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Xiaozhong Liu and Bingru Huang

Summer decline in turf quality of creeping bentgrass (Agrostis palustris Hud.) is a major problem in golf course green management. The objective of this study was to examine whether seasonal changes and cultivar variations in turf performance are associated with changes in photosynthesis and respiration rates for creeping bentgrass. The study was conducted on a USGA-specification putting green in Manhattan, Kans., during 1997 and 1998. Four creeping bentgrass cultivars, `L-93', `Crenshaw', `Penncross', and `Providence', were examined. Grasses were mowed daily at 4 mm and irrigated on alternate days to replace 100% of daily water loss. In both years, turf quality, canopy net photosynthetic rate (Pn), and leaf photochemical efficiency (Fv/Fm) were high in May and June and decreased to the lowest levels in July through September. Whole-plant respiration rate (R) and canopy minus air temperature (▵T) increased during summer months. In October, turf quality and Pn increased, whereas R and T decreased. During summer months, turf quality was highest for `L-93', lowest for `Penncross', and intermediate for `Providence' and `Crenshaw'. Seasonal changes and cultivar variations in turf quality were associated with the decreasing photosynthetic rate and increasing respiration rate.

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Xiaozhong Liu and Bingru Huang

Low mowing increases ball roll distance on putting greens, but may affect growth and physiological responses to summer heat stress. The objective of this study was to examine whether the effect of mowing heights on turf summer performance was associated with changes in photosynthetic activities and respiration rate for two creeping bentgrass [Agrostis palustris (L.) Huds] cultivars, `Crenshaw' and `Penncross'. Both cultivars were grown under USGA-specification putting green conditions from 1997 to 1998. Grasses were mowed daily at a 3-mm (low mowing) or 4-mm (high mowing) height. Turf quality, net photosynthesis rate (Pn), and leaf photochemical efficiency (Fv/Fm) declined, whereas respiration rate of whole plants, canopy minus air temperature, and soil temperatures increased under low mowing compared to those at the high mowing height. The decline or increase in those parameters under low mowing was more pronounced in summer than in spring or fall months. The results showed that turf quality was better at the 4-mm mowing height, especially during summer months. Better quality at the higher mowing height could be related to the maintenance of higher photosynthetic activities and lower respiration rate. Mowing at the lower height had more adverse effects on turf growth and photosynthetic capacity for `Penncross' than `Crenshaw', particularly during summer months.

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Xiaozhong Liu and Bingru Huang

Previous studies found that high soil temperature is more detrimental than high air temperature for the growth of creeping bentgrass (Agrostis palustris L.). The objective of the study was to investigate changes in fatty acid composition and saturation levels in leaves and roots for creeping bentgrass exposed to high soil temperature. Shoots and roots of `Penncross' plants were subjected to a differential air/soil temperature of 20/35 °C in a growth chamber. Soil temperature was controlled at 35 °C using an immersion circulating heater in water bath. Shoot injury induced by high soil temperature was evaluated by measuring level of lipid peroxidation expressed as malonyldialdehyde (MDA) content, chlorophyll content, and photochemical efficiency (Fv/Fm) of leaves. MDA content increased while chlorophyll content and Fv/Fm decreased at high soil temperature. The content of total fatty acids and different species of fatty acids were analyzed in both leaves and roots. Total fatty acid content in leaves increased initially at 5 days of high soil temperature and then decreased at 15 days, while total fatty acid content in roots decreased, beginning at 5 days. Linolenic acid was the major fatty acid in leaves and linoleic acid and palmitic acid were the major fatty acids in roots of creeping bentgrass. Leaf content of all fatty acid components except oleic acid increased initially and then decreased at high soil temperature. Root content of all fatty acid components except palmitoleic acid and oleic acid decreased, beginning at 5 d of high soil temperature. Oleic acid in leaves and palmitoleic and oleic acid in roots did not change during the entire experimental period. Leaf content of saturated fatty acids and unsaturated fatty acids increased during the first 5 to 10 days of high soil temperature and decreased at 15 and 25 days, respectively. Root content of saturated fatty acids and unsaturated fatty acids decreased beginning at 5 days of high soil temperature. Double bond index decreased in both leaves and roots. High soil temperature induced changes in fatty acid composition and saturation levels in leaves and roots, and this could be associated with physiological damages in leaves even though only roots were exposed to high temperature.

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Xiaozhong Liu and Bingru Huang

Understanding physiological factors that may confer heat tolerance would facilitate breeding for improvement of summer turf quality. The objective of this study was to investigate whether carbohydrate availability contributes to changes in turf quality and root mortality during heat stress in two creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw. (syn. A. palustris Huds.)] cultivars, `L-93' and `Penncross', that contrast in heat tolerance. Grasses were grown at 14-hour days and 11-hour nights of 22/16 °C (control) and 35/25 °C (heat stress) for 56 days in growth chambers. Turf quality decreased while root mortality increased under heat-stress conditions for both cultivars, but to a greater extent for `Penncross' than `L-93'. The concentrations of total nonstructural carbohydrate (TNC), fructans, starch, glucose, and sucrose in shoots (leaves and stems) and roots decreased at 35/25 °C. The reduction in carbohydrate concentrations of shoots was more pronounced than that of roots. Shoot glucose and sucrose concentrations were more sensitive to heat stress than other carbohydrates. `L-93' maintained significantly higher carbohydrate concentrations, especially glucose and sucrose, than `Penncross' at 35/25 °C. Results suggest that high carbohydrate availability, particularly glucose and sucrose, during heat stress was an important physiological trait associated with heat-stress tolerance in creeping bentgrass.

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Yali He, Xiaozhong Liu, and Bingru Huang

The acclimation of plants to moderately high temperature plays an important role in inducing plant tolerance to subsequent lethal high temperatures. This study was performed to investigate the effects of heat acclimation and sudden heat stress on protein synthesis and degradation in creeping bentgrass (Agrostis palustris Huds.). Plants of the cultivar Penncross were subjected to two temperature regimes in growth chambers: 1) heat acclimation—plants were exposed to a gradual increase in temperatures from 20 to 25, 30, and 35 °C for 7 days at each temperature level before being exposed to 40 °C for 28 days; and 2) sudden heat stress (nonacclimation)—plants were directly exposed to 40 °C for 28 days from 20 °C without acclimation through the gradual increase in temperatures. Heat acclimation increased plant tolerance to subsequent heat stress, as demonstrated by lower electrolyte leakage (relative EL) in leaves of heat-acclimated plants compared to nonacclimated plants at 40 °C. Heat acclimation induced expression of some heat shock proteins (HSPs), 57 and 54 kDa, detected in a salt-soluble form (cystoplasmic proteins), which were not present in unacclimated plants under heat stress. However, HSPs of 23, 36, and 66 kDa were induced by both sudden and gradual exposure to heat stress. In general, total protein content decreased under both heat acclimation and sudden heat stress. Cystoplasmic proteins was more sensitive to increasing temperatures, with a significant decline initiated at 25 °C, while sodium dodecyl sulphate (SDS)-soluble (membrane) protein content did not decrease significantly until temperature was elevated to 30 °C. The results demonstrated that both a gradual increase in temperature and sudden heat stress caused protein degradation and also induced expression of newly synthesized HSPs. Our results suggested that the induction of new HSPs during heat acclimation might be associated with the enhanced thermotolerance of creeping bentgrass, although direct correlation of these two factors is yet to be determined. This study also indicated that protein degradation could be associated with heat injury during either gradual increases in temperature or sudden heat stress.

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Yali He, Xiaozhong Liu, and Bingru Huang

Various physiological processes may deteriorate in response to increasing temperatures, contributing to the decline in turf quality for cool-season turfgrasses during heat stress. This study was performed to investigate metabolic changes (membrane lipid peroxidation, total protein content, amino acid content, and protease activity) associated with turf quality decline for creeping bentgrass (Agrostis stolonifera Huds.) in response to gradually increasing temperatures for a short duration and prolonged exposure to lethally high temperature. Plants were subjected to increasing temperatures of 20, 25, 30, 35, and 40 °C for 7 days at each level of temperature [gradual heat stress (GHS)] or exposed to high temperature of 40 °C for 28 days [prolonged heat stress (PHS)] in growth chambers. During the GHS treatment, significant decline in turf quality occurred when plants were exposed to 30 °C for 7 days; simultaneously, malondialdehyde (MDA) content increased and total protein content in shoots decreased significantly compared to those at 20 °C. Protease activity increased at 25 °C and then decreased as temperature was elevated from 30 to 40 °C during the GHS treatment. Amino acid content decreased under GHS, beginning at 25 °C. Under the PHS treatment, turf quality declined and MDA content increased significantly, beginning at 14 days of PHS, while total protein content decreased at 7 days of PHS. Protease activity and amino acid content increased at 7 days of PHS, and then declined with longer stress duration. Our results indicated that protease activity, and amino acid and total protein content were more responsive to GHS or PHS than that of lipid peroxidation and turf quality. Changes in metabolic parameters of protease activity, amino acid and total protein content, and lipid peroxidation may contribute to leaf senescence and poor turf performance under severe or prolonged heat stress conditions for creeping bentgrass.