Differential enzymatic defense mechanisms in leaves and roots of two true mangrove species under long- term salt stress

To elucidate the role of antioxidative defense system against salt stresses, the levels of antioxidative enzymes and lipid peroxidation were quantified in leaves and roots of 2-year old Kandelia candel and Rhizophora stylosa seedlings subjected to 0, 5, 15 and 30 parts per thousand (ppt) of salt concentrations during one and two months treatments (1MT and 2MT). The data revealed differential responses between the species to different salt concentrations, while no significant differences observed between the treatment periods. In leaves, higher salinity decreased superoxide dismutase (SOD) and monodehydroascorbate reductase (MDHAR) activities in both species and catalase (CAT) activity in K. candel, while ascorbate peroxidase (APX) and guaiacol peroxidase (GPX) activities in both species and CAT in R. stylosa were increased. In roots, salt stresses increased the activities of most of the tested enzymes. Except for the roots K. candel, the malondialdehyde (MDA) contents were decreased significantly at higher salt concentrations. In K. candel leaves, moderate to strong negative correlations existed between APX, GPX and DHAR activities with MDA content, while SOD, CAT and MDHAR showed positive correlations. Contrarily, in R. stylosa leaves, CAT, APX and sometimes GPX and DHAR showed inverse correlations with respective MDA, while other enzymes showed reverse correlations. With few exceptions, all tested enzymes in roots showed negative correlations with MDA. These results suggested that most of the antioxidant enzymes in roots acted coordinately for effective stress mitigation, while leaf tissues might adopt many other mechanisms for salt tolerance of mangrove plants where antioxidant enzymes played minor role.