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Yiwei Jiang and Bingru Huang

Heat and drought are two major factors limiting growth of cool-season grasses during summer. The objective of this study was to compare the effects of heat stress alone (H) or in combination with drought (H+D) on photosynthesis, water relations, and root growth of tall fescue (Festuca arundinacea L.) vs. perennial ryegrass (Lolium perenne L.). Grasses were exposed to H (35 °C day/30 °C night) or H+D (induced by withholding irrigation) in growth chambers for 35 days. Soil water content declined under H+D for both grasses but to a greater extent for fescue than for ryegrass. Declines in canopy net photosynthetic rate (Pn), leaf photochemical efficiency (Fv/Fm), and leaf relative water content (RWC) and the increase in electrolyte leakage (EL) were much more severe and occurred earlier for ryegrass than fescue subjected to both H and H+D and for both species than under H+D then H. Evapotranspiration (ET) rate increased to above the control level within 3 or 6 days of H and H+D for both species, but fescue had a higher ET rate than ryegrass at 3 and 6 days of H and 6 days of H+D. Root dry weight and viability in all soil layers decreased under H and H+D for both species. However, fescue had higher root dry weight and viability than ryegrass in the 20-40 cm layer under H and in both the 0-20 and 20-40 cm layers under H+D. The results indicated that maintenance of higher Pn, Fv/Fm, ET, RWC, and root growth and lower EL would help cool-season turfgrass survive summer stress, and that their characteristics could be used for selecting stress tolerant species or cultivars.

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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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Michelle DaCosta and Bingru Huang

Previous investigations identified velvet bentgrass (Agrostis canina L.) as having higher drought resistance among bentgrass species. This study was designed to determine whether species variation in drought resistance for colonial bentgrass (A. capillaris L.), creeping bentgrass (A. stolonifera L.), and velvet bentgrass was associated with differences in antioxidant enzyme levels in response to drought. Plants of ‘Tiger II’ colonial bentgrass, ‘L-93’ creeping bentgrass, and ‘Greenwich’ velvet bentgrass were maintained in a growth chamber under two watering treatments: 1) well-watered control and 2) irrigation completely withheld for 28 d (drought stress). Prolonged drought stress caused oxidative damage in all three bentgrass species as exhibited by a general decline in antioxidant enzyme activities and an increase in lipid peroxidation. Compared among the three species, velvet bentgrass maintained antioxidant enzyme activities for a greater duration of drought treatment compared with both colonial bentgrass and creeping bentgrass. Higher antioxidant enzyme capacity for velvet bentgrass was associated with less lipid peroxidation and higher turf quality, leaf relative water content, and photochemical efficiency for a greater duration of stress compared with colonial bentgrass and creeping bentgrass. These results suggest that bentgrass resistance to drought stress could be associated with higher oxidative scavenging ability, especially for velvet bentgrass.

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Yan Xu and Bingru Huang

Leaf senescence can be induced by many environmental stresses, including supraoptimal temperatures. The objectives of this study were to evaluate leaf senescence induced by heat stress for two Agrostis species contrasting in heat tolerance and to examine whether heat-induced leaf senescence in both species was associated with changes in three major senescence-related hormones: ethylene, abscisic acid (ABA), and cytokinins. Plants of heat-tolerant rough bentgrass (Agrostis scabra Willd.) and heat-sensitive creeping bentgrass (Agrostis stolonifera L.) were exposed to 35/30 °C (day/night) (high temperature) or 20/15 °C (control) for 35 d in growth chambers. Turf quality, photochemical efficiency (Fv/Fm), and the contents of two pigments (chlorophyll and carotenoid) for both species decreased under high temperature; however, heat-tolerant A. scabra exhibited delayed and less severe decline in all parameters compared with heat-sensitive A. stolonifera. Ethylene production rate increased in both species at 35 °C, but the increase was observed 21 days later in A. scabra compared with that in A. stolonifera. ABA content increased at the initiation of heat stress and then declined in both species after prolonged heat stress. However, the timing of the increase was delayed for 7 days and the highest level of ABA content was less in A. scabra (4.0 times that of the control) than that in A. stolonifera (5.9 times that of the control). Decreases in both forms of cytokinins (transzeatin/zeatin riboside and isopentenyl adenosine) were also delayed for 14 days and less pronounced in A. scabra. Correlation analysis revealed that leaf senescence induced by heat stress was negatively correlated to ethylene and ABA accumulation and positively correlated to cytokinin production. Delayed leaf senescence in A. scabra under heat stress could be related to slower and less magnitude of changes in ethylene, ABA, and cytokinins.

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Michelle DaCosta and Bingru Huang

Abscisic acid (ABA) and cytokinins are two groups of plant hormones that play important roles in regulating plant responses to decreases in soil water availability. The primary objective for this study was to determine whether species variability in drought survival and recovery for colonial bentgrass (Agrostis capillaris L.), creeping bentgrass (A. stolonifera L.), and velvet bentgrass (A. canina L.) were related to changes in ABA and cytokinin content. Plants of ‘Tiger II’ colonial bentgrass, ‘L-93’ creeping bentgrass, and ‘Greenwich’ velvet bentgrass were subjected to two soil moisture treatments: 1) well-watered controls, irrigated three times per week; and 2) drought, irrigation completely withheld for 16 days. For recovery, previously drought-stressed plants were rewatered and irrigated three times per week to evaluate the recovery potential for each species. Drought stress resulted in significant declines in turf quality (TQ), shoot extension rates, canopy net photosynthetic rate (Pn), daily evapotranspiration rate (ET), and cytokinin content, and significant increases in ABA content for all three bentgrass species. Velvet bentgrass exhibited less severe drought injury, as exhibited by higher TQ, Pn, and daily ET rate compared with colonial bentgrass and creeping bentgrass. Velvet bentgrass also had significantly less ABA accumulation, which could allow for continued gas exchange and sustained plant survival during drought stress compared with colonial bentgrass and creeping bentgrass. Upon rewatering after drought stress, colonial bentgrass exhibited more rapid recovery in turfgrass growth and water use compared with creeping bentgrass and velvet bentgrass. The higher recuperative ability of colonial bentgrass could be associated with its more rapid decline in ABA content and increases in cytokinin content compared with creeping bentgrass and velvet bentgrass.

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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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Michelle DaCosta and Bingru Huang

Osmotic adjustment (OA) is a major physiological mechanism associated with maintenance of cell turgor in response to dehydration stress. The objectives of this study were to examine changes in capacity for OA in relation to plant tolerance to drought stress for two cool-season turfgrass species, creeping bentgrass (Agrostis stolonifera L.) and velvet bentgrass (A. canina L.), and to determine major solutes contributing to OA in these grass species. Plants of `L-93' creeping bentgrass and `Greenwich' velvet bentgrass were grown in a growth chamber in polyvinyl chloride (PVC) tubes (5 cm diameter, 40 cm high) filled with a 1:3 (v/v) sterilized mixture of sand and sandy loam soil. The experiment consisted of two soil moisture treatments: 1) well-watered control, irrigated three times per week to maintain soil moisture near pot capacity; and 2) drought stress, irrigation completely withheld. Velvet bentgrass exhibited higher drought tolerance compared to creeping bentgrass, as manifested by higher visual turfgrass quality (TQ) and leaf relative water content (RWC) under drought stress. Both creeping bentgrass and velvet bentgrass exhibited OA in response to drought stress; however, velvet bentgrass exhibited 50% to 60% higher magnitude of OA, which could be related to the maintenance of higher leaf RWC and TQ for greater drought duration compared to creeping bentgrass. OA for both creeping bentgrass and velvet bentgrass was associated with accumulation of water soluble carbohydrates during the early period of drought and increases in proline content following prolonged period of drought; however, inorganic ion content (Ca2+ and K+) did not considerably change under drought stress and did not seem to contribute to OA in these species.

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Michelle DaCosta and Bingru Huang

Efficient carbon distribution and utilization may enhance drought survival and recovery ability for perennial grasses. The objectives of this study were to examine changes in carbon partitioning and carbohydrate accumulation patterns in shoots and roots of colonial bentgrass (Agrostis capillaris L.), creeping bentgrass (A. stolonifera L.), and velvet bentgrass (A. canina L.) in response to drought and re-watering following drought, and to determine whether species variation in drought tolerance and recuperative potential is related to differences in the patterns of carbon partitioning and accumulation. The experiment consisted of three treatments: 1) well-watered control; 2) drought, irrigation completely withheld for 18 days; and 3) drought recovery, a group of drought-stressed plants were re-watered at the end of the drought treatment (18 days). Drought tolerance and recuperative ability of three species was evaluated by measuring turf quality and leaf relative water content. These parameters indicated that velvet bentgrass was most drought tolerant while colonial bentgrass had highest recuperative ability among the three species. Plants were labeled with 14CO2 to determine carbon partitioning to shoots and roots. Carbohydrate accumulation was assessed by total nonstructural carbohydrate (TNC) content. The proportion of newly photosynthesized 14C partitioned to roots increased at 12 days of drought compared to the pre-stress level, to a greater extent for velvet bentgrass (45%) than for colonial bentgrass (35%) and creeping bentgrass (30%). In general, the proportion of 14C was highest in roots, intermediate in stems, and lowest in leaves at 12 days of drought treatment for all three bentgrass species. As drought duration and severity increased (18 days), 14C partitioning increased more in leaves and stems relative to that in roots for all three species. Stem TNC content was significantly greater for drought-stressed plants of colonial bentgrass and velvet bentgrass compared to their respective well-watered control plants, whereas no differences in stem TNC content were observed between drought-stressed and well-watered creeping bentgrass. Our results suggest that increased carbon partitioning to roots during initial drought stress represented an adaptive response of bentgrass species to short-term drought stress, and increased carbon partitioning and carbohydrate accumulation in stems during prolonged period of drought stress could be beneficial for rapid recovery of turf growth and water status upon re-watering.

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Yali He and Bingru Huang

Understanding antioxidant mechanisms for heat stress is important for improving heat tolerance in cool-season plant species. The objective of this study was to identify antioxidant enzymes associated with cultivar variations in heat tolerance in kentucky bluegrass (Poa pratensis) by comparing heat responses of activity and isoforms of antioxidant enzymes in two cultivars contrasting in heat tolerance. Plants of heat-tolerant ‘Eagleton’ and heat-sensitive ‘Brilliant’ were exposed to 20 °C (control) or 40 °C (heat stress) for 28 days in growth chambers. Chlorophyll (Chl) a content remained unchanged and Chl b content increased in ‘Eagleton’, while both of them decreased in ‘Brilliant’, and by 28 days, ‘Eagleton’ had significantly higher Chl a and b content than ‘Brilliant’. The activities of superoxide dismutase (SOD) were significantly higher in ‘Eagleton’ than in ‘Brilliant’ by 28 days of heat stress. An isozyme SOD2 was induced early during heat stress in ‘Eagleton’, while isozyme SOD3 degraded, to a lesser extent in ‘Eagleton’ than in ‘Brilliant’. Catalase (CAT) activity significantly increased in ‘Brilliant’ but remained constant in ‘Eagleton’, and ‘Brilliant’ had a significantly higher CAT activity and isozyme CAT1 than ‘Eagleton’ during heat stress. Significant increases in ascorbate peroxidase (APX) activities occurred under heat stress, to a greater extent in ‘Eagleton’, whereas isozymes did not exhibit difference between cultivars. Guaiacol-peroxidase (POD) activity declined during heat stress in both cultivars. The intensity of POD isozymes in ‘Brilliant’ remained constant, while ‘Eagleton’ showed a transient increases in POD1 at 7 days of heat stress. Our results indicated that antioxidant defense mechanisms for heat tolerance in kentucky bluegrass could be mainly associated with changes in activity and forms of isozymes of SOD for O2 scavenging and APX activity for H2O2 scavenging under heat stress.

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Patrick Burgess and Bingru Huang

Elevated CO2 may contribute toward plant tolerance to prolonged drought stress. The objective of this study was to investigate changes in protein abundance associated with mitigation of drought stress by elevated CO2 in leaves of a cool-season grass species used as fine turfgrass. Plants of creeping bentgrass (Agrostis stolonifera cv. Penncross) were grown at either ambient CO2 concentration (400 µL·L−1) or elevated CO2 concentration (800 µL·L−1) for 35 days under well-irrigated and fertilized conditions and then subjected to drought stress for 21 days by withholding irrigation. Plants exposed to elevated CO2 concentration maintained higher leaf water content, membrane stability, and visual turf quality (TQ) under drought stress compared with plants grown under ambient CO2 conditions. The abundance of proteins involved in photosynthetic carbon fixation and assimilation, including chloroplastic glyceraldehyde phosphate dehydrogenase A (GAPDH-A) and ribulose 1,5-bisphosphate carboxylase (RuBisCO) decreased less and the abundance of proteins involved in respiratory metabolism (i.e., cytosolic GAPDH) increased less during drought due to elevated CO2. The results suggest that elevated CO2 lessened growth and physiological damages during drought by facilitating ribulose 1,5-bisphosphate regeneration and adenosine triphosphate (ATP) production in photosynthesis and downregulating factors contributing to respiratory metabolism.