Abscisic Acid, Calmodulin Response to Short Term and Long Term Salinity and the Relevance to NaCl-induced Antioxidant Defense in Two Mangrove Species

In this study we investigated the effects of short term (ST, 24 h, 100 mM NaCl) and long term (LT) salinity (4 weeks, NaCl increased weekly from 100 to 400 mM) on leaf abscisic acid (ABA), calmodulin (CaM) and activity of antioxidant enzymes (superoxide dismutase, SOD; peroxidase, POD) in 1-year-old seedlings of two mangrove species, Kandelia candel and Bruguiera gymnorrhiza. The gas exchange and salt (Na + and Cl  ) accumulation upon a LT stress were compared between the two species. Results show that stomatal conductance, net photosynthetic rates and unit transpiration rates of the two species were significantly decreased by LT salinity and the inhibitory effects were more pronounced in K. candel, especially at high saline conditions. Leaf Na + and Cl  concentrations in both species steadily increased with increasing the period of salt stress, but K. candel exhibited a higher capacity for salt exclusion. Malondialdehyde content and membrane permeability did not significantly increase in the two mangroves during the prolonged period of salt exposure. NaCl up-regulated ABA, CaM and activity of SOD and POD in the two species, but different trends were observed. (1) Upon a ST stress, leaf ABA in K. candel increased rapidly and reached the peaking levels after 4 h, and activity of antioxidant enzymes correspondingly increased to the peaking values after 8 hours of stress. In B. gymnorrhiza leaves, SOD and POD activity exhibited a coincident increase after the initiation of salt exposure and leaf CaM markedly increased after 8 hours. (2) Under a LT salinity, K. candel maintained high levels of leaf ABA and POD activity, whereas B. gymnorrhiza retained high CaM levels during the period of stress, and SOD activity was markedly elevated at high salinity (400 mM NaCl). Therefore, we conclude that the two mangrove species were able to up-regulate the activity of antioxidant enzymes to avoid excess reactive oxygen species and the subsequent oxidative stress despite a NaCl buildup in salinised plants. The elevation of antioxidant enzymes is likely associated with the salt- induced rise of ABA and CaM since the acceleration effect of NaCl on antioxidant enzymes were inhibited by ABA synthesis inhibitor, tungstate (sodium form) and CaM inhibitor, trifluoperazine (TFP).