Cyclic Electron Transport via the NDH Complex Sustains Photosynthesis and Productivity Under Fluctuating and Sub-Optimal Environments.

The chloroplast NADH dehydrogenase-like (NDH) complex mediates cyclic electron transport (CET) around photosystem I (PSI) and contributes to photosynthetic regulation and photoprotection under various environmental stresses. Although NDH function has been extensively characterized under controlled conditions, NDH-deficient mutants often show only subtle phenotypes in such environments, leaving its physiological importance to photosynthesis in plants grown under naturally fluctuating field conditions poorly understood. Here, we evaluated growth, yield, and photosynthetic performance of NDH-deficient rice cultivated in outdoor fields. Mutant plants exhibited reduced biomass accumulation and grain yield compared with wild type (WT). Detailed physiological analyses in field-grown plants revealed that NDH deficiency markedly decreased PSI electron transport and CO2 assimilation, particularly under low temperature and sub-saturating irradiance. At moderate and high temperatures, reductions in CO2 assimilation were largely confined to low-light conditions, whereas at low temperatures, impairment extended across nearly the entire light response range. Under repetitive fluctuating light regimes, NDH-deficient plants showed progressive declines in CO2 assimilation accompanied by a decrease in PSI and PSII photochemical capacity, indicating photoinhibition of both PSI and PSII. These findings demonstrate that NDH-dependent CET plays a crucial role in sustaining crop productivity in field environments by stabilizing photosynthetic efficiency under sub-optimal conditions through maintaining PSI redox balance.