GROWTH AND LEAVES MEMBRANE LIPID PEROXIDATION OF BRUGUIERA GYMNORRHIZA(L.)LAMK. SEEDLINGS UNDER LONG-AND SHORT-TERM SALINITY

The aim of the present study was to investigate the effects of severity and duration of the salinity on the growth and leaves membrane lipid peroxidation,antioxidative systems,electrolyte leakage,succulence and chlorophyll content of mangrove Bruguiera gymnorrhiza(L. )Lamk. seedlings. Mature B. gymnorrhiza hypocotyls with similar lengths and fresh weights were cultured hydroponically in sand beds at salinities of 0,10,20,30,40,50g/L under natural photoperiods and mean day/night temperatures of 32/25℃. Tap water was added daily to keep the salinity constant. The salt salutions were changed once 15 days. At 1 and 40 days after the first pair of leaves of all seedlings unfolded,the first pair of leaves for each treatment were collected randomly at different time. The superoxide dismutase(SOD)activity,malondialdehyde MDA content,succulence,electrolyte leakage and chlorophyll content of each leaf were measured. The changes in growth,leaf SOD activity, MDA content,electrolyte leakage,succulence and chlorophyll content under short-term(1day) and long-term(40days)salinity were as follows:Moderate salt stress can stimulate the growth of seedlings, and the optimum salinity for growth was 10g/L and 20g/L,respectively. When the substrate salinity was higher than the optimum salinity,growth of seedlings declined. On the contrary,SOD activity and MDA content decreased under a moderate salt stress(10-20g/L). When the substrate salinity was higher than 30-40g/L,SOD activity and MDA content increased significantly. With the increase of salinity,electrolyte leakage increased constantly,but chlorophyll content did not change significantly. These results indicated that changes in growth,leaf SOD activity,MDA content,succulence,electrolyte leakage and chlorophyll content under short-term salinity were similar to those under long-term salinity,thus the B. gymnorrhiza seedlings had similar salt tolerant mechanism under long and short-term salinity in our study. On the other hand,with the prolongation of salt stress,certain changes occurred in B. gymnorrhiza seedlings to adapt themselves better to salt stress. Under short-term salinity,the optimum salinity for the growth of B. gymnorrhiza seedlings was 10g/L, while it was 20g/L under long-term salinity. The leaves under long-term salinity had lower electrolyte leakage. With the increase of salinity, no direct linear correlation between MDA contents and electrolyte leakage under short-term salt-stress, nor did under long-term salt-stress. These result indicated that membrane lipid peroxidation was not the main reason for the destroy of B.gymnorrhiza leaves under salt stress.