data for nitrogen deposition and W derived from stable isotopes in tree rings. AI and atmospheric CO₂ account for most of the variance in W of trees across the globe, while cumulative nitrogen deposition has a significant effect only in regions without strong legacies of atmospheric pollution. The relation of aridity and W displays a clear discontinuity. W and AI are strongly related below a threshold value of AI ≈ 1 but are not related where AI > 1. Tree ring data emphasise that effective demarcation of water-limited from non-water-limited behaviour of stomata is critical to improving hydrological models that operate at regional to global scales.

A meta-analysis of mesophyll conductance to CO2 in relation to major abiotic stresses in poplar species

Mesophyll conductance (gm) determines the diffusion of CO2 from the substomatal cavities to the site of carboxylation in the chloroplasts and represents a critical component of the diffusive limitation of photosynthesis. In this study, we evaluated the average effect sizes of different environmental constraints on gm in Populus spp., a forest tree model. We collected raw data of 815 A-Ci response curves from 26 datasets to estimate gm, using a single curve-fitting method to alleviate method-related bias. We performed a meta-analysis to assess the effects of different abiotic stresses on gm. We found a significant increase in gm from the bottom to the top of the canopy that was concomitant with the increase of maximum rate of carboxylation and light-saturated photosynthetic rate (Amax). gm was positively associated with increases in soil moisture and nutrient availability, but was insensitive to increasing soil copper concentration and did not vary with atmospheric CO2 concentration. Our results showed that gm was strongly related to Amax and to a lesser extent to stomatal conductance (gs). Moreover, a negative exponential relationship was obtained between gm and specific leaf area, which may be used to scale-up gm within the canopy.

Stomatal conductance limited the CO2 response of grassland in the last century

BACKGROUND

The anthropogenic increase of atmospheric CO2 concentration (ca) is impacting carbon (C), water, and nitrogen (N) cycles in grassland and other terrestrial biomes. Plant canopy stomatal conductance is a key player in these coupled cycles: it is a physiological control of vegetation water use efficiency (the ratio of C gain by photosynthesis to water loss by transpiration), and it responds to photosynthetic activity, which is influenced by vegetation N status. It is unknown if the ca-increase and climate change over the last century have already affected canopy stomatal conductance and its links with C and N processes in grassland.

RESULTS

Here, we assessed two independent proxies of (growing season-integrating canopy-scale) stomatal conductance changes over the last century: trends of δ18O in cellulose (δ18Ocellulose) in archived herbage from a wide range of grassland communities on the Park Grass Experiment at Rothamsted (U.K.) and changes of the ratio of yields to the CO2 concentration gradient between the atmosphere and the leaf internal gas space (ca - ci). The two proxies correlated closely (R2 = 0.70), in agreement with the hypothesis. In addition, the sensitivity of δ18Ocellulose changes to estimated stomatal conductance changes agreed broadly with published sensitivities across a range of contemporary field and controlled environment studies, further supporting the utility of δ18Ocellulose changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ18Ocellulose differed strongly between plots and indicated much greater reductions of stomatal conductance in grass-rich than dicot-rich communities. Reductions of stomatal conductance were connected with reductions of yield trends, nitrogen acquisition, and nitrogen nutrition index. Although all plots were nitrogen-limited or phosphorus- and nitrogen-co-limited to different degrees, long-term reductions of stomatal conductance were largely independent of fertilizer regimes and soil pH, except for nitrogen fertilizer supply which promoted the abundance of grasses.

CONCLUSIONS

Our data indicate that some types of temperate grassland may have attained saturation of C sink activity more than one century ago. Increasing N fertilizer supply may not be an effective climate change mitigation strategy in many grasslands, as it promotes the expansion of grasses at the disadvantage of the more CO2 responsive forbs and N-fixing legumes.