Drought stress reduces crop carbon gain due to delayed photosynthetic induction under fluctuating light conditions

Drought stress is a major limiting factor for crop growth and yield. Water availability in the field can cyclically change between drought and rewatering conditions, depending on precipitation patterns. Concurrently, light intensity under field conditions can fluctuate, inducing dynamic photosynthesis and transpiration during the crop growth period. The present study aimed to characterize carbon gain and water use in fluctuating light under drought and rewatering conditions in two major crops, namely rice and soybean. We conducted gas exchange measurements under fluctuating light conditions with rice and soybean plants exposed to drought treatment (9-13 days) imposed by withholding water and subsequent rewatering treatment (8-9 days). Drought stress significantly reduced the maximum CO₂ assimilation rate (A) in soybean but not in rice. Under drought conditions, A increased after a step increase in light and then gradually decreased in both crops, resulting in the significant reduction of steady-state A in rice and soybean. Moreover, drought stress delayed photosynthetic induction in both crops even when it had relatively small impact on maximum A. These results suggest that the drought effects on photosynthesis should be evaluated based on induction, maximum, and steady states. The delayed photosynthetic induction under drought owing to the reduced gas diffusional conductance via stomata resulted in a substantial loss of leaf carbon gain under fluctuating light conditions. Meanwhile, rewatering, after drought, completely recovered photosynthesis under fluctuating light in both crops. Therefore, the stability of photosynthetic induction can be a promising target to improve drought tolerance during crop breeding in the future.