Effects of foliar application of nutrients on heat tolerance of creeping bentgrass

This study was designed to investigate physiological responses of creeping bentgrass (Agrostis palustris Huds.) to foliar application of nutrients under heat stress. "Penncross" plants were exposed to two temperature regimes in growth chambers: optimum temperature control (20/15°C, day/night) and heat stress (35/30°C). A 10mL solution of CaCl₂ (10mM), KH₂PO₄ (10mM), NH₄NO₃ (124mM), or water (untreated control) was sprayed to the foliage of plants at 0, 14, and 28d of heat stress. Turf quality, shoot growth rate, canopy net photosynthetic rate (P_n), leaf chlorophyll content, and photochemical efficiency (F_v/F_m) declined during heat stress, regardless of nutrient treatments. The activities of superoxide dismutase (SOD), catalase (CAT), and hydrogen peroxidase (POD) were suppressed, but the content of a lipid peroxidation product, malondialdehyde (MDA) was increased with heat stress. Foliar application of CaCl₂ and KH₂PO₄ helped to maintain a higher F_v/F_m, shoot growth rate, and turf quality than untreated plants. Foliar application of NH₄NO₃ increased P_n, chlorophyll content, F_v/F_m, and turf quality under heat stress. None of the nutrient solutions had significant effects on SOD activity during heat stress; however all three nutrients increased CAT and POD activity and reduced MDA content at different times of heat stress. These results demonstrated that foliar application CaCl₂, KH₂PO₄, or NH₄NO₃ improved heat tolerance of creeping bentgrass to some extent by slowing leaf senescence and maintaining photosynthetic activities. The alleviating effects of foliar application of CaCl₂, KH₂PO₄, or NH₄NO₃ on heat injury in creeping bentgrass was related to the maintenance of the scavenging ability of antioxidants and inhibition of lipid peroxidation.