Impact of Drought Stress on Photosynthetic Electron Transport in Rice Seedlings

To investigate photosynthetic physiological response mechanisms of rice (Oryza sativa L.) seedling leaves under drought stress, a pot experiment with controlled watering was conducted. Four treatments were applied-control (CK), mild drought (LD), moderate drought (MD), and severe drought (SD)-to four cultivars (H17B7, HY3015, LH1H, WNSM) at seedling stage. The results showed that, after drought stress, the morphological indicators and chlorophyll content of the four rice cultivars all showed decreased trend, but a compensatory effect appeared in some varieties under the LD treatment. As drought intensified, the chlorophyll fluorescence induction kinetics (OJIP) curves' polyphasic rise was inhibited, mainly affecting the J-I and I-P phases. Under SD, the P-phase declined by 44.36%, 36.75%, 32.61% and 50.62% compared with CK, and positive L-band, K-band, and J-band. Chlorophyll fluorescence parameters showed dynamic changes. Most chlorophyll fluorescence parameters (e.g., F_v/F_m) decreased, whereas TR_o/CS_o and DI_o/CS_o exhibited cultivar-dependent variations. Principal components of the chlorophyll fluorescence parameters in seedling leaves of the four rice cultivars showed differences and were clearly distinguished. Plant height, chlorophyll content, etc., are highly significantly positively correlated with F_o, DI_o/CS_o, etc., as well as chlorophyll content with DI_o/CS_o. The OJIP/JIP-test responses suggest impairment of the PQ pool and downstream electron transport, with the extent of damage varying among cultivars.