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TURFGRASS SCIENCE

Growth and Carbohydrate Metabolism of Creeping Bentgrass Cultivars in Response to Increasing Temperatures

Dep. of Horticulture, Forestry, and Recreation Resources, Kansas State Univ., Manhattan, KS 66506-5506 USA

bhuang{at}oz.oznet.ksu.edu

	ABSTRACT

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High temperature is a major factor limiting growth of creeping bentgrass (Agrostis palustris Huds.). Physiological causes of turf growth and quality decline with increasing temperature is not well understood. The objective of the study was to examine responses of growth and carbohydrate metabolisms to increasing temperatures in three creeping bentgrass cultivars. Sods of `Penncross', `ISI-AP-89150', and `SR 1020' were grown in growth chambers and exposed sequentially for 20 d to each of the following temperatures: 20, 24, 30, 34, and 38°C. Evaluation and measurements were made at 10 and 20 d after each sequential temperature increase. Decreased root viability and root dry matter production of all cultivars was observed after a 10-d exposure at 30°C and continued to decline with increasing temperatures. A decline in turf quality and leaf chlorophyll content (Chl) was observed at a 20-d exposure to 30°C. Turf quality, Chl content, and root viability of SR 1020 were higher than those of Penncross after a 10-d exposure at 30°C and 20 d at 34°C, and 10 d at 38°C, respectively. Canopy net photosynthetic rate (P_n) decreased with temperature in all cultivars. Dark respiration rates of whole plants (R_plant) increased with temperature up to 34°C, and then declined at 38°C. Daily carbon consumption to production ratio increased dramatically with temperature after 30°C, and R_plant exceeded P_n when temperature increased to 34 or 38°C in all cultivars. Plants grown at 30, 34, and 38°C had lower total nonstructural carbohydrate than those grown at 20 or 24°C. Results suggest that a decline in root activity of creeping bentgrass occurred before a decline in turf quality at temperatures above 30°C, and could be related to the imbalance between photosynthesis and respiration, and limited carbohydrate availability.

Abbreviations: Chl, chlorophyll content • LSD, least significance difference • P_n, net photosynthetic rate • R_plant, whole-plant respiration rate • R_plant+soil, respiration rate of plants and soil • R_soil, bare soil respiration rate

	INTRODUCTION

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INDIRECT HIGH TEMPERATURE STRESS is one of the major factors limiting use of cool-season grasses in transitional and warm climatic regions (Beard and Daniel, 1965; Carrow, 1996; Beard, 1997). The optimum temperature ranges for cool-season turfgrass are 15 to 24°C for shoot growth and 10 to 18°C for root growth (Beard, 1973). However, air and soil temperatures often exceed the optimal ranges for prolonged periods during mid-summer in these regions. Growth inhibition, and even death, can occur at suparoptimal temperatures in many cool-season grass species (Baker and Jung, 1968; Duff and Beard, 1974; Wehner and Watschke, 1981; Martin and Wehner; 1987). Kentucky bluegrass (Poa pratensis L.) produces maximum shoot growth at 22°C. Root growth declines as temperature increases to 25°C, and shoot dry weight at 35°C is less than half that at 22°C (Baker and Jung, 1968). Krans and Johnson (1974) observed chlorosis of creeping bentgrass after a few days of exposure to high temperatures from 35 to 45°C. Huang et al. (1998a, b) reported severe reduction in shoot and root growth and photosynthetic capacity of creeping bentgrass at 35°C.

Creeping bentgrass is one of the most widely used cool-season species on putting greens. Decline in turf quality and root dieback in creeping bentgrass during summer months are common, especially when grasses are closely mowed (Lucas, 1995; Carrow, 1996; Beard, 1997). Turfgrass quality decline in summer has been attributed to high temperatures (Beard and Daniel, 1965; Minner et al., 1983; Carrow, 1996; Beard, 1997). However, the critical temperature at which growth declines, and the physiological causes of growth decline in creeping bentgrass with increasing temperatures under close mowing are not well understood.

Carbohydrate metabolism is among the key physiological processes controlling plant growth because it provides energy and carbon skeletons. Some investigations suggest that growth reduction at high temperatures is related to limited carbohydrate availability (Baker and Jung, 1968; Watschke et al. 1970, 1972; DiPaola and Beard, 1992; Hull, 1992). Watschke et al. (1970) found carbohydrate reserves in Kentucky bluegrass decreased when day/night temperatures were increased from 10/10°C to 35/20°C. However, Duff and Beard (1974) reported that carbohydrate levels of creeping bentgrass increased by more than 50% at a day/night temperature of 40/30°C compared with 20/10°C. Whether declines in turf quality and root growth in creeping bentgrass under high temperatures and close mowing are related to changes in carbohydrate metabolism remains unclear.

This study was designed to: (i) investigate the responses of shoot and root growth, photosynthesis, respiration, and carbohydrate accumulation to increasing temperatures in two creeping bentgrass cultivars; and (ii) examine whether declines in shoot and root growth as temperature increases could be associated with an imbalance between photosynthesis and respiration and limited carbohydrate availability.

	Materials and methods

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Plant Materials and Growth Conditions
Creeping bentgrass cultivars Penncross, ISI-AP-89150, and SR 1020 were examined. Five-year-old sods of each cultivar were collected from a sand-based putting green in the Rocky Ford Turfgrass Research Center, Manhattan, KS. Sods were washed free of sand and established in an autoclaved sand and fritted clay mix (Profile, ALMCOR, Deerfield, IL) (9:1, v/v) in a greenhouse. The soil medium was contained in polyvinyl chloride (PVC) tubes measuring 5-cm diam by 60 cm deep. Plants were maintained in a greenhouse for 60 d and then were transferred to growth chambers with an average day/night temperature of 20/15°C, a photosynthetically active radiation of 600 µmol m^-2 s^-1 at canopy level, and a 13-h photoperiod. Plants were mowed daily to the lowest height possible (about 4 mm) with an electric hand-held clipper. Turf was watered daily to prevent drought stress and fertilized weekly with half-strength Hoagland's solution (Hoagland and Arnon, 1950).

Treatments, Experimental Design, and Statistical Analysis
After acclimating to the growth chamber conditions for 30 d, plants were sequentially exposed for 20 d to 20°C (constant day/night, control), 24, 30, 34, and 38°C. At 20 d of each temperature treatment, plants in four containers were destructively harvested for shoot and root analyses. Plants in the remaining containers were exposed to the next temperature treatment. Each temperature regime was conducted in four growth chambers. Cultivars were arranged randomly within each temperature regime. All measurements were made on plants in four containers per treatment. The experimental design was a completely randomized split-plot design with temperature as the main plot and cultivar as the subplot in four replicates. Effects of temperature and cultivar were determined by analysis of variance according to the general linear model procedure of the Statistical Analysis System (SAS Institute Inc., Cary, NC). To define how each cultivar responded to the different temperature regimes, temperature effect analysis was conducted separately within each cultivar. Differences between treatment means and cultivars were separated by the least significance difference (LSD) test at the 0.05 probability level.

Measurements
After 10 and 20 d at each temperature regime, various shoot and root parameters were measured. Turf quality was rated visually on the basis of color, uniformity, and density on a 0 to 9 scale, where 0 = worst quality, 6 or above = acceptable, and 9 = best quality. Leaf chlorophyll was extracted by soaking 0.1 g of leaves in 8 mL dimethyl sulfoxide for 72 h as described in Hiscox and Israelstam (1979). Absorbance of extractants was measured at 663 nm and 645 nm with a spectrophotometer (Spectronic Instruments, Rochester, NY). Leaf chlorophyll (Chl) content was calculated by the formula of Arnon (1949). Canopy net photosynthetic rate (P_n) was measured during photoperiod from 1000 to 1400 h with a LI-6400 portable gas exchange system (LI-COR Inc.,Lincoln, NE). Dark respiration rate of whole plants (shoots and roots) and soil (R_plant+soil), and that of soil without grasses growing (bare soil R_soil), were measured during night period from 1900 to 2300 h at a 10- and 20-d exposure to 20, 24, 34, and 38°C, and a 10-d exposure at 30°C. Respiration rate was not measured at a 20-d exposure at 30°C due to the instrument break-down. Bare soils (sand and fritted clay mix) without grasses growing was filled in PVC tubes at the same time, and received the same amount of water and fertilizers as for grassed soil columns. The soil mix contained no organic matters and was autoclaved before filling in the tubes to minimize soil microbial respiration. Respiration rates of whole plants (R_plant) was estimated as the difference between R_plant+soil and R_soil. The P_n and R_plant were expressed as CO₂ uptake and evolution per unit turf canopy area, respectively. Daily carbon consumed in respiration as a proportion of that produced in photosynthesis was calculated with the data of P_n and R_plant integrated over a 13-h photoperiod and an 11-h dark period, respectively.

After shoot measurements at the end of each temperature treatment period (20 d), plants in four containers were randomly sampled and destructively harvested. Shoots were separated from roots and dried in an 85°C oven for 48 h. Dry shoots were ground with a tissue grinder and stored in sealed vials for carbohydrate analysis. Total nonstructural carbohydrate (TNC) content of shoots was measured using the method described by Smith (1981). All roots in each container were washed free of soil. Root viability was determined on all roots from a container by measuring dehydrogenase activity with the triphenyltetrazolium chloride (TTC) reduction technique (Knievel, 1973; McMichael and Burke, 1994). After root viability measurement, roots were dried in an 85°C oven for 48 h, and root dry weight was determined.

	Results

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Turf Quality and Leaf Chlorophyll Content
Turf quality began to decline significantly from the respective control levels (at 20°C) for all cultivars after a 10-d exposure at 34°C or a 20-d exposure at 30°C for Penncross and ISI-AP-89150 (Fig. 1) . None of the cultivars exhibited acceptable quality after 10 d at 34°C. However, when plants were exposed to 30°C for 20 d, 34°C for 10 or 20 d, or 38°C for 10 d, SR 1020 turf quality was significantly better than that of Penncross, but not ISI-AP-89150 except after a 20-d exposure at 34°C. After a 20-d exposure at 38°C, turf quality of all cultivars declined to the lowest level and no differences were observed among cultivars.

View larger version (20K): [in this window] [in a new window]   Fig. 1 Turf quality response to increasing temperature for Penncross, ISI-AP-89150, and SR 1020 creeping bentgrass at 10 and 20 d. Dotted, horizontal line indicates the control level averaged over the three cultivars. Solid, vertical lines indicate LSD values for cultivar comparisons within a given temperature treatment. Dotted, vertical lines indicate LSD values for temperature treatment comparisons within a given cultivar. `NS' indicates no significant difference at

View larger version (21K): [in this window] [in a new window]   Fig. 2 Leaf chlorophyll content response to increasing temperature for Penncross, ISI-AP-89150, and SR 1020 creeping bentgrass at 10 and 20 d. Dotted, horizontal line indicates the control level averaged over the three cultivars. Solid, vertical lines indicate LSD values for cultivar comparisons within a given temperature treatment. Dotted, vertical lines indicate LSD values for temperature treatment comparisons within a given cultivar. `NS' indicates no significant difference at

View larger version (22K): [in this window] [in a new window]   Fig. 3 Root dry matter production response to increasing temperature for Penncross, ISI-AP-89150, and SR 1020 creeping bentgrass at 10 and 20 d. Dotted, horizontal line indicates the control level averaged over the three cultivars. Solid, vertical lines indicate LSD values (P = 0.05) for cultivar comparisons within a given temperature treatment. Dotted, vertical lines indicate LSD values for temperature treatment comparisons within a given cultivar. `NS' indicates no significant difference at

View larger version (22K): [in this window] [in a new window]   Fig. 4 Root viability, expressed as TTC reduction, in response to increasing temperature for Penncross, ISI-AP-89150, and SR 1020 at creeping bentgrass 10 and 20 d. Dotted, horizontal line indicates the control level averaged over the three cultivars. Solid, vertical lines indicate LSD values for cultivar comparisons within a given temperature treatment. Dotted, vertical lines indicate LSD values for temperature treatment comparisons within a given cultivar. `NS' indicates no significant difference at

View larger version (25K): [in this window] [in a new window]   Fig. 5 Net photosynthetic rate of the canopy (Pn) and dark respiration rate of whole plants (Rplant) in response to increasing temperatures for Penncross, ISI-AP-89150, and SR 1020 creeping bentgrass at 10 and 20 d. Solid and dotted lines indicate LSD values for temperature treatment comparisons of Pn and Rplant, respectively, within a given cultivar

View larger version (26K): [in this window] [in a new window]   Fig. 6 Daily carbon consumed in respiration, as a proportion of that produced in photosynthesis, in response to increasing temperatures for Penncross, ISI-AP-89150, and SR 1020 creeping bentgrass at 10 and 20 d. Horizontal line indicates the level at which carbon consumption equals production. Vertical bars indicate LSD values for treatment and cultivar comparisons. `NS' indicates no significant difference at

View larger version (49K): [in this window] [in a new window]   Fig. 7 Carbohydrate accumulation response to increasing temperature for Penncross, ISI-AP-89150, and SR 1020 creeping bentgrass at 20 d. Columns marked with the same letters within a given cultivar were not significantly different based on the LSD test

	Discussion

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Root growth and viability also decreased with increased temperatures, but at lower temperatures and at shorter stress duration (10 d of 30°C) than turf quality, indicating that roots were more sensitive to high temperatures than shoots. Schmidt and Blaser (1967) measured root and shoot growth of `Cohansey' bentgrass at 12, 24, and 36°C. They found that high temperature did not have adverse effects on shoot growth, but root weights of plants grown at 36°C were reduced by 30 and 46% compared with those of plants grown at 24°C and 12°C, respectively.

The declines in shoot and root growth for all cultivars with increasing temperature, and cultivar variations in growth responses to high temperature, could be explained by the effects of high temperature on photosynthesis, respiration, and carbohydrate accumulation processes. Photosynthesis in most species is extremely sensitive to high temperatures, and is often the first process that is affected (Paulsen, 1994). Along with declines in turf quality and root growth with increasing temperatures, canopy P_n also declined significantly for all cultivars, but did not differ among cultivars. Watschke et al. (1970) and (1972) reported that Kentucky bluegrass cultivars varied in P_n under high temperature conditions and cultivars that were adapted to high temperatures exhibited the highest P_n during heat stress. High temperatures also adversely affect shoot and root growth by affecting respiration (Paulsen, 1994). The R_plant of whole plants increased with temperature to 34°C and then declined as temperature increased to 38°C. Under high temperature conditions (34 and 38°C), R_plant exceeded P_n in all three cultivars, which resulted in daily carbon consumption that was several times higher than carbon production. Although cultivars did not differ in the absolute levels of P_n and R_plant, daily carbon consumption to production ratios for Penncross were higher than those for SR 1020. In a previous study, Huang et al. (1998a) found higher R_plant than P_n under high temperatures (35°C), with greater ratios of R_plant to P_n for a relatively heat-sensitive cultivar (Penncross) than a relatively heat-tolerant cultivar (Crenshaw). Watschke et al. (1972) concluded that best performing Kentucky bluegrass cultivars under high temperatures maintain not only high P_n, but also low respiration rate. These studies pointed out the importance of the balance between photosynthesis and respiration during high temperature stress.

The imbalance between photosynthesis and respiration during high temperature stress could lead to limited carbohydrate availability. Shoots grown at 30, 34, and 38°C had significantly lower TNC than those grown at 20 and 24°C. Schmidt and Blaser (1967) also reported adverse effects of high temperatures on carbohydrate accumulation in creeping bentgrass, which was attributed to increased respiration rate. Carbohydrates constitute the energy currency within turfgrass plants. Growth, differentiation, and maintenance are supported by carbohydrates obtained from current photosynthate or that stored in tissues (Hull, 1992). Therefore, the maintenance of balanced photosynthesis and respiration and adequate carbohydrate accumulation is important for maintaining high quality turf during heat stress. Low canopy photosynthetic rates are inevitable on a creeping bentgrass green because of the limited amount of leaf area available for photosynthesis under close mowing. Raising mowing heights at a time when golfers demand a faster putting surface is not practical. However, improving photosynthetic efficiency or/and reducing carbohydrate consumption by maintaining low respiration rates could improve turf survival during the summer. This could be achieved by reducing soil temperature. Lowering soil temperature to 20°C while maintaining shoots under high air temperature (35°C) conditions enhanced canopy photosynthetic rate and turf quality and reduced respiration rate for several cultivars of creeping bentgrass (Huang, 1999, unpublished data).

In summary, our results demonstrated that 30°C could be a critical temperature leading to a decline in turf quality and root growth of creeping bentgrass cultivars. These declines appeared to be related to the imbalance in photosynthesis and respiration, and reduced carbohydrate availability under high temperature conditions. Management practices and breeding programs that help turf to conserve carbohydrates, or stimulate carbohydrate production, would enhance high-temperature tolerance of creeping bentgrass.Martin Wehner 1987

	ACKNOWLEDGMENTS

 
Research funds were provided by Kansas Turfgrass Foundation and the United States Golf Association. The authors thank Mr. Yiwei Jiang for assistance with the carbohydrate analysis. Thanks also go to Drs. Jack Fry and Mary Beth Kirkham for reviewing the manuscript.

	NOTES

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Contribution No.99-264-J from Kansas Agric. Exp. Stn.

Received for publication April 2, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 

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