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Zhaolong Wang and Bingru Huang

Drought is a major limiting factor for turfgrass growth. Understanding genetic variations in physiological responses of turfgrass to drought stress would facilitate breeding and management programs to improve drought resistance. This study was designed to evaluate responses of abscisic acid (ABA) accumulation, water relations, and gas exchange to drought stress in four Kentucky bluegrass (Poa pratensis L.) cultivars differing in drought resistance. Plants of `Midnight' and `A82-204' (drought resistant) and `Brilliant' and `RSP' (drought susceptible) were grown under well-watered (control) or drought stress conditions for 25 days in growth chambers. Turf quality, leaf water potential (Ψleaf), relative water content (RWC), leaf net photosynthesis rate (Pn), and stomatal conductance (gs) declined, while electrolyte leakage (EL) increased during drought progression in all cultivars. The magnitudes of the change in these parameters were greater for `RSP' and `Brilliant' than for `Midnight' and `A82-204'. Leaf ABA content in `RSP' and `Brilliant' increased sharply after 2 days of stress to as much as 34 times the control level at 10 days of drought. Leaf ABA content in `Midnight' and `A82-204' also increased with drought, but to a lesser extent than in the other two cultivars. Leaf ABA level was negatively correlated with Ψleaf and gs. `A82-204' had a significantly lower ABA accumulation rate with changes in Ψleaf during drought compared to `Midnight', `RSP' and `Brilliant'; however, no differences in ABA accumulation rate were detected among the latter three cultivars. In addition, leaf gs was more sensitive to changes in ABA accumulation in `Midnight' and `A82-204' than in `RSP' and `Brilliant'. These results demonstrated that drought tolerant cultivars of Kentucky bluegrass were characterized by lower ABA accumulation and less severe decline in Ψleaf, Pn, gs, and turf quality during drought stress than drought sensitive cultivars. Drought tolerance of Kentucky bluegrass could be related to sensitivity of stomata to endogenous accumulation of ABA under drought stress conditions.

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Qingzhang Xu, Bingru Huang and Zhaolong Wang

Turf quality of creeping bentgrass (Agrotis palustris L.) often declines during summer months. Reducing soil temperature alleviates bentgrass quality decline at supraoptimal air temperatures. The objective of this study was to investigate whether reducing soil temperature during the night is more effective than during the day in improving shoot and root growth when air temperature was supraoptimal for creeping bentgrass. The experiment was conducted in growth chambers using water baths to manipulate soil temperatures. Plants were exposed to the following temperature treatments: 1) optimal air and soil temperature during the day and night (20/20 °C, day/night, control); 2) high air and soil temperature during the day and night (35/35 °C, day/night); 3) lower soil temperatures during the day (20/35, 25/35, and 30/35 °C, day/night); and 4) lower soil temperature during the night (35/20, 35/25, and 35/30 °C) while air temperature was maintained at 35 °C during the day and night. Turf quality (on 1-9 scale) increased to the level of 6.5, 3.0, and 2.5 by reducing day soil temperature to 20, 25, or 30 °C, respectively, at 28 days of treatment, compared to the quality of 2.0 at 35/35 °C. Turf quality increased from 2.0 at 35/35 °C to 7.0, 6.0, and 4.5, respectively, by 28 days of exposure to night temperatures of 20, 25, and 30 °C. Chlorophyll content, root number, and root weight also were increased by reducing day or night soil temperature, and the increases were more pronounced for reduced night temperatures than day temperatures. These results demonstrated that reduced night soil temperature was more effective than reduced day soil temperature in improving shoot and root growth in creeping bentgrass under high air temperature conditions.

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John Pote, Zhaolong Wang and Bingru Huang

Knowledge of the level of soil temperatures that is detrimental for shoot and root growth for cool-season grasses may help develop heat-tolerant plants and effective management practices to improve summer performance. The objectives of this study were to determine the level and duration of high temperatures in the root zone that will induce decline for various growth and physiological parameters and to compare the responses of different physiological parameters and cultivars to high root-zone temperatures. Nine creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw.] cultivars were subjected to eight root-zone temperatures (20, 21, 22, 23, 25, 27, 31, 35 °C) in water baths while exposed to a constant air temperature of 20 °C for 54 days. Root number, dry weight, and depth, active root biomass, turf quality, leaf cytokinin content, and canopy net photosynthetic rate (Pn), decreased in all nine cultivars as root-zone temperature increased from 20 to 35 °C, but the time and temperature at which the decline occurred varied for each parameter measured. Pn, cytokinin content, root number, and turf quality declined at 23, 27, 27, and 35 °C, respectively, after 28 days of exposure. Active root biomass, root number, root dry weight, turf quality, and rooting depth declined at 23, 25, 25, 25, and 35 °C, respectively, at 54 days. At a 31 °C root-zone temperature the decline in root number, cytokinin content, and turf quality occurred at 19, 37, and 47 days, respectively. The results suggest that root-zone temperatures of 23 °C or above this level were detrimental to root activities, Pn, and overall turf growth. Root and Pn decline at lower temperatures and earlier in the study than turf quality suggest that the disturbance of physiological activities of roots and leaves could lead to turfgrass quality decline at high root-zone temperatures.

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Hongmei Du, Zhaolong Wang and Bingru Huang

Heat stress may limit the growth of turfgrasses through the induction of oxidative stress, causing cellular and physiological damage. The objective of the study was to examine the association of heat and oxidative stresses between warm-season (C4) and cool-season (C3) turfgrasses. Plants of zoysiagrass (Zoysia matrella L. Merr. cv. Manila) (C4) and tall fescue (Festuca arundinacea Shreber cv. Barlexus) (C3) were exposed to optimal temperature conditions (24 °C for tall fescue and 34 °C for zoysiagrass) or heat stress (10 °C above the respective optimal temperature for each species) in growth chambers. Zoysiagrass exhibited less severe decline in turf quality and photochemical efficiency and less severe oxidative damage in cellular membranes as demonstrated by lower membrane electrolyte leakage and lipid peroxidation compared with tall fescue when both were exposed to heat stress. The activities of superoxide dismutase (SOD) and peroxidase (POD) declined with heat stress for both species, but to a lesser extent in zoysiagrass than in tall fescue, whereas catalase activity did not change significantly under heat stress and did not exhibit species variation. Our results demonstrate that the superior heat tolerance in zoysiagrass in comparison with tall fescue was associated with greater oxidative scavenging capacity as a result of the maintenance of higher SOD and POD activities.

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Qingzhang Xu, Bingru Huang and Zhaolong Wang

High air and soil temperatures are major factors limiting growth of cool-season grasses. A previous study by the authors reported that a soil temperature reduction of only 3 °C when air temperature was maintained at 35 °C significantly improved shoot and root growth of creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw. (syn. A. palustris Huds.)]. This study was designed to investigate the responses of photosynthetic activities of creeping bentgrass to lowered root-zone temperatures from the supraoptimal level when shoots were exposed to high air temperature. Two cultivars of creeping bentgrass, `L-93' and `Penncross', were exposed to the following air/root-zone temperature regimes in growth chambers and water baths: 1) optimal air and soil temperatures (20/20 °C, control); 2) lowering soil temperature by 3, 6, and 11 °C from 35 °C at high air temperatures (35/32, 35/29, and 35/24 °C); and 3) high air and soil temperatures (35/35 °C). Soil temperature was reduced from 35 °C by circulating cool water (18 °C) in water baths at variable flow rates. Both cultivars had similar responses to high or low root-zone temperatures with high air temperature. High air and root-zone temperatures caused significant reductions in canopy photosynthetic rate (Pcanopy), single-leaf photosynthetic rate (Pleaf), leaf chlorophyll content, photochemical efficiency (Fv/Fm), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, beginning on day 1 of high air and soil temperature stress for Pcanopy and Pleaf, and day 7 for chlorophyll content, Fv/Fm, and Rubisco activity. The 3 °C reduction in root-zone temperature at high air temperature had no effect on those photosynthetic parameters, except chlorophyll content. Reducing root-zone temperature by 6 °C or 11 °C while maintaining air temperature at 35 °C significantly improved Pcanopy, Poleaf, leaf chlorophyll content, Fv/Fm, and Rubisco activity. Single leaf photosynthetic rate at 35/24 °C was not different from the control level, but Pcanopy at 35/24 °C was lower than the control level. A reduction in root-zone temperature enhanced canopy and single-leaf photosynthetic capacity even though shoots were exposed to supraoptimal air temperature, which could contribute to improved turfgrass growth.

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Zhaolong Wang, Bingru Huang and Qingzhang Xu

Abscisic acid (ABA) is an important hormone regulating plant response to drought stress. The objective of this study was to investigate effects of exogenous ABA application on turf performance and physiological activities of kentucky bluegrass (Poa pratensis L.) in response to drought stress. Plants of two kentucky bluegrass cultivars, `Brilliant' (drought susceptible) and `Midnight' (drought tolerant), were treated with ABA (100 μm) or water by foliar application and then grown under drought stress (no irrigation) or well-watered (irrigation on alternate days) conditions in a growth chamber. The two cultivars responded similarly to ABA application under both watering regimes. Foliar application of ABA had no effects on turf quality or physiological parameters under well-watered conditions. ABA application, however, helped maintain higher turf quality and delayed the quality decline during drought stress, compared to the untreated control. ABA-treated plants exposed to drought stress had higher cell membrane stability, as indicated by less electrolyte leakage of leaves, and higher photochemical efficiency, expressed as Fv/Fm, compared to untreated plants. Leaf water potential was not significantly affected, whereas leaf turgor pressure increased with ABA application after 9 and 12 d of drought. Osmotic adjustment increased with ABA application, and was sustained for a longer period of drought in `Midnight' than in `Brilliant'. The results suggested that exogenous ABA application improved turf performance during drought in both drought-sensitive and tolerant cultivars of kentucky bluegrass. This positive effect of ABA could be related to increased osmotic adjustment, cell turgor maintenance, and reduced damage to cell membranes and the photosynthetic system.

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Zhaolong Wang, John Pote and Bingru Huang

This study was designed to examine whether shoot injury induced by high root-zone temperature is associated with changes in shoot detoxifying metabolism and to determine the level and duration of high root-zone temperatures that would induce physiological changes in two cultivars of creeping bentgrass (Agrostis stolonifera var. palustris Huds) differing in heat tolerance. Plants of `Penn A-4' (heat tolerant) and `Putter' (heat susceptible) were grown in sand and exposed to root-zone temperatures of 20 (control), 21, 22, 23, 25, 27, 31, and 35 °C in water baths while air temperature was maintained at 20 °C in a growth chamber. Turf quality, leaf cytokinin content, and antioxidant enzyme activities declined at increased soil temperatures and the duration of treatment for both cultivars. A decline in turf quality occurred following 40 days of exposure to 35 °C for `Penn A-4' and 26 days of exposure to 31 °C for `Putter'. The root-zone temperature causing the decline of isopentenyl adenosine and zeatin cytokinins was 25 °C at 37 d for `Putter' and 27 °C at 47 days for `Penn A-4'. The temperature causing the decline of superoxide dismutase and catalase activities was 25 °C and 27 °C at 33 days for `Putter' and 27 °C and 31 °C at 43 days for Penn A-4, respectively. Malondialdehyde content increased at 27 °C for `Putter' and 31 °C for `Penn A-4' at 43 days of treatment. The decline in cytokinin content and antioxidant enzyme activity occurred at a lower soil temperature and earlier during the treatment than the decline in turf quality, possibly contributing to turf quality decline. The root-zone temperatures causing the decline in turf quality, cytokinin content, and oxidative damage were higher in the heat-tolerant cultivar than heat-susceptible cultivar.

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Qingzhang Xu, Bingru Huang and Zhaolong Wang

Heat injury in creeping bentgrass (Agrostis stolonifera var. palustris Huds) has been associated with decreases in carbohydrate availability. Extending light duration may increase carbohydrate availability and thus improve growth of creeping bentgrass under heat stress. The objective of this study was to investigate whether turf performance and carbohydrate status could be improved by extending daily light duration for creeping bentgrass exposed to supraoptimal temperature conditions. `Penncross' plants were initially grown in growth chambers set at a day/night temperature of 20/15 °C and 14-hour photoperiod and then exposed to a day/night temperature of 33/28 °C (heat stress) and three different light durations: 14 (control), 18, and 22 hours (extended light duration) for 30 days. Turf quality and tiller density decreased with the duration of heat stress, as compared to the initial level at 20 °C, regardless of the light duration. However, both parameters increased with extended light duration from 14 to 18 or 22 hours. Extended light duration, particularly to 22 hours, also improved canopy net photosynthetic rate from -1.26 to 0.39 μmol·m-2·s-1 and daily total amount of carbon assimilation from -6.4 to 31.0 mmol·m-2·d-1, but reduced daily total amount of carbon loss or consumption to 50% through dark respiration compared to 14 hours treatment by the end of experiment. In addition, extending light duration from 14 to 22 hours increased water-soluble carbohydrate content in leaves both at the end of light duration and the dark period. These results demonstrated that extending light duration improved turf performance of creeping bentgrass under heat stress, as manifested by the increased tiller density and turf quality. This could be related to the increased carbohydrate production and accumulation. Supplemental lighting could be used to improve performance if creeping bentgrass is suffering from heat stress.

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Yiming Liu, Hongmei Du, Kai Wang, Bingru Huang and Zhaolong Wang

Salinity is a detrimental abiotic stress for plant growth in salt-affected soils. The objective of this study was to examine photosynthetic responses to salinity stress in two warm-season turfgrasses differing in salinity tolerance. Salt-tolerant species seashore paspalum (Paspalum vaginatum) and salt-sensitive species centipedegrass (Eremochloa ophiuroides) were exposed to salinity at three NaCl concentrations (0, 300, and 500 mm) in a growth chamber. Turf quality, relative water content (RWC), and leaf photochemical efficiency (Fv/Fm) declined, whereas electrolyte leakage (EL) increased under the two NaCl regimes for both grass species, and the changes were more dramatic in centipedegrass than that in seashore paspalum as well as in the higher salinity concentration. Two grass species showed different phytosynthetic responses to salinity stress. The earlier inhibition of photosynthesis in seashore paspalum was mainly associated with stomatal closure. As salinity increased and salinity stress prolonged, the inhibition of photosynthesis in seashore paspalum was mainly associated with non-stomatal factors. The inhibition of photosynthesis in centipedegrass was associated with both stomatal closure and non-stomatal factors at both salinity levels. The sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis demonstrated the Rubisco large subunit had no obvious decrease during the whole stress period under the 300-mm and 500-mm treatments in seashore paspalum, whereas it significantly decreased in centipedegrass under both the 300-mm and 500-mm treatments. The results indicated that the superior salinity tolerance in seashore paspalum, compared with centipedegrass, could be attributed to its maintenance of Rubisco stability, chlorophyll content, photochemical efficiency as well as photosynthetic rate (Pn) capacity under salinity stress.

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Diheng Zhong, Hongmei Du, Zhaolong Wang and Bingru Huang

Fatty acid metabolism may be involved in plant adaptation to drought stress. The objective of this study was to identify saturated and unsaturated fatty acids associated with leaf dehydration tolerance by comparing fatty acid composition and unsaturation levels at equivalent leaf water status of two bermudagrass genotypes contrasting in drought resistance. A drought-resistant hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) genotype (‘Tifway’) and a drought-sensitive bermudagrass (C. dactylon) genotype (‘C299’) were maintained under well-watered (control) or water-withheld (drought) conditions. Drought treatment was imposed until soil water content decreased to 5% or leaf relative water content (RWC) dropped to 28% to 29%. ‘Tifway’ maintained higher RWC and lower electrolyte leakage (EL) at 5 and 10 days of drought stress. Leaves of ‘Tifway’ maintained lower EL when RWC of both genotypes declined to the same level of water deficit (28% to 29%) by the end of drought periods. The degree of fatty acid unsaturation, expressed as the double bond index, decreased in both genotypes during drought stress, which was mainly associated with the decline in linoleic (C18:2) and linolenic acids (C18:3) and an increase in palmitic (C16:0) and stearic acids (C18:0). A lipid composition characterized by a greater amount of unsaturated fatty acids was detected in ‘Tifway’ relative to ‘C299’ exposed to the same level of water deficit, mainly as a result of a greater content of C18:2 and a lower content of C16:0 and C18:0. Our results suggest that the ability to maintain a greater composition of unsaturated fatty acids in membrane lipids may contribute to superior leaf dehydration tolerance in bermudagrass.