Photosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell Plants are often subjected to water deficits and high soil salinity during their life cycle. Understanding how plants respond to drought, salt, and combined stresses is crucial for improving crop performance under these conditions and protecting natural vegetation. Photosynthesis, along with cell growth, is one of the primary processes affected by water or salt stress. The effects of drought and salt stress on photosynthesis can be direct (e.g., diffusion limitations through stomata and mesophyll, alterations in photosynthetic metabolism) or secondary (e.g., oxidative stress from multiple stresses). The carbon balance of a plant during stress and recovery depends on the speed and extent of photosynthetic recovery, as well as the degree and speed of photosynthesis decline during water depletion. Current knowledge about physiological limitations to photosynthetic recovery after different intensities of water and salt stress is limited. Transcript-profiling studies show that plants quickly alter gene expression in response to stress, even under mild conditions. Salt stress affects more genes than drought, possibly due to combined effects of dehydration and osmotic stress. Drought and salinity share a physiological water deficit, but prolonged salt stress also involves hyper-ionic and hyper-osmotic stress. Both stresses can induce osmotic adjustment, which helps maintain water uptake and cell turgor. The effects of drought and salinity on photosynthesis range from CO₂ diffusion limitations to alterations in leaf photochemistry and carbon metabolism. These effects vary with stress intensity, duration, and leaf age. Salt stress also involves specific effects, such as ion toxicity. Some plants prevent salt entry or compartmentalize salt in vacuoles to avoid toxic effects. Under salt stress, primary metabolites increase, playing a role in osmotic adjustment and protection against reactive oxygen species. CO₂ diffusion through stomata and mesophyll is affected by stress, leading to reduced photosynthetic efficiency. Stomata close in response to leaf turgor decline, high vapour pressure deficit, or root signals. Reduced stomatal conductance can have protective effects under mild stress. Mesophyll conductance to CO₂ is also affected, with changes linked to structural and biochemical alterations. These changes are regulated biochemically, not just by anatomical traits. Carbonic anhydrase and aquaporins are involved in regulating CO₂ diffusion. Biochemical and photochemical limitations to photosynthesis include enzyme deactivation, reduced ATP production, and metabolic limitations due to high Na⁺ and Cl⁻ concentrations. Under salt stress, metabolic limitations occur, and photorespiration may help protect the photosynthetic apparatus. Photoprotective mechanisms, such as thermal dissipation in light-harvesting complexes, help reduce light damage. Anthocyanins in senescing leaves protect against photo-oxidative damage. Signalling pathways, including ABA, ROS, and nitric oxide, play a role in plant responses toPhotosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell Plants are often subjected to water deficits and high soil salinity during their life cycle. Understanding how plants respond to drought, salt, and combined stresses is crucial for improving crop performance under these conditions and protecting natural vegetation. Photosynthesis, along with cell growth, is one of the primary processes affected by water or salt stress. The effects of drought and salt stress on photosynthesis can be direct (e.g., diffusion limitations through stomata and mesophyll, alterations in photosynthetic metabolism) or secondary (e.g., oxidative stress from multiple stresses). The carbon balance of a plant during stress and recovery depends on the speed and extent of photosynthetic recovery, as well as the degree and speed of photosynthesis decline during water depletion. Current knowledge about physiological limitations to photosynthetic recovery after different intensities of water and salt stress is limited. Transcript-profiling studies show that plants quickly alter gene expression in response to stress, even under mild conditions. Salt stress affects more genes than drought, possibly due to combined effects of dehydration and osmotic stress. Drought and salinity share a physiological water deficit, but prolonged salt stress also involves hyper-ionic and hyper-osmotic stress. Both stresses can induce osmotic adjustment, which helps maintain water uptake and cell turgor. The effects of drought and salinity on photosynthesis range from CO₂ diffusion limitations to alterations in leaf photochemistry and carbon metabolism. These effects vary with stress intensity, duration, and leaf age. Salt stress also involves specific effects, such as ion toxicity. Some plants prevent salt entry or compartmentalize salt in vacuoles to avoid toxic effects. Under salt stress, primary metabolites increase, playing a role in osmotic adjustment and protection against reactive oxygen species. CO₂ diffusion through stomata and mesophyll is affected by stress, leading to reduced photosynthetic efficiency. Stomata close in response to leaf turgor decline, high vapour pressure deficit, or root signals. Reduced stomatal conductance can have protective effects under mild stress. Mesophyll conductance to CO₂ is also affected, with changes linked to structural and biochemical alterations. These changes are regulated biochemically, not just by anatomical traits. Carbonic anhydrase and aquaporins are involved in regulating CO₂ diffusion. Biochemical and photochemical limitations to photosynthesis include enzyme deactivation, reduced ATP production, and metabolic limitations due to high Na⁺ and Cl⁻ concentrations. Under salt stress, metabolic limitations occur, and photorespiration may help protect the photosynthetic apparatus. Photoprotective mechanisms, such as thermal dissipation in light-harvesting complexes, help reduce light damage. Anthocyanins in senescing leaves protect against photo-oxidative damage. Signalling pathways, including ABA, ROS, and nitric oxide, play a role in plant responses to