What is it about?
Photosynthesis is affected by high temperature and stronger evaporative demand, that is, greater air dryness. These two environmental factors often occur together, making it difficult to understand their separate effects on photosynthesis. In this study, we separated the effects of heat and air dryness across different CO2 levels in three C3 crop species: cotton, sunflower, and dwarf bean. Using simultaneous gas-exchange and chlorophyll fluorescence measurements, we explored how stomatal, mesophyll, and biochemical processes contribute to photosynthetic responses. We found that mesophyll conductance responds to heat and air dryness in directions that differ from those of stomatal conductance. This coordination helps buffer the effects of heat and dryness on CO2 diffusion and maintain a relatively conservative CO2 environment within chloroplasts, where photosynthesis occurs. We also found that, in the species and environmental conditions studied, photosynthesis is co-limited by Rubisco carboxylation capacity and RuBP regeneration capacity near current atmospheric CO2 levels. This suggests that both capacities are used efficiently, without either being in surplus.
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Why is it important?
This study reveals the coordinated stomatal, mesophyll, and biochemical functions in photosynthetic responses to heat and air dryness. By separating the effects of heat and air dryness, we identified conservatism in the chloroplastic CO2 environment and in photosynthetic biochemical co-limitation. These findings help deepen our understanding of plant strategies for coping with climate change and using resources efficiently. They are also helpful for improving predictions of plant photosynthesis and water use under climate change and provide insights for practical applications such as agricultural management.
Perspectives
When we first started this study, many people felt that photosynthetic responses to temperature and air dryness had already been extensively studied. However, most previous findings reflected the mixed effects of these two factors, rather than their separate effects. In addition, a common view is that high evaporative demand reduces stomatal conductance to minimize water loss, thereby limiting CO2 diffusion into the leaf and suppressing photosynthesis. As this study progressed, we gradually came to appreciate the important but long-overlooked role of mesophyll conductance in buffering the effects of high temperature and air dryness on the CO2 environment inside chloroplasts. To me, one of the most fascinating aspects of scientific research is that new evidence can take something we think we already understand and move it a little further forward.
Xingyu Hu
Read the Original
This page is a summary of: Coordinated stomatal, mesophyll, and biochemical functions in photosynthetic responses to heat and dryness, Proceedings of the National Academy of Sciences, May 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2605032123.
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