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Dep. of Horticulture, Forestry & Recreation Resources, 2021 Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506. Contribution no. 07-185-J from the Kansas Agric. Exp. Station
* Corresponding author (bremer{at}ksu.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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), dry matter production, and lower electrolyte leakage and soil surface temperatures than KBG and TF under high temperature. Cumulative P during the study was 16 and 24% greater in HBG than in KBG and TF, respectively. Green leaf area index (LAI) in HBG was not affected by high temperature, but LAI was reduced by 29% in KBG and 38% in TF. Differences in drought resistance were negligible among species. The combination of high temperature and drought caused rapid declines in visual quality and dry matter production, but HBG generally performed better. Results indicated greater heat resistance, but not drought resistance, in HBG than in KBG or TF.
Abbreviations: DOT, days of treatment • EL, electrolyte leakage • ET, evapotranspiration • HBG, a hybrid between Kentucky and Texas bluegrass • KBG, Kentucky bluegrass • LAI, leaf area index • P, gross photosynthesis • TF, tall fescue
Dep. of Horticulture, Forestry & Recreation Resources, 2021 Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506. Contribution no. 07-185-J from the Kansas Agric. Exp. Station
* Corresponding author (bremer{at}ksu.edu).
High temperature and drought stresses may reduce quality in cool-season turfgrasses during summer months in the transition zone. This growth chamber study was conducted to evaluate effects of high temperature and drought on physiology and growth of ‘Apollo’ Kentucky bluegrass (Poa pratensis L.) (KBG), ‘Dynasty’ tall fescue (Festuca arundincea Schreb.) (TF), and ‘Thermal Blue’, a hybrid (HBG) between KBG and Texas bluegrass (Poa arachnifera Torr.). Turfgrasses were exposed for 48 d to supra-optimal (high temperature; 35/25°C, 14-h day/10-h night) and optimal (control; 22/15°C, 14-h day/10-h night) temperatures under well-watered (100% evapotranspiration [ET] replacement) and deficit (60% ET replacement) irrigation. Heat resistance was greater in HBG, which had greater visual quality, gross photosynthesis (P), dry matter production, and lower electrolyte leakage and soil surface temperatures than KBG and TF under high temperature. Cumulative P during the study was 16 and 24% greater in HBG than in KBG and TF, respectively. Green leaf area index (LAI) in HBG was not affected by high temperature, but LAI was reduced by 29% in KBG and 38% in TF. Differences in drought resistance were negligible among species. The combination of high temperature and drought caused rapid declines in visual quality and dry matter production, but HBG generally performed better. Results indicated greater heat resistance, but not drought resistance, in HBG than in KBG or TF.
Abbreviations: DOT, days of treatment • EL, electrolyte leakage • ET, evapotranspiration • HBG, a hybrid between Kentucky and Texas bluegrass • KBG, Kentucky bluegrass • LAI, leaf area index • P, gross photosynthesis • TF, tall fescue
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Hybrid bluegrasses (HBG), which are genetic crosses between native Texas bluegrass and Kentucky bluegrass (KBG), may have greater heat and drought resistance than other cool-season grasses. Hybrid bluegrasses have similar visual qualities to KBG, which is a fine-textured, cool-season turfgrass commonly used in lawns and golf courses in the United States (Read et al., 1999; Turgeon, 2002). Consequently, new cultivars of HBG are being investigated as potential water-saving, heat-resistant alternatives to current cool-season turfgrasses.
Abraham et al. (2004) evaluated, in a growth chamber study, the drought resistance of 30 HBG and their genetic parents and concluded that their drought resistance varied significantly. Those researchers also determined that highly drought resistant hybrids could be achieved by selecting first generation hybrids with good drought resistance and backcrossing them with elite drought resistant genotypes of KBG. In field tests in Colorado, USA, ‘Reveille’ HBG used significantly less water while maintaining higher quality than ‘Bensun’s A-34' KBG (Supplick-Ploense and Qian, 2005). Bremer et al. (2006) reported little difference in the general performance or drought resistance among two HBG (Thermal Blue and ‘Dura Blue’) and a KBG (Apollo) in a field study in Kansas, USA. Dura Blue HBG and Apollo KBG were the most desirable among five cool-season turfgrasses under different N rate levels in Tennessee, USA (Teuton et al., 2005). Thermal Blue and Dura Blue HBG may be acceptable replacements for KBG in the Upper Midwest, USA, because they had similar quality and earlier spring green-up than KBG (Stier et al., 2005). However, little information is available about the effects of both high temperature and drought on HBG.
The objectives of this study were to evaluate the effects of high temperature and drought stress on the growth, appearance, and physiology of HBG, KBG, and tall fescue (TF).
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Lysimeters were then transferred to growth chambers at 22/15°C, 14-h day/10-h night with photosynthetically active radiation of 580 µmol m–2 s–1 during the daylight period. The turfgrasses were kept well-watered in the growth chambers for 14 d to allow for acclimation to growth chambers before treatments. Turfgrasses were mowed and watered every 3 d and fertilized every 6 d with a solution supplying 20 kg N ha–1 (15N–13P–12K) during the treatments.
Treatments and Experimental Design
Thermal Blue HBG, Apollo KBG, and Dynasty TF were exposed to the following treatments in growth chambers: (i) supra-optimal temperature (high temperature; 35/25°C, 100% evapotranspiration [ET]); (ii) drought (22/15°C, 60% ET); (iii) high temperature and drought (35/25°C, 60% ET); and (iv) optimal temperature (control; 22/15°C, 100% ET).
Six whole plots (two temperature treatments x three replications) and six subplots per whole plot (three species x two irrigation treatments), for a total of 36 subplots, were arranged in a split-plot design. Whole plots were the growth chambers, where the supra-optimal and optimal temperature treatments were applied in separate chambers in a randomized complete block design. Subplots, which were the lysimeters, were arranged in a completely randomized design within each whole plot. Because only two growth chambers were available, whole plots could not be replicated concurrently and had to be replicated sequentially in time. The first whole-plot replication was from March to May 2004, the second replication from May to July 2004, and the third from July to September 2004. To ensure that turf age was similar in each replication, turfgrasses were seeded in lysimeters 5 mo before treatments were applied. Therefore, lysimeters for the first, second, and third replications (blocks) were seeded in October 2003, December 2003, and January 2004, respectively.
Evapotranspiration (100%) was measured gravimetrically with the well-watered lysimeters (Bremer, 2003). Using this method, lysimeters were irrigated, allowed to drain until free drainage ceased, sealed, and weighed. Lysimeters were then weighed after 3 d and the water loss was attributed to ET. Lysimeters were irrigated every 3 d according to their ET losses. Cumulative ET (mm) for each treatment was determined as the sum of all ET during the study.
Turfgrass Quality, Photosynthesis, Electrolyte Leakage, and Canopy Temperature
Turf visual quality was rated on a scale of 1 to 9 (1 = poorest quality, 6 = minimally acceptable, and 9 = highest quality) according to color, texture, density, and uniformity (Emmons, 2000). Quality ratings were recorded every 6 d by the same individual during the entire study.
Photosynthesis was measured every 6 d at about 8 h into the daily light cycle, with a LI-6400 portable gas exchange system (LI-COR Inc., Lincoln, NE) using a custom surface chamber described by Bremer and Ham (2005). According to their Eq. [5] and [6], sunlit chamber measurements determine Pg – (Rc + Rs') and shaded chamber measurements determine Rc + Rs', where Pg is gross photosynthesis, Rc is canopy respiration, and Rs' is residual soil respiration in a pressurized chamber; all values are positive and units are µmol m–2 s–1. Shaded chamber measurements were obtained by covering the chamber with a black cloth that completely blocked solar radiation from the chamber. Gross photosynthesis was calculated using Eq. [8] in Bremer and Ham (2005): Pg = sunlit chamber + shaded chamber. Cumulative Pg for each treatment was calculated by summing the products of mean Pg on each measurement day and the number of hours between samples when lights were turned on (i.e., 14-h daily period when plants were photosynthesizing).
Leaf electrolyte leakage (EL) was determined by the method of Blum and Ebercon (1981) and Marcum (1998) with modifications. Five living leaves about the same age were collected from each lysimeter at 0, 3, 15, 27, 39, and 45 d of treatment (DOT). Each leaf was cut into two to three 2-cm segments and rinsed three times with distilled deionized water. All rinsed leaf segments from each lysimeter were placed in a test tube containing 20 mL deionized water. Test tubes were shaken on a shaker table at 120 rpm (Lab-Line Instruments Inc., Melrose Park, IL) for 24 h to dissolve electrolytes that had leaked from cells (e.g., due to membranes damaged by heat or drought stress treatments). After measuring conductivity (C1) with a conductivity meter (Model 32, Yellow Spring Instrumental Inc., Yellow Spring, OH), the test tubes with leaf samples were placed in an autoclave at 140°C for 20 min to destroy all cell membranes, shaken for 24 h to extract all electrolytes from the cells, and then the conductivity (C2) was measured again. The percentage of the total electrolytes that had leaked from cells during treatments was calculated as C1/C2 x 100. Lower EL indicated greater resistance to stresses.
Canopy temperature was measured every 6 d with three infrared thermometers (model IRTP5, Apogee, Logan, UT). The infrared thermometers were positioned at 0.2 m above the turf canopy. Measurements were automatically logged every 5 s and averaged and recorded every 30 min with a micrologger (CR10x, Campbell Scientific, Logan, UT). Measurements for each lysimeter were conducted for about 1.5 to 2.5 h.
Dry Matter Production, Soil Surface Temperature, and Volumetric Soil Water Content
Turfgrasses were mowed every 3 d, and all clippings were collected. Clippings were dried in a forced-air oven for 48 h at 70°C and then weighed. Cumulative dry matter production for each treatment was determined by summing the dry weights of all clippings during the 48-d study. Daily dry matter production was calculated as the clipping weight at each mowing divided by the number of days since the previous mowing.
Soil surface temperature was measured with soil-encapsulated thermocouples using the method of Ham and Senock (1992). To evaluate potential cumulative heat effects among treatments during the most stressful periods, heat units (degree-hours) were calculated as the sum of soil surface temperatures during the final 8 h of each daily light cycle. Our data indicated that this was the period of maximum soil surface temperatures, which may have had important physiological impacts on the turfgrasses (e.g., on meristematic activity in the crowns).
Volumetric soil water content (
v) and temperature inside the lysimeters at 5, 25, and 45 cm were measured automatically using the dual-probe heat-pulse technique (Campbell et al., 1991; Tarara and Ham, 1997; Song et al., 1998). Sensors were fabricated in the laboratory as described by Basinger et al. (2003) and Bremer (2003). Measurements of v were logged twice daily at 0800 and 2000 h CST and soil temperatures were logged every 60 min. All data acquisition and control were accomplished with a micrologger and accessories (CR10x, two AM16/32's, and one AM25T, Campbell Scientific, Logan, UT).
Green Leaf Area Index, Aboveground Biomass, and Root Mass Density
At the end of each 48-d replication, aboveground biomass was harvested from each lysimeter and separated into living and dead components. Green leaves were separated from green shoots and the area of the leaves was measured with an area meter (LI-3100, LI-COR, Lincoln, NE). All green and dead tissue was then dried in a forced-air oven for 48 h at 70°C and weighed separately. Green leaf area index (LAI) was calculated as the ratio of the green leaf area to ground surface, and total aboveground biomass for each treatment was calculated as the sum of the dry weights of all living and dead tissue.
After aboveground biomass was harvested, lysimeters were laid horizontally and cut into three sections (0–15, 15–35, and 35–57.5 cm). The soil was washed from the roots in each section and roots were dried in a forced-air oven for 48 h at 70°C and then weighed. Root mass density of each section was calculated as dry root mass divided by the volume of soil inside each respective section of lysimeter.
All data were analyzed with the mixed procedure of SAS (SAS Institute, Cary, NC). Variation was partitioned into grass species, temperature treatment, and irrigation level. Interactions among species, temperature, and irrigation level were not significantly different on a given day after treatment initiation (days of treatment). Therefore, the comparison of species under temperature and/or drought treatments was statistically analyzed. Differences between means were separated by the least significance difference (LSD) test at P = 0.05.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The combination of high temperature and drought stresses caused a more rapid decline in visual quality among species than individual treatments of high temperature or drought (Fig. 1A, 1B, and 1C). For example, visual quality of TF fell below 6 by 6 DOT, and HBG and KBG fell below 6 by 18 DOT. Nevertheless, the visual quality of HBG was consistently higher among species late in the study (i.e., after 18 DOT), albeit significantly higher only than TF.
In the control, visual quality was consistently higher in HBG and KBG than in TF although differences were not always significant; visual qualities were similar between HBG and KBG (Fig. 1D). Tall fescue had a lower visual quality than HBG and KBG in the control primarily because of its coarser texture.
Photosynthesis
High temperature reduced cumulative Pg by 21% in HBG, 30% in KBG, and 27% in TF, compared with the control (Fig. 2
). In the high temperature treatment, however, cumulative Pg of HBG was 16% greater than KBG and 24% greater than TF. During the study, high temperature caused a general decline in daily Pg among species although Pg was consistently higher in HBG than in TF and KBG late in the study (Fig. 3
). Daily Pg was significantly higher in HBG than in TF from 24 through 42 DOT, and higher in HBG than in KBG on 42 DOT. These data indicate greater resistance to heat in HBG than in TF and KBG.
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Under combined high temperature and drought stress, cumulative Pg of HBG, KBG, and TF was reduced by 51, 56, and 60%, respectively, compared with the control (Fig. 2). The combination of high temperature and drought stresses reduced cumulative Pg more than individual treatments of high temperature or drought. This result is similar to other reports where the combined stresses of heat and drought caused photosynthesis to decline in KBG (Jiang and Huang, 2000) and in TF (Jiang and Huang, 2001). Wang and Huang (2004) reported that the combination of high temperature and drought stresses decreased leaf photochemical efficiency and chlorophyll content of two KBG cultivars (‘Midnight’ and ‘Brilliant’) and had a detrimental effect on the photosynthesis system for KBG. Cumulative Pg in this study was greater in HBG than in TF under the combined stresses of high temperature and drought.
Electrolyte Leakage
High temperature had no effect on electrolyte leakage (EL) in well-watered HBG, but EL increased in well-watered KBG on 27 DOT, and remained higher than HBG thereafter (Fig. 4A
). Mean EL also increased in TF on 27 DOT and remained higher than HBG thereafter, but differences were significant only on 39 DOT. Drought stress under optimal temperatures had no significant effect on EL among species, although mean EL was consistently higher in TF among species beginning on 15 DOT (Fig. 4B). The combination of high temperature and drought caused EL to increase in all species (Fig. 4C). Interestingly, EL decreased in HBG after 27 DOT and was significantly lower than in KBG and TF thereafter. In the control, EL remained consistently low and was similar among the three turfgrasses throughout the study (Fig. 4D).
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Dry Matter Production
In all treatments and the control, cumulative dry matter production was greater in HBG than in KBG and TF (albeit not significantly in the combination high temperature and drought treatment) and similar between KBG and TF (Fig. 5
). Within each species, however, cumulative dry matter production was significantly reduced by high temperature, drought, and the combination of high temperature and drought. High temperature reduced cumulative dry matter production in KBG, HBG, and TF by 88, 74, and 91% respectively, which was greater than the reduction under drought; drought reduced cumulative dry matter production in KB, HBG, and TF by 49, 48, and 52%, respectively. Lower cumulative dry matter production in high temperature than in drought was a result, in part, of a dramatic decrease in clippings after the initial 3 d of treatments compared with the more gradual decline in daily dry matter production under drought as soils dried (data not shown). The combination of high temperature and drought reduced cumulative dry matter production in KBG, HBG, and TF by 95, 86, and 93%, respectively. Within each species, the combination of heat and drought had no additional effect on cumulative dry matter production compared to high heat only.
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Drought alone reduced ET compared with the control, but similar ET among species indicated no differences in drought resistances (Fig. 6A). Similarly, no differences in ET were observed among species in the well-watered, optimal temperatures of the control. Our results contradict those of Supplick-Ploense and Qian (2005), who found that a HBG (Reveille) used less water and exhibited greater drought resistance in the field than a KBG (Bensun's A-34). Our results also differ from Minner and Butler (1985), who found that TF used significantly more water than KBG by using lysimeters filled with sand to measure ET under field conditions. High variability in water use among individual HBG and cultivars of KBG and TF may partially explain the contrasts between results from those studies and ours; the specific hybrids and cultivars used in their studies were different from those used in ours. Research by others has indicated significant variability in water use among individual cultivars of KBG and TF, respectively (Shearman, 1986; Bowman and Macaulay, 1991). Furthermore, considerable variability in relative water content and drought resistance, which suggests variability in water use, was found among 30 HBG, and some HBG exhibited less drought resistance than KBG (Abraham et al., 2004).
Green LAI in HBG was not affected by high temperature compared with the control, but LAI in HBG was reduced (i.e., 56 to 64%) by drought and by the combination of high temperature and drought (Fig. 6B). High temperature reduced green LAI in KBG compared with the control, and high temperature combined with drought caused further reductions in LAI in KBG compared with high temperature only. Green LAI in KBG was 29, 55, and 61% lower in high temperature, drought, and combined high temperature and drought treatments, respectively, than in the control. Green LAI in TF was reduced by 38 to 68% among treatments compared with the control, but differences in green LAI in TF were not significant among stress treatments. Green LAI was similar among all three species in the control.
Canopy and Soil-Surface Temperatures and Volumetric Soil Water Content
In the drought and the combination drought and high temperature treatments, canopy temperatures increased steadily by 4.1 to 4.8°C during the study (data not shown). Presumably, decreasing transpiration as the soils dried caused a corresponding decrease in evaporative cooling of leaves and increased drought stress on leaf tissue (Sifers and Beard, 1993). Within each treatment, canopy temperatures were generally similar among species (data not shown).
In the high temperature treatment, mean soil surface temperatures were consistently cooler in HBG than in KBG and TF, by about 1°C (data not shown). Cumulative heat (degree-hours) at the soil surface was significantly lower in HBG than in KBG and TF under high temperature (Fig. 6C), which suggests that long-term heat impacts on meristematic activity may have been reduced in HBG compared with KBG and TF. Under combined high temperature and drought stress, the soil surface also was consistently cooler in HBG than in KBG and TF on a daily basis (data not shown). Cumulative heat at the soil surface was lower in HBG than in TF, but was statistically similar to KBG. Cooler soil surface temperatures in HBG exposed to high temperature may have resulted from greater shading of the soil surface, caused by faster growth in HBG between mowing as discussed earlier. Faster growth between mowing may have increased transpiration of the canopy and thus, cooled the air above the soil surface in HBG (Bonos and Murphy, 1999). Under drought alone, there was no difference in soil surface temperatures among species, which was similar to the control.
Volumetric soil water content inside the lysimeters was similar among species under all treatments and the control (data not shown). At lower depths (i.e., 25 and 45 cm) under the combination of high temperature and drought, however, volumetric soil water content was consistently lower in TF and HBG than in KBG, which indicated that TF and HBG were using water from deeper in the profile than KBG.
Aboveground Biomass and Root Biomass Density
In all treatments and the control, total aboveground biomass at the end of the study was greatest in TF, except under high temperature, although more than half was dead (Table 1
). In all treatments and the control, dead aboveground biomass was greater in TF than in HBG. Conversely, in HBG, the living biomass as a percentage of the total was greater than in TF under high temperature, indicating that HBG has a greater resistance to heat than TF.
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At shallower depths (i.e., 0–15 cm and 15–35 cm), root biomass was generally similar among species within each treatment (Table 2). Root biomass was greater in TF than in KBG, however, in the middle profile (15–35 cm) of the control and under the combination of high temperature and drought. Conversely, root biomass was greater in KBG than in TF in the surface layer under the combination of high temperature and drought. Total root biomass at all depths was similar among species within each treatment with the exception of the control, where total root biomass was greater in TF than in KBG.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In this growth chamber study, HBG exhibited a higher resistance to heat than KBG and TF, which suggests that HBG may be better suited for areas where frequent high temperatures may damage other cool-season turfgrasses. Differences in drought resistance were negligible among HBG, KBG, and TF, however, indicating no advantage of this HBG over KBG and TF under dry conditions. Field studies are needed to further investigate the heat and drought resistance of HBG, including additional cultivars of HBG, under the actual dynamic and stressful environments where turfgrasses are grown.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication December 12, 2006.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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