Crop Science 41:1878-1883 (2001)
© 2001 Crop Science Society of America
TURFGRASS SCIENCE
Lowering Soil Temperatures Improves Creeping Bentgrass Growth under Heat Stress
Qingzhang Xu and
Bingru Huang*
Dep. Plant Sci., Rutgers University, New Brunswick, NJ 08901
* Corresponding author (huang{at}aesop.rutgers.edu)
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ABSTRACT
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High soil temperature is a major factor limiting growth of cool-season grasses. The objectives of this study with creeping bentgrass (Agrostis palustris Huds.) were to examine growth responses to lowering soil temperatures from the supraoptimal level under high air temperature conditions, and to determine the minimum reduction in soil temperature that could improve shoot and root growth. Two creeping bentgrass cultivars, L-93 and Penncross, were exposed to the following air and soil temperature regimes in growth chambers and water baths: (i) optimal air and soil temperatures (20/20°C, control); (ii) lowering soil temperature by 3, 6, and 11°C from 35°C at high air temperatures (35/32, 35/29, 35/24°C); and (iii) 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 rates. Turf quality, leaf chlorophyll content, shoot growth rate, and root/shoot ratio (R/S) ratio increased as soil temperature was reduced from 35 to 32°C to a greater extent for Penncross than for L-93. Significant increases in tiller density, clipping yield, root number, and fresh weight were not observed until soil temperature was reduced to 29°C. When soil temperature was reduced to 24°C, turf quality, shoot growth rate, and R/S were maintained at the same levels as in the control regime. These results suggested that reducing soil temperature by 3°C or more is effective in improving turf quality and shoot and root growth of creeping bentgrass exposed to high air temperatures.
Abbreviations: R/S, root/shoot ratio
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INTRODUCTION
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CREEPING BENTGRASS is a widely used cool-season grass on golf greens in the northern region and transitional zone of the United States. Cool-season grasses maintain their maximum growth rate at temperatures ranging from 15 to 24°C for shoots and 10 to 18°C for roots (Beard, 1973). Turf quality of creeping bentgrass often decreases when temperatures exceed the optimal levels for extended time periods (Carrow, 1996; Xu and Huang, 2000a). Supraoptimal temperatures reduce leaf photosynthetic rate, increase shoot and root respiration, decrease total nonstructural carbohydrate contents in shoots and roots, and inhibit C allocation to roots in creeping bentgrass (Huang et al., 1998; Xu and Huang, 2000a, 2000b; Liu et al., 2001, unpublished data).
Supraoptimal soil temperature is more detrimental than air temperature for root and shoot growth (Ramcharam et al., 1991; Ruter and Ingram, 1990, 1992; Paulsen, 1994; Xu and Huang, 2000a,b, 2001) and nutrient uptake (Klock et al., 1997; Huang and Xu, 2000). High soil temperatures increased respiration rate in roots (Klock et al., 1997; Xu and Huang, 2000b), which might cause carbohydrate starvation and root dieback (Carrow, 1996). Reducing soil temperature from a supraoptimal level of 35°C to an optimal level of 20°C while maintaining air temperature at 35°C increased canopy photosynthesis, total nonstructural carbohydrate content in roots and shoots, and root growth and shoot growth and improved turf quality of creeping bentgrass up to the same level as that at optimal air and soil temperatures (Xu and Huang, 2000a). Reducing soil temperature also has been reported to improve growth and photosynthesis in other species (Kuroyanagi and Paulsen, 1988; Ruter and Ingram, 1992).
Even though reducing soil temperature from a supraoptimal level such as 35°C to an optimal level such as 20°C is effective in improving plant growth, it is not practical on golf greens and could be costly. However, soil temperature or turf canopy temperature can be reduced by 2 to 5°C through routine management practices such as syringing (Dipaola, 1984) and raising mowing height (Beard and Sifers, 1997), use of fans (Taylor et al., 1993; Beard, 1998), and subsurface cooling and aeration (Dodd et al., 1999; Camberato et al., 1999). The minimum reduction in soil temperature from the supraoptimal level that would be effective in improving root growth, shoot growth, and turf quality under high air temperatures has not been determined for creeping bentgrass. Finding that temperature level could be of great significance for developing effective management practices to maintain high quality bentgrass greens during summer.
Therefore, the objectives of this study were to examine growth responses of two creeping bentgrass cultivars to decreased soil temperatures and to determine the minimum reduction in soil temperature required for maintaining turf quality at high air temperatures.
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MATERIALS AND METHODS
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Sod pieces of creeping bentgrass (L-93 and Penncross) were collected from the Rocky Ford Turfgrass Research Center at Manhattan in Kansas and transplanted into a mixture (9:1, v/v) of sand and fritted clay (Profile Products Ltd., Deerfield, IL) in clear polyethylene bags (5 cm in diameter and 40 cm in length, with eight holes drilled at the bottom for drainage). The polyethylene bags were placed in opaque polyvinylchloride tubes of the same diameter and length, which were installed vertically in water baths with the lower open end exposed from the bottom of the water bath for drainage (Fig. 1)
. These were designed in a way that enabled plant growth in well-drained soil in polyethylene bags, while soil temperature was controlled at a constant, predetermined level. Plants were grown in growth chambers at 20/15°C (day/night), 400 mmol m-2 s-1 photosynthetic photon flux density, and a 14-h photoperiod for 60 d before differential air/soil temperature treatments were imposed. Before and during temperature treatments, turf was mowed daily at a 3- to 4-mm height with an electric hair clipper, watered daily until soil moisture reached field capacity (when free drainage ceased from the bottom of the plant containers), and fertilized weekly with 50 mL of full-strength Hoagland's nutrient solution (Hoagland and Arnon, 1950).
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Fig. 1. Diagram of a water bath. The soil temperatures were regulated by the circulating water (18°C) at variable flow rate. Low soil temperature was achieved by increasing flow rate.
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Plants were exposed to the following air/soil temperature regimes: (i) optimum air and soil temperatures (20/20°C, control); (ii) lowering soil temperature by 3, 6, or 11°C from 35°C at high air temperature (35/32, 35/29, and 35/24°C); and (iii) high air and high soil temperatures (35/35°C, heat stress). Shoots were maintained at the ambient temperature (35°C) of the growth chambers, which was regulated by a temperature controller. Soil temperatures were controlled by maintaining the entire root zone (40-cm-long soil column in a polyethylene bag) in water baths with circulating cool water (18°C) around the plant containers at variable rates to generate different soil temperatures required (Fig. 1). Water was circulated continuously to maintain a constant and uniform temperature. Water levels were maintained at the top edges of both water baths during the experimental period.
Air temperatures at 5 cm from the canopy, canopy temperature, and soil temperatures at different depths from the surface were measured with thermocouples connected to a thermometer (Fig. 2) . In the control, air temperature was maintained at approximately 20°C and soil temperatures at 18 to 20°C. In the heat stress treatment, air temperature was 35°C and soil temperature at the 0- to 40-cm depth ranged from 32 to 34°C. In the reduced soil temperature treatments, air temperature was 35°C and soil temperatures ranged from 22 to 26°C for the 35/24°C treatment, 27 to 31°C for the 35/29°C treatment, and 30 to 33°C for the 35/32°C treatment.
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Fig. 2. Temperature profiles in soil and air under five differential air and soil temperature regimes. Bars on data points indicate standard errors; some were too small to be shown.
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Temperatures and cultivars were arranged in a split-plot randomized design with temperature as the main plot and cultivar as the subplot. Measurements were made at various times to study time response to temperature treatments. Cultivars were arranged randomly in each temperature regime in each chamber. All measurements were taken on two tubes (two subsamples) sampled randomly in each treatment at various times of treatments (last harvest at 21 d of treatment). The experiment was run three times in different growth chambers and water baths as three replications. Six samples were used for each measurement in this experiment. Growth chambers and water baths were swapped among temperature treatments to minimize the equipment errors at each run. Effects of temperature, cultivar, time of treatment, and their interactions were determined by analysis of variance according to the general linear model procedure of Statistical Analysis System (SAS Institute, Cary, NC). Differences among treatments and cultivars were determined by the least significance difference (LSD) test at the 0.05 probability level.
Turf quality based on color, density, and uniformity was visually rated weekly after treatments were initiated, on a scale of zero (worst, plants dead and brown) to nine (best, plants healthy and green). Grasses rated at six or above were considered to have acceptable quality. Shoot growth rate was estimated by the difference in canopy height in a 1-d period using a ruler. Clipping yield was measured as the fresh weight of leaves after clipping.
Plants were harvested weekly to determine leaf chlorophyll content, tiller density, and fresh weight and number of roots. Leaf chlorophyll content was measured according to the method of Arnon (1949). Fresh leaf samples (0.05 g) were extracted in dimethyl sulfoxide for 48 h in darkness before measurement. Thirty to 40 plants were selected randomly from each tube to estimate tiller number per plant. Plants from two tubes at each treatment in each replication (total six tubes in three replications) were harvested to determine fresh weight and number of roots. Fresh weight of all roots in each tube was determined after roots were washed free of soil and blotted dry with paper towels. The number of crown roots (roots directly attached to the crown) in each tube was counted by hand. Root/shoot ratio was calculated from the fresh weights of roots and shoots in each tube.
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RESULTS
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Most of main effects of treatments, date, and cultivars were significant at P < 0.05 for the parameters examined (Table 1), except the main effects of cultivars for clipping yield and R/S and the main effects of dates for R/S. The interactions between treatment and cultivars were significant only for tiller density and shoot growth rate, while all interaction between treatment and dates were significant.
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Table 1. Analysis of variance (ANOVA) for various parameters as affected by reducing soil temperature in two creeping bentgrass cultivars.
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Shoot Growth
Turf quality at 35/35°C declined to below the control level beginning at 7 d for both cultivars (Fig. 3)
. Reducing soil temperatures from 35°C to 32, 29, and 24°C while maintaining air temperature at 35°C significantly increased turf quality at 14 and 21 d for both L-93 and Penncross; 35/32 and 35/29°C were equally effective. Turf quality of grasses at 35/24°C stayed at the level of the control (20/20°C) at 14 and 21 d for L-93 and at 21 d for Penncross.
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Fig. 3. The responses of turf quality of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for quality comparison over time within a temperature.
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Leaf chlorophyll content at 35/35°C decreased to below the control level at 7 d for Penncross and at 14 d for L-93 (Fig. 4) . Reducing soil temperature increased leaf chlorophyll content at 14 and 21 d of treatment for both cultivars beginning at 35/32°C. The effectiveness increased with further reduction in soil temperatures. Chlorophyll content was maintained at the control level at 35/24°C for L-93 and at 35/29 and 35/24°C for Penncross during the entire treatment period.
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Fig. 4. The responses of leaf chlorophyll content of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for leaf chlorophyll comparison over time within a temperature.
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Tiller density of L-93 increased with reduction in soil temperature from 35°C to 32, 29, or 24°C during the entire treatment period (Fig. 5)
. For Penncross, tiller density at 14 and 21 d of 35/24°C and at 21 d of 35/32 and 35/29°C was higher than that at the same time of 35/35°C. By the end of the treatment (21 d), tiller density had not returned to the control level with reduced soil temperature of 24°C for either cultivar.
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Fig. 5. The responses of tiller density of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for tiller density comparison over time within a temperature.
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Shoot growth rate at 35/35°C declined rapidly to below the control level, beginning at 2 d of treatment for both cultivars (Fig. 6)
. Reducing soil temperature significantly enhanced shoot growth rate beginning at 2 d of 35/32°C for both cultivars, but the increase was more pronounced for Penncross than L-93. Shoot growth rate at 35/24°C reached the same level as at 20/20°C for L-93 during most of the experimental period and for Penncross at 2, 8, 14, and 21 d.
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Fig. 6. The responses of shoot growth rate of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for shoot growth rate comparison over time within a temperature.
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Significant declines in clipping yield were observed at 4 d of 35/35°C for both cultivars (Fig. 7)
. Clipping yield at reduced soil temperature was higher than that at 35/35°C, beginning at 4 d of all low soil temperature treatments for L-93 and at 4 d of 35/24°C and 12 d of 35/29 and 35/32°C for Penncross. Clipping yields of both cultivars at 35/24°C were not different from that of control plants at 4 d of treatment and thereafter declined to below the control level, but were still significantly higher than those at 35/35°C.
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Fig. 7. The responses of clipping yield of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for clipping yield comparison over time within a temperature.
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Root Growth
Crown root number at 35/35°C decreased to below the control level at 7 d for both cultivars (Fig. 8)
. Reducing soil temperature from 35°C to 32, 29, or 24°C increased root number to levels higher than that at 35/35°C at 14 and 21 d for L-93. For Penncross, reducing soil temperature to 29 and 24°C increased root number at 7 d; reducing soil temperature to 32°C had no significant effect on root number.
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Fig. 8. The responses of root number of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for root number comparison over time within a temperature.
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A significant decline in root fresh weight was observed at 7 d of 35/35°C for both cultivars (Fig. 9)
. Root fresh weight increased with reducing soil temperature from 35 to 24°C for both cultivars, but never reached the control level. Reducing soil temperature to 29 or 32°C had no effect on root fresh weight for either cultivar.
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Fig. 9. The responses of root fresh weight of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for root fresh weight comparison over time within a temperature.
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Root/Shoot Ratio
High air and soil temperatures reduced the R/S ratio beginning at 7 d of treatment for both cultivars (Fig. 10)
. The R/S ratio increased with reducing soil temperature at 14 and 21 d for Penncross and at 21 d for L-93 when temperature was reduced to 29 or 32°C and during the entire treatment period for both cultivars when temperature was reduced to 24°C. The R/S ratio at 35/24°C was not significantly different from that of the control during the entire treatment period for L-93 and at 7 and 21 d for Penncross.
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Fig. 10. The responses of root/shoot (R/S) ratio of two creeping bentgrass cultivars to reducing root-zone temperature at different times of treatment. Vertical bars on the top indicate LSDs (P = 0.05) for treatment comparison at a given time of treatment. Vertical bars on the right indicate LSDs (P = 0.05) for R/S comparison over time within a temperature.
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DISCUSSION
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Reducing soil temperature by only 3°C while maintaining air temperature at 35°C significantly improved turf quality, chlorophyll content, tiller density, shoot growth rate, and clipping yield during most of the treatment period for both L-93 and Penncross. The effectiveness of lowering soil temperature for improved growth increased as temperature was reduced towards the optimal range. Dodd et al. (1999) and Cambernato et al. (1999) reported that subsurface cooling and aeration in field tests decreased soil temperature at 5 and 20 cm by 2.2°C, which did not affect turf visual quality and rooting depth, but did result in a 23% increase shoot density at the end of a hot summer.
The response to soil temperature reduction varied with cultivars and growth parameters. Performance of L-93 appeared to be better than that of Penncross. Specifically, turf quality (Fig. 3), tiller density, (Fig. 5), and root fresh weight (Fig. 9) of Penncross were enhanced to a greater extent by a 3°C reduction soil temperature. Toubakaris and McCarty (2000) reported that older bentgrass cultivars (particularly Penncross) were more responsive to temperature changes than newer cultivars (including L-93). For both cultivars in our study, turf quality, leaf chlorophyll content, shoot growth rate, and R/S ratio increased with reducing soil temperature at the initiation of the treatments and were maintained at the control level during most of the treatment period at 35/24°C. Tiller density, clipping yield, root number, and fresh weight also increased with reducing soil temperature but never reached the control level for either cultivar.
Reducing soil temperature increased photosynthetic rate and carbohydrate availability in leaves (Xu and Huang, 2000b), which could be related to the increased turf quality, chlorophyll content, and leaf growth rate. The improvement in shoot growth by reducing soil temperature could be mediated by root physiological activities including cytokinin synthesis and water and nutrient uptake. Various studies (Yeager et al., 1991; Hood and Mills, 1994; Klock et al., 1997; Huang and Xu, 2000) have shown that nutrient uptake capacity increased with reduced soil temperature. Root hydraulic conductivity also has been enhanced with low soil temperature at high air temperatures (Graves et al., 1991). Reducing soil temperature from 35 to 20°C not only increased the cytokinin synthesis in roots, but also enhanced cytokinin supply from roots to shoots in creeping bentgrass (Liu et al., 2001, unpublished data).
In conclusion, our results suggest that high quality creeping bentgrass could be maintained during periods of supraoptimal air temperature by reducing soil temperature 3°C or more.
Received for publication January 11, 2001.
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ABSTRACT
INTRODUCTION
