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

While waterlogging stress slows photosynthetic rate (Asat), the underlying processes remain poorly understood. Here, we aimed to characterize the limitations to photosynthesis imposed by stomatal conductance (gs), mesophyll conductance (gm), 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 Asat, gs, gm, and the maximum carboxylation rate (Vcmax) 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, CO2 concentration at carboxylation site decreased in parallel with gs and gm, indicating that photosynthesis was mainly limited by diffusional factors (combination of gs and gm). After drainage, diffusional limitation persisted during early reoxygenation, whereas biochemical limitation due to reduced Vcmax became dominant after 7 days of reoxygenation. Therefore, maintaining high diffusional conductances and Vcmax during waterlogging and reoxygenation, respectively, is important for enhancing photosynthetic tolerance to waterlogging stress. Overall, our results demonstrate that Asat under waterlogging and reoxygenation is dynamically constrained by multiple factors, emphasizing the need for comprehensive assessment of gas diffusion and carbon assimilation processes.