Photosynthetic and physiological responses of Kandelia candel L. Druce seedlings to duration of tidal immersion in artificial seawater

The present study demonstrates the influence of the duration of periodical waterlogging with artificial seawater on the photosynthetic and physiological responses of Kandelia candel L. Druce seedlings, the pre-dominant species of subtropical mangroves in China. Artificial tidal fluctuations applied here closely mimicked the twice daily tidal inundation which mangroves experience in the field. All the seedlings were immersed in artificial seawater during 70-day cultivation. Similar trends with increasing duration of immersion occurred in photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration, where significant decreases occurred only in long time treatments of 10 or 12 h. Water used efficiency and chlorophyll contents showed lower in medium periods and higher in long periods of immersion. This indicates that the increase in pigment contents of leaves was ineffective in promoting Pn under long time immersion. Light saturation points under short time waterlogging (0-4 h) occurred at light intensities of 800-1000 μmol/m2/s, and at around 400 μmol/m2/s in long time treatments (8-12 h). Long periods of tidal immersion therefore significantly inhibited photosynthesis of mature leaves. Alcohol dehydrogenase and oxidase activity in roots both increased under longer immersion periods, suggesting that roots are sensitive to anaerobiosis under long term waterlogging. The activities of peroxidase and superoxide dismutase in mature leaves increased in 8 h and 10 h treatments, respectively. The content of malondialdehyde in mature leaves increased under long time treatments. Abscisic acid accumulation in mature leaves also had a sharp increase from 8 h to 12 h inundation. Even though the anti-oxidative enzymes were induced by waterlogging, this was not sufficient to protect the seedlings from senescence. The results suggested that K. candel seedlings completely tolerated tidal immersion by seawater up to about 8 h in each cycle, which matches the natural distribution of K. candel in inter-tidal zones of China.