Effects of calcium on antioxidant activities and water relations associated with heat tolerance in two cool-season grasses

Calcium (Ca²⁺) 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 mM) or H₂O by foliar application and then exposed to heat stress (35/30 °C) in growth chambers. Some of the Ca²⁺-untreated plants were maintained at 20/15 °C 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 external Ca²⁺ treatment in both species. Osmotic potential increased with heat stress, but external 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 (CAT), ascorbate peroxidase (AP), and glutathione reductase (GR) of both species decreased during heat stress. Plants treated with Ca²⁺ under heat stress had higher CAT, GR and AP activities than untreated plants. Lesser amounts of malondialdehyde (MDA) accumulated in Ca²⁺treated plants than in untreated plants during extended periods of heat stress. The results suggested that exogenous Ca²⁺ treatment enhanced heat tolerance in both tall fescue and Kentucky bluegrass. This enhancement was related to the maintenance of antioxidant activities and a decrease in membrane lipid peroxidation, but not to the regulation of osmotic potential and osmotic adjustment.