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Qingzhang Xu and Bingru Huang

Roots play important roles in plant responses to environmental changes. The objective of this study was to investigate seasonal changes and cultivar variation in root growth, respiratory activity, nitrogen uptake, and carbon allocation in relation to turf performance for two cultivars of creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.)] under field conditions. Two cultivars, `Penncross' and `L-93', were managed under USGA-specification putting green conditions, with daily irrigation and mowing at a 3-mm height from May to November in 1999 and 2000. Turf quality of both cultivars declined from the highest rating of 7 to 9 in May to 4 to 5 in August and September, and recovered to above 7 in October and November in both years. This corresponded to seasonal changes in root dry weight, dehydrogenase activity, nitrate reductase (NR) activity, carbon allocation to roots, and 15N uptake. Compared to Penncross, L-93 generally maintained better turf quality, as well as higher root dry weight, 15N uptake, NR activity, and carbon allocation during summer months. Previous studies often emphasize the important of a large, extensive root system. The results in the present study demonstrated that root metabolic activities followed the same seasonal pattern and cultivars variation as turf performance, and suggested that decline in root metabolic activities could be contributed to summer decline in turf quality for creeping bentgrass.

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Bingru Huang and Qingzhang Xu

This study was designed to compare and determine root growth and nutritional responses of creeping bentgrass cultivars that differ in heat tolerance to deferential, supraoptimal shoot and root temperatures. Shoots and roots of `Penncross' (heat sensitive) and `L-93' (heat tolerant) were exposed to four differential air/soil temperature regimes (20/20-control, 20/35, 35/20, and 35/35 °C) in water baths and growth chambers. Exposing roots to supraoptimal root temperature (35 °C) while maintaining shoots at normal temperature (20 °C), or at 35 °C in particular, reduced root fresh weight, root number, the content of N, P, and K in shoots and roots, and accelerated root death for both cultivars. High root temperature had a greater detrimental effects on root growth and nutrient accumulation than high shoot temperature for both cultivars. Reducing root temperature at supraoptimal shoot temperature improved root growth, reduced root mortality, and increased N, P, and K content in shoots and roots. Among the three nutrient elements, K was the most sensitive to changes in root temperature. L-93 generally maintained higher root fresh weight and number, and N, P, K content in shoots and roots, particularly K in roots, under high root (20/35 °C) or shoot/root (35/35 °C) temperatures. The results indicated that root growth and nutrient accumulation, particularly K, played an important role in creeping bentgrass tolerance to heat stress imposed to shoots by high air temperature or to roots by high soil temperatures. Reducing root temperature under supraoptimal ambient temperatures enhanced root growth and nutrient relations, and thus could lead to the improved shoot growth in cool-season grasses as reported previously.

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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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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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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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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.