SALT AND OXIDATIVE STRESS - SIMILAR AND SPECIFIC RESPONSES AND THEIR RELATION TO SALT TOLERANCE IN CITRUS

Salt damage to plants has been attributed to a combination of several factors including mainly osmotic stress and the accumulation of toxic ions. Recent findings in our laboratory showed that phospholipid hydro peroxide glutathione peroxidase (PHGPX), an enzyme active in the cellu lar antioxidant system, was induced by salt in citrus cells and mainly in roots of plants. Following this observation we studied the two mos t important enzymes active in elimination of reactive oxygen species, namely, superoxide dismutase (SOD) and ascorbate peroxidase (APX), to determine whether a general oxidative stress is induced by salt. While Cu/Zn-SOD activity and cytosolic APX protein level were similarly ind uced by salt;and methyl viologen, the response of PHGPX and other APX isozymes was either specific to salt or methyl viologen, respectively. Unlike PHGPX, cytosolic APX and Cu/Zn-SOD were not induced by exogeno usly added abscisic acid. Salt induced a significant increase in SOD a ctivity which was not matched by the subsequent enzyme APX. We suggest that the excess of H2O2 interacts with lipids to form hydroperoxides which in turn induce and are removed by PHGPX. Ascorbate peroxidase se ems to be a key enzyme in determining salt tolerance in citrus as its constitutive activity in salt-sensitive callus is far below the activi ty observed in salt-tolerant callus, while the activities of other enz ymes involved in the defence against oxidative stress, namely SOD, glu tathione reductase and PHGPX, are essentially similar.