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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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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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Jinmin Fu and Bingru Huang

Growth of cool-season grasses declines with increasing temperatures. The objective of this study was to determine the effects of elevated night temperature on turf quality, root mortality, and carbohydrate metabolism in creeping bentgrass (Agrostis stoloniferous L. var. palustris (Huds.) Farw (syn. A. palustris Huds.). Plants of `Penncross' were exposed to two night temperature regimes: 24 °C (higher night temperature); and 19 °C (lower temperature control) under the same day temperature (24 °C) in growth chambers for 45 days. Prolonged exposure of plants to higher night temperature reduced turf quality, canopy photosynthetic rate, whole-plant and root respiration rates during the day, translocation of newly fixed 14C assimilate to roots, and total nonstructural carbohydrate content in shoots and roots (including dead and live roots). Elevated night temperature increased root mortality and whole-plant and root respiration rates at night. Our results indicated that a decline in turf quality and increase in root dieback with high night temperature was mainly associated with increased night respiration rates of whole plant and roots and reduced carbohydrate availability.

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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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Bingru Huang and Hongwen Gao

To investigate shoot physiological responses to drought stress of six tall fescue (Festuca arundinacea) cultivars representing several generations of turfgrass improvement, forage-type `Kentucky-31', turf-type `Phoenix', `Phoenix', and `Houndog V', and dwarf-type `Rebel Jr` and `Bonsai' were grown in well-watered or drying soil for 35 days in a greenhouse. Net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), relative water content (RWC), and photochemical efficiency (Fv/Fm) declined during drought progression in all cultivars, but the time and the severity of reductions varied with cultivars and physiological factors. Pn, RWC, gs, and Tr decreased significantly for `Rebel Jr', `Bonsai', and `Phoenix' when soil water content declined to 20% after 9 days of treatment (DOT) and for `Falcon II', `Houndog V', and `Kentucky-31' when soil water content dropped to 10% at 15 DOT. A significant decrease in Fv/Fm was not observed in drought-stressed plants until 21 DOT for `Rebel Jr', `Bonsai', and `Phoenix' and 28 DOT for `Houndog V', `Kentucky-31', and `Falcon II'. The decline in Pn was due mostly to internal water deficit and stomatal closure under short-term or mild drought-stress conditions. After a prolonged period of drought (35 DOT), higher Pn in `Falcon II', `Houndog V', and `Kentucky-31' could be attributed to their higher Fv/Fm.

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Bingru Huang and Hongwen Gao

Drought is among the most limiting factors for turfgrass growth. Understanding genetic variations and physiological mechanisms in turfgrass drought resistance would facilitate breeding and management programs to improve drought resistance. The experiment was designed to investigate shoot physiological responses of six tall fescue (Festuca arundinacea Schreb.) cultivars representing several generations of turfgrass improvement to drought stress. Grasses were grown in well-watered or drying (nonirrigated) soil for 35 days in the greenhouse. Net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), relative water content (RWC), and photochemical efficiency (Fv/Fm) declined during drought progression in all cultivars, but the time and the severity of reductions varied with cultivar and physiological factors. The values of Pn, RWC, gs, and Tr decreased significantly for `Rebel Jr', `Bonsai', and `Phoenix' when soil water content declined to 20% after 9 days of treatment (DOT) and for `Houndog V', `Kentucky-31', and `Falcon II' when soil water content dropped to 10% at 15 DOT. A significant decrease in Fv/Fm was not observed in drought-stressed plants until 21 DOT for `Rebel Jr', `Bonsai', and `Phoenix' and 28 DOT for `Houndog V', `Kentucky-31', and `Falcon II'. The decline in Pn resulted mainly from internal water deficit and stomatal closure under mild drought-stress conditions. After a prolonged period of drought stress (35 DOT), `Falcon II', `Houndog V', and `Kentucky-31' maintained higher Pn than did `Rebel Jr', `Bonsai', and `Phoenix', which could be attributed to their higher Fv/Fm. This study demonstrated variation in drought resistance among tall fescue cultivars, which was related to their differential responses in photosynthetic capacity and water relations.

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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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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.