Responses of Rice Photosynthetic Carboxylation Capacity to Drought–Flood Abrupt Alternation: Implications for Yield and Water Use Efficiency

Investigating how drought and flooding stresses interact during drought–flood abrupt alternation events and their impact on rice photosynthetic carboxylation capacity ($V_{cmax}$) is critical for improving crop growth and yield models under environmental stress conditions. However, there is limited research on the specific role of these combined stresses on $V_{cmax}$ in rice. This study aims to address this gap by examining the effects of drought and flooding on rice $V_{cmax}$. Using data from drought–flood experiments conducted in 2017 and 2018, we calculated $V_{cmax}$ by combining observed gas exchange parameters with photosynthetic biochemical models. The results revealed that $V_{cmax}$ damage caused by drought and flooding stresses was eventually repaired. Notably, $V_{cmax}$ recovered more quickly when mild drought preceded flooding stress. In contrast, severe and moderate drought treatments showed synergistic effects, where the preceding drought and subsequent flooding exacerbated the damage to $V_{cmax}$. However, the pre-mild drought stress antagonistically mitigated the damage to $V_{cmax}$ of rice induced by flooding stress, showing an antagonistic effect. Additionally, rice increased intrinsic water use efficiency (${WUE}_i$; $A_n/g_s$) by increasing investment in $V_{cmax}$ after drought and flooding stress, but rice yield was not improved. The preceding drought is probably beneficial for yield of rice experiencing subsequent flooding stress at relatively low $V_{cmax}$, while subsequent flooding stress exacerbated the reduction in yield of rice experiencing preceding drought stress. This research enhances our understanding of how the interaction between drought and flooding affects rice’s photosynthetic capacity and emphasizes that appropriate drought and flooding management may have potential optimizing effects on rice yield and water use, and provides an important theoretical basis and practical guidance for paddy water management.