EFFECT OF CALCIUM AND CALMODULIN ON INTRINSIC HEAT TOLERANCE IN RELATION TO ANTIOXIDANT SYSTEMS IN MAIZE SEEDLINGS

Pretreatment of maize (Zen mays L.) seeds with CaCl2 solution signific antly raised calcium and calmodulin levels and enhanced intrinsic heat tolerance in maize seedlings. This enhancement is specific to Ca2+ an d cannot be substituted by Mg2+. In contrast, treatments with the Ca2 chelator EGTA or plasma membrane Ca2+ channel blockers La3+ or verapa mil lowered the intrinsic heat tolerance. Concurrent treatment with Ca 2+ and Ca2+-channel blockers (La3+, verapamil) also decreased the Ca2-enhanced heat tolerance. In addition, calmodulin antagonists chlorpro mazine or W7 treatment eliminated the Ca2+-enhanced heat tolerance, wh ilst W5, a less active analogue of W7, had little effect on the Ca2+-e nhanced heat tolerance. Measurement of antioxidant enzyme activities a nd lipid peroxidation showed that heat stress induced an oxidative str ess in maize seedlings. External Ca2+ treatment enabled the seedlings to keep relatively higher activities of superoxide dismutase (SOD), ca talase (CAT) and ascorbate peroxidase (APX) and lower levels of lipid peroxidation than Ca2+-deficient treatments; on the contrary, EGTA tre atment led to more rapid loss of SOD, CAT and APX activities and highe r levels of lipid peroxidation in the seedlings under heat stress. In addition, concurrent Ca2+ and W7 treatment weakened the effects of Ca2 + treatment on SOD, CAT and APX activities and caused more severe lipi d peroxidation. These results suggested that external Ca2+ can enhance the intrinsic heat tolerance of maize seedlings. This enhancement req uires the entry of external Ca2+ into cells across plasma membranes an d the mediation of intracellular calmodulin, and is associated with th e increase of antioxidant system activity.