DROUGHT DURATION ON POST-REHYDRATION PHYSIOLOGICAL RECOVERY OF NOSTOC SPHAEROIDES AT DIFFERENT DEVELOPMENTAL STAGES

To elucidate the effects of drought duration on physiological and metabolic recovery of Gexianmi (Nostoc sphaeroides) at different developmental stages after rehydration, three developmental stages (hormogonia, micro-colony, and small colony with 1—2 mm diameter) were used to simulate soil drying for 1d, 7d, and 21d in paddy fields followed by rehydration. Indicators including cell viability, nitrogen fixation activity, photosynthetic oxygen evolution rate, photosystem II (PSⅡ) maximum photochemical efficiency (F_v/F_m), activities of antioxidant enzymes (SOD and CAT), and contents of reactive oxygen species (ROS) and malondialdehyde (MDA) were determined. The results showed that after short-term drought (1d) and subsequent rehydration, cell viability and SOD activity of micro-colonies exhibited the most significant increase, while photosynthetic oxygen evolution activity of small colonies had the highest increase. With prolonged drought to 7—21d, the cell viability, nitrogen fixation activity, and F_v/F_m of hormogonia after 24h of rehydration were all significantly lower than the initial levels before desiccation. Nitrogen fixation system was the most sensitive to drought stress. Among the three developmental stages, micro-colonies had the strongest tolerance to short-term drought but lost its protective advantage under long-term drought, whereas hormogonia were most inhibited by drought yet exhibited the least oxidative damage. This study reveals that drought duration and developmental stage jointly regulate the physiological recovery of N. sphaeroides after rehydration, providing a theoretical basis for water management and artificial cultivation of N. sphaeroides in paddy fields.