photosynthesis under high temperature conditions has been associated with summer bentgrass decline, particu-Turf quality of creeping bentgrass [Agrostis stolonifera var. pallarly under low mowing conditions (Liu and Huang, ustris (Huds.) Farw.] often declines during summer months. Whether summer bentgrass decline is associated with oxidative stress is unclear. 2001; Xu and Huang, 2003). The objectives of this study were to investigate seasonal changes of Leaf senescence and damage to cell membranes are antioxidant activities and lipid peroxidation under different mowing typical symptoms of summer bentgrass decline under heights for two creeping bentgrass cultivars (L-93 and Penncross) high temperature conditions, and these have been attribdiffering in heat tolerance and to determine whether such changes uted to lipid peroxidation of cell membranes and the could be associated with turf quality decline during summer. Penncross suppression of antioxidant enzymes (Liu and Huang, and L-93 were grown under USGA-specification putting green condi-2001). Plants exposed to environmental stresses such as tions at the Turfgrass Research Center at Manhattan, KS, in 1999 heat, cold, drought, and salinity produce active oxygen and 2000. Turf was mowed every day at 3 mm or 4 mm. Turf quality, species such as superoxide radical ( · O 2 Ϫ ), hydrogen perleaf chlorophyll (CHL), and protein content declined while electrolyte oxide (H 2 O 2 ), hydroxyl free radical ( · OH), and singlet leakage (EL) and lipid peroxidation increased from May to July through September for both cultivars at both mowing heights. Activi-oxygen ( 1 O 2 ) (Scandalios, 1993; Zhang and Kirkham, ties of superoxide dismutase (SOD), catalase (CAT), hydrogen peroxi-1996). Those active oxygen species are highly cytotoxic dase (POD), and ascorbate peroxidase (APX) increased from May and can react with unsaturated fatty acids to cause perto July when temperatures increased from optimum (Ͻ20؇C) to modoxidation of essential membrane lipids in plasmalemma erate level (≈26؇C), then declined to the lowest level in August when and intercellular organelles (Scandalios, 1993). Malontemperature reached the highest level (Ͼ30؇C), and recovered by dialdehyde (MDA) is the product of membrane lipid October. Similar seasonal patterns of antioxidant activities were obperoxidation. Increasing MDA content in plant cells served for both cultivars and mowing heights. In conclusion, summerindicates damage of cell membranes (Scandalios, 1993; bentgrass decline was related to the decline in antioxidant activities Zhang and Kirkham, 1996), which can lead to inhibition and lipid peroxidation. Both cultivars were able to acclimate to inof photosynthesis and respiration processes, and thus creasing temperatures by increasing the activities of antioxidant enzymes to scavenge active oxygen species in the early summer, but lost plant growth. Plant cells are protected against the attack the scavenging ability by midsummer.