Photosynthesis and drought: can we make metabolic connections from available data? C. Pinheiro and M. M. Chaves Abstract: Photosynthesis is a key process affected by water deficits, primarily through reduced CO₂ diffusion to chloroplasts and metabolic constraints. The impact of these limitations varies with stress intensity, the presence of superimposed stresses, and species. Total plant carbon uptake is further reduced due to growth inhibition. Leaf carbohydrate status, directly or indirectly altered by water deficits, acts as a metabolic signal. Other signals include abscisic acid (ABA), which affects stomatal aperture and gene regulation; other hormones that act concurrently or antagonistically with ABA; and redox control of energy balance in photosynthetic cells deprived of CO₂. A meta-analysis of >450 papers over 15 years shows the interplay of sugars, reactive oxygen species (ROS), and hormones with photosynthetic responses to drought. However, results are fragmented and non-comparable, making it hard to relate molecular events to plant physiological status. Genes ABI1 and ABI3 show similar responses to water shortage in Arabidopsis and barley, regardless of stress type or intensity. ABI1 is up-regulated, while ABI3 is usually down-regulated. ABI3 is hypothesized to be essential for drought recovery. Introduction: Plants respond rapidly to water stress via physiological, cellular, and molecular events. Responses are modulated by stress intensity, duration, and rate. Methodological challenges include defining and monitoring drought in experiments. Artificial stress imposition systems may not reflect field conditions. Molecular and metabolic responses to combined stresses are unique and cannot be extrapolated from individual stress studies. Drought is defined by its effects on yield. Photosynthesis is central to plant performance under drought. Leaf net carbon uptake decreases, leading to changes in root-to-shoot ratio and carbon and nitrogen metabolism. Sugars are key in integrating cellular responses. They act as substrates and modulators of enzyme activity and gene expression. Sugars interact with redox and hormone signals. Starch is a major integrator of plant metabolism and growth. Revisiting drought constraints to photosynthesis: Decreased CO₂ diffusion is the main cause of reduced photosynthesis under mild to moderate water limitation. Mesophyll conductance (g_m) is affected by water stress. Stomata act as pressure regulators, preventing xylem cavitation. Stomatal closure under drought is mediated by ABA. Under field conditions, the timing and duration of stomatal closure indicate stress levels. When combined with high irradiance, leaves may experience excess incident energy, leading to photoinhibition. Photoprotective mechanisms, such as the xanthophyll cycle and lutein cycle, help protect photosynthesis. These mechanisms compete with photochemistry for energy, leading to downregulation of photosynthesis. If CO₂ assimilation is limited, other sinks for absorbed energy, such as photorespiration or thePhotosynthesis and drought: can we make metabolic connections from available data? C. Pinheiro and M. M. Chaves Abstract: Photosynthesis is a key process affected by water deficits, primarily through reduced CO₂ diffusion to chloroplasts and metabolic constraints. The impact of these limitations varies with stress intensity, the presence of superimposed stresses, and species. Total plant carbon uptake is further reduced due to growth inhibition. Leaf carbohydrate status, directly or indirectly altered by water deficits, acts as a metabolic signal. Other signals include abscisic acid (ABA), which affects stomatal aperture and gene regulation; other hormones that act concurrently or antagonistically with ABA; and redox control of energy balance in photosynthetic cells deprived of CO₂. A meta-analysis of >450 papers over 15 years shows the interplay of sugars, reactive oxygen species (ROS), and hormones with photosynthetic responses to drought. However, results are fragmented and non-comparable, making it hard to relate molecular events to plant physiological status. Genes ABI1 and ABI3 show similar responses to water shortage in Arabidopsis and barley, regardless of stress type or intensity. ABI1 is up-regulated, while ABI3 is usually down-regulated. ABI3 is hypothesized to be essential for drought recovery. Introduction: Plants respond rapidly to water stress via physiological, cellular, and molecular events. Responses are modulated by stress intensity, duration, and rate. Methodological challenges include defining and monitoring drought in experiments. Artificial stress imposition systems may not reflect field conditions. Molecular and metabolic responses to combined stresses are unique and cannot be extrapolated from individual stress studies. Drought is defined by its effects on yield. Photosynthesis is central to plant performance under drought. Leaf net carbon uptake decreases, leading to changes in root-to-shoot ratio and carbon and nitrogen metabolism. Sugars are key in integrating cellular responses. They act as substrates and modulators of enzyme activity and gene expression. Sugars interact with redox and hormone signals. Starch is a major integrator of plant metabolism and growth. Revisiting drought constraints to photosynthesis: Decreased CO₂ diffusion is the main cause of reduced photosynthesis under mild to moderate water limitation. Mesophyll conductance (g_m) is affected by water stress. Stomata act as pressure regulators, preventing xylem cavitation. Stomatal closure under drought is mediated by ABA. Under field conditions, the timing and duration of stomatal closure indicate stress levels. When combined with high irradiance, leaves may experience excess incident energy, leading to photoinhibition. Photoprotective mechanisms, such as the xanthophyll cycle and lutein cycle, help protect photosynthesis. These mechanisms compete with photochemistry for energy, leading to downregulation of photosynthesis. If CO₂ assimilation is limited, other sinks for absorbed energy, such as photorespiration or the