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

Unsaturation of Leaf Air Spaces Sheds New Light on the Role of Aquaporins

Aquaporins have long been known to facilitate water transport across membranes inside leaves. However, their role in regulating the water potential (ψ) difference between the cytosol and cell wall has been questioned, as the ψ of the cytosol and cell wall would be in equilibrium under the assumption of water‐vapour‐saturated leaf intercellular air spaces. Recent advances suggest that intercellular air spaces are unsaturated at high vapour pressure deficit (VPD) and that aquaporins that up‐regulate water transport might simultaneously down‐regulate CO2 transport in a competitive manner. Therefore, the currently assumed mechanisms of CO2 and water transport in the mesophyll under varying VPD must be re‐examined. We incorporated the competitive aquaporin hypothesis into the leaf gas exchange pathways with unsaturated intercellular air spaces. We show that the putative competitive control of CO2‐ and water‐facilitating aquaporins is fully effective only when there is a large ψ gradient between the cytosol and cell wall at high VPD. In this context, the down‐regulation of water‐facilitating aquaporins and the up‐regulation of CO2‐facilitating aquaporins could protect the cytosol from drying and maintain the CO2 supply for photosynthesis, respectively. While it remains unclear whether unsaturation drives aquaporin activity or vice versa, we identify research challenges that need to be addressed.

New avenues in photosynthesis: from light harvesting to global modeling

Photosynthesis underpins life on Earth, serving as the primary energy source while regulating global carbon and water cycles, thereby shaping climate and vegetation. Advancing photosynthesis research is essential for improving crop productivity and refining photosynthesis models across scales, ultimately addressing critical global challenges such as food security and environmental sustainability. This minireview synthesizes a selection of recent advancements presented at the 2nd European Congress of Photosynthesis Research, focusing on improving photosynthesis efficiency and modelling across the scales. We explore strategies to optimize light harvesting and carbon fixation, leading to canopy level improvements. Alongside synthetic biology, we examine recent advances in harnessing natural variability in key photosynthetic traits, considering both methodological innovations and the vast reservoir of opportunities they present. Additionally, we highlight unique insights gained from plants adapted to extreme environments, offering pathways to improve photosynthetic efficiency and resilience simultaneously. We emphasize the importance of a holistic approach, integrating dynamic modeling of metabolic processes to bridge these advancements. Beyond photosynthesis improvements, we discuss the progress of improving photosynthesis simulations, particularly through improved parametrization of mesophyll conductance, crucial for enhancing leaf-to-global scale simulations. Recognizing the need for greater interdisciplinary collaboration to tackle the grand challenges put on photosynthesis research, we highlight two initiatives launched at the congress-an open science platform and a dedicated journal for plant ecophysiology. We conclude this minireview with a forward-looking outline, highlighting key next steps toward achieving meaningful improvements in photosynthesis, yield, resilience and modeling.

Reductions in mesophyll conductance under drought stress are influenced by increases in cell wall chelator-soluble pectin content and denser microfibril alignment in cotton

Plants commonly undergo leaf morphoanatomy and composition modifications to cope with drought stress, and these tend to reduce mesophyll conductance to CO2 diffusion (gm), a key limitation to photosynthesis. The cell wall appears to play a crucial role in this reduction, yet the specific effect of cell wall component on gm and the underlying regulatory mechanisms of cell wall thickness (Tcw) variation are not well understood. In this study, we subjected cotton plants to varying levels of water deficit to investigate the impact of leaf cell wall component and the arrangement patterns of microfibrils within cell walls on Tcw and leaf gas exchange. Drought stress resulted in a significant thickening of cell walls and a decrease in gm. Concurrently, drought stress increased the content of chelator-soluble pectin and cellulose while reducing hemicellulose content. The alignment of cellulose microfibrils became more parallel and their diameter increased under drought conditions, suggesting a decrease in cell wall effective porosity which coincides with the observed reduction in gm. This research demonstrates that reduced gm typically observed under drought stress is related not only to thickened cell walls, but also to ultra-anatomical and compositional variations. Specifically, increases in cellulose content, diameter, and a highly aligned arrangement of cellulose microfibrils collectively contributed to an increase in Tcw, which, together with increases in chelator-soluble pectin content, resulted in an increased cell wall resistance to CO2 diffusion.

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.

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.

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.