Stomatal, mesophyll, and biochemical limitation to photosynthesis of soybeans under waterlogging and reoxygenation

While waterlogging stress slows photosynthetic rate ( A _sat ), the underlying processes remain poorly understood. Here, we aimed to characterize the limitations to photosynthesis imposed by stomatal conductance ( g _s ), mesophyll conductance ( g _m ), and biochemical processes under waterlogging and subsequent reoxygenation. Two soybean cultivars ( Glycine max L. cv. Fukuyutaka and Iyodaizu) were subjected to 6 days of waterlogging, after which excess water was drained. The responses of A _sat , g _s , g _m , and the maximum carboxylation rate ( V _cmax ) were investigated. In both cultivars, A declined significantly within 4 days of waterlogging and did not recover completely by two weeks of reoxygenation. During waterlogging, CO ₂ concentration at carboxylation site decreased in parallel with g _s and g _m , indicating that photosynthesis was mainly limited by diffusional factors (combination of g _s and g _m ). After drainage, diffusional limitation persisted during early reoxygenation, whereas biochemical limitation due to reduced V _cmax became dominant after 7 days of reoxygenation. Therefore, maintaining high diffusional conductances and V _cmax during waterlogging and reoxygenation, respectively, is important for enhancing photosynthetic tolerance to waterlogging stress. Overall, our results demonstrate that A _sat under waterlogging and reoxygenation is dynamically constrained by multiple factors, emphasizing the need for comprehensive assessment of gas diffusion and carbon assimilation processes.