Physiological And Biochemical Mechanisms Of Salt Resistance In Two Mangrove Seedlings-Kandelia Candel And Bruguiera Gymnorrhiza

Dynamic variation of accumulation of several metal elements in different tissues, membrane lipid peroxidation, photosynthesis and chlorophyll a fluorescence parameters and signal transduction were examined in 1-year-old seedlings of two typical salt excluders, Kandelia candel and Bruguiera gymnorrhiza under an increasing salinity (from 100mmol/1 to 400mmol/l NaCl) to investigate the mechanism of physiological and biochemical of two mangroves under salt-stress. Matured hypocotyls of K. candel and B. gymnorrhiza were obtained from Dongzhai Harbor Mangrove Forest Reserve of Hainan Province, China. The results were shown as follows:1. Cl and Na were largely accumulated in tissues of K .candel and B. gymnorrhiza seedlings under salt-stress, They enhanced osmotic adjustment of plants by accumulating more metal ions at high salinity, Meanwhile, the experiment showed that K. candel and B. gymnorrhiza have common Na+-exclusion mechanism, Namely, Na+ transportation was controlled by roots, But two mangrove seedlings had different Cl"-exclusion mechanism: Cl" absorption in K. candel and Cl" transportation in B. gymnorrhiza was controlled by roots. Keep lower salinity overground was common characters of K. candel and B. gymnorrhiza under the condition of satisfication osmotic adjustment2. Dynamic variation of leaf abscisic acid (ABA), calmodulin (CaM) and activities of superoxide dismutase (SOD) and peroxidase (POD) were examined in 1-year-old seedlings of two mangrove species, K.candel and B.gymnorrhiza under a short (24 hours, 100 mmol/1 NaCl) and longer-term of salt stress (28 days, external NaCl increased from 100 to 400 mmol/1). The results were shown as follows:(l). In a short-term stress, leaf ABA in K. candel increased rapidly and reached the peaking levels after 4 hours of NaCl stress, while CaM content reached the maximum at 24 hours. Activities of SOD and POD in K. candel increased upon salinity and both reached the peaking value after 8 hours of stress. Compared with K. candel, there was no significant increase of ABA in salt-stressed B.gymnorrhiza plant, whereas leaf CaM in B. gymnorrhiza increased rapidly after 4 hours of stress. Leaf SOD and POD activities in B. gymnorrhiza exhibited a harmonious increase after salt exposure. (2). In a longer-term stress, leaf Na+ and Cl" concentrations increased corresponding to the increasing salinity in both genotypes, but there were no significant changes of malondialdehyde (MDA) content and membrane permeability during the period of salt exposure. MDA content and membrane permeability in K.candel and B.gymnorrhiza increased by 20-39% under 400mmol/l NaCl. K.candel remained higher levels of leaf ABA and POD activity upon increasing NaCl stress. In comparison, B. gymnorrhiza maintained higher levels of CaM during the period of stress, and leaf SOD activity was markedly elevated under high salinity.3. The effects of salt stress on photosynthesis and chlorophyll a fluorescence parameters were investigated in K.candel and B.gymnorrhiza. The results showed that the predominant limiting factor of photosynthesis was stomatal conductance in two mangrove plants under natural conditions (control). While the decrease of net photosynthesis rate (Pn) was the result of the stomatal and non-stomatal factors including the inhibition of photosynthetic electron transport and the decrease of ribulose bisphosphate carboxylase -oxygenase(Rubisco) activity under NaCl stress. Meanwhile, the results also indicated that B. gymnorrhiza had higher salt tolerance than K candel.