Efects of Calcium on Antioxidant Acivities and Water Relations Associated with Heat Tolerance in Two Cool-season Turfgrasses

Calcium (Ca+) may be involved in plant tolerance to heat stress by regulating antioxidant metabolism or/and water relations. This study was designed to examine whether external Ca2+ treatment would improve heat tolerance in two C3, cool-season grass species, tall fescue (Festuca arundinacea L.) and Kentucky bluegrass (Poa pratensis L.), and to determine the physiological mechanisms of Ca2+ effects on grass tolerance to heat stress. Grasses were treated with CaCl2 (10 roM) or H20 by foliar application and then exposed to heat stress (35°C/30°C) in growth chambers. Some of the Ca2+-untreated plants were maintained at 20°C /lSoC as the temperature control. Heat stress reduced grass quality, relative water content (RWC), and chlorophyll (ChI) content of leaves in both species, but Ca2+ treatment increased all three factors under heat stress. The Ca2+ concentration in cell saps increased with heat stress and with Ca2+ treatment in both species. Osmotic potential increased with heat stress, but Ca2+ treatment had no effect. Osmotic adjustment increased during short-term heat stress but then decreased with a prolonged period of stress; it was not influenced by Ca2+treatment. The activity of superoxide dismutase (SOD) in both species increased transiently at 12 d of heat stress and then remained at a level similar to that of the control. External Ca2+ treatment had no effect on SOD activity. The activities of catalase