Glutamate acts as a repressor for heat-induced leaf senescence involving chlorophyll degradation and amino acid metabolism in creeping bentgrass

Abstract

Leaf senescence is accelerated by prolonged exposure to high temperatures in cool-season plants and is characterized by loss of chlorophyll and adjustment in amino acid metabolism. The objectives of this study included ascertaining how glutamate may affect the tolerance of cool-season grass species to heat stress in relation to leaf senescence through the regulation of leaf chlorophyll (Chl) and amino acid metabolism. Creeping bentgrass (<i>Agrostis stolonifera</i> L.) plants were maintained for 35d under optimal growth temperature (22/18 °C, day/night, non-stress control) or heat stress (35/30 °C, day/night) in growth chambers, and glutamate (60 mM) or water (untreated control) was applied weekly as a foliar treatment. Under heat stress, turf quality (TQ), photochemical efficiency (F_v/F_m), and Chl content were significantly higher in plants treated with glutamate at 14-35 d, 7-28 d, and 7-35 d, respectively, compared to untreated controls. Glutamate application significantly reduced the activities of chlorophyll-degrading peroxidase and chlorophyllase, two Chl-degrading enzymes, at 14, 21, and 35 d of heat stress, whereas activity of the rate-limiting Chl synthesis enzyme, porphobilinogen deaminase, was not significantly affected. Glutamate treatment significantly increased the endogenous content of serine, threonine, lysine, aspartate, glutamate, and GABA under heat stress, while methionine, asparagine, isoleucine, and histidine were significantly up-regulated exclusively under heat stress. These findings suggest that glutamate could serve as a repressor for heat-accelerated leaf senescence by suppressing Chl degradation and activating amino acid metabolism involved in energy production, antioxidant defense, and nitrogen balance.