Effects of Drought Stress and Rewatering on the Photosynthetic Physiology of Rice Seedlings

To clarify the photosynthetic and physiological responses of rice seedlings to drought stress and subsequent rewatering, four rice cultivars (HHZ, PH34, WGZ, and LD24) were employed as experimental materials. Three water treatments were applied, including a well-watered control (CK), drought stress (DS), and rewatering (RW) following drought exposure. This study focused on leaf chlorophyll content and quantified variations in photosystem II (PSII) functionality using the JIP-test to analyze rapid chlorophyll a fluorescence induction kinetics (OJIP transients). Drought stress significantly reduced leaf chlorophyll content; while rewatering partially restored chlorophyll levels, they remained significantly lower than those in the control group. Distinct treatment-specific differences were observed in the OJIP curves after the J step: in both the DS and RW groups, the ascending trends of the J-I and I-P phases were attenuated, with the fluorescence intensity from the I to P phase being markedly lower than that in the CK group. This reduction was most prominent under drought stress conditions. The occurrence of distinct L-, K-, and J-bands in the differential curves indicated damage to the oxygen-evolving complex (OEC) and impaired electron donation on the donor side of PSII. Furthermore, both drought stress and rewatering decreased Vj and Fv/Fm values, as well as the performance indices reflecting overall PSII activity (PIabs, PItotal) and energy distribution per cross-section (TRo/CSo, ETo/CSo). In contrast, ABS/RC, DIo/RC, ETo/RC, and TRo/RC were elevated, whereas RC/CSm was reduced. These changes suggested an accumulation of inactivated reaction centers and an increased excitation load on the remaining active reaction centers. Collectively, these results demonstrate that drought stress strongly inhibits the photosynthetic function of rice seedlings by impairing photoprotective mechanisms and damaging PSII reaction centers, thereby restricting seedling growth. Although rewatering triggers partial physiological compensation, the photosynthetic capacity fails to fully recover to the normal level after severe drought stress.