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Dep. of Plant Biology and Pathology, Rutgers Univ., New Brunswick, NJ 08901
* Corresponding author (huang{at}aesop.rutgers.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: CID, carbon isotope discrimination • ET, evapotranspiration • ETa, actual evapotranspiration • SWD, soil water depletion • PVC, polyvinyl chloride • TDR, time-domain reflectometry • WUE, water use efficiency
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Turfgrass consumptive water use is defined as the total amount of water required for growth plus the amount of water lost through ET, where ET is comprised of the sum of soil evaporation and plant transpiration (Beard, 1973). Water use rankings of cool-season grasses under nonlimiting soil moisture conditions have been reported previously: tall fescue (Festuca arundinacea Schreb.), Kentucky bluegrass (Poa pratensis L.), annual bluegrass (P. annua L.), and creeping bentgrass had highest ET rates; rough bluegrass (P. trivialis L.) and perennial ryegrass (Lolium perenne L.) ranked intermediate; and chewings [F. rubra L. subsp. fallax (Thuill.) Nyman], hard (F. brevipila R. Tracey), and red (F. rubra L. subsp. rubra) fescues had lowest ET rates (Minner, 1984; Aronson et al., 1987; Beard and Kim, 1989). Water use may also vary between cultivars of the same species (Shearman, 1986; Kopec et al., 1988; Shearman, 1989; Bowman and Macaulay, 1991; Salaiz et al., 1991; Ebdon and Petrovic, 1998). However, it is important to note that the comparative water use rankings of different species and cultivars may change across different environments, climatic conditions, and cultural regimes (Aronson et al., 1987).
Irrigation quantity may have significant impact on plant water use. Deficit irrigation, a practice of irrigating in an amount below that of the plant's maximum water demand, has been utilized to reduce water use in cool- and warm-season turfgrass species such as tall fescue and bermudagrass (Cynodon dactylon (L.) Pers.; Fu et al., 2004), buffalograss [Bouteloua dactyloides (Nutt.) Columbus], and St. Augustinegrass [Stenotaphrum secundatum (Walter) Kuntze] (Qian and Engelke, 1999). Deficit irrigation practices have been associated with increases in WUE in nonturf species, resulting in maintenance of photosynthetic assimilation with less water (English and Raja, 1996). Water use efficiency is commonly calculated from gas-exchange measurements as the molar ratio of CO2 assimilated during photosynthesis to the amount of water lost through transpiration (Hopkins, 1999). Gas-exchange WUE of maize (Zea mays L.) increased significantly when irrigated at 55% of field capacity compared with well-watered control plants under field conditions (Kang et al., 2000). Field gas-exchange WUE of wheat (Triticum aestivum L.) also increased when irrigated at 45% of available soil water (Panda et al., 2003). Measurements of CID have also been utilized to estimate WUE. The composition of naturally occurring carbon isotopes (12C and 13C) within plants differ from that of atmospheric CO2. In general, there is less abundance of 13C relative to 12C in plant tissues compared with that in air, indicating that CID occurs during the assimilation of CO2 into plants (Farquhar et al., 1989; Lambers et al., 1998). Water stress induces stomatal closure, resulting in reduced internal CO2 concentrations and less discrimination against the heavier isotope of carbon by Rubisco. In C3 plants, CID is related to the ratio of carbon assimilation rate to transpiration (inherent gas-exchange WUE), such that lower discrimination is correlated with higher WUE. Negative correlations between CID and WUE have been reported in Kentucky bluegrass (Ebdon et al., 1998; Ebdon and Kopp, 2004), tall fescue (Johnson and Bassett, 1991; Johnson, 1993), and perennial ryegrass (Johnson and Bassett, 1991).
In recent years, there is increasing demand for the use of bentgrass species on golf course fairways. We previously reported that velvet and creeping bentgrasses maintained higher turf quality than colonial bentgrass under the same level of deficit irrigation (DaCosta and Huang, 2006). However, the effects of deficit irrigation on water use characteristics of bentgrass species, such as soil moisture depletion rate and WUE, were not well understood. Furthermore, whether differences in irrigation requirements among different bentgrass species are related to variability in ET rate, soil moisture depletion rate, and WUE is not clear.
The objectives of this study were to investigate the effects of deficit irrigation on water use characteristics of creeping, colonial, and velvet bentgrasses and to compare water use among three bentgrass species. Water use characteristics were evaluated by measuring ET rates, soil moisture depletion, gas-exchange WUE, and CID.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The experiment was conducted in 2002 and 2003 under a fully automated, mobile rainout shelter (10.7 by 20.1 m) (modified commercially available greenhouse, Stuppy Greenhouse Manufacturing, North Kansas City, MO; designed and constructed by Agricultural Engineering staff, New Jersey Agricultural Experiment Station, Rutgers University). A Campbell Scientific data acquisition system (Model CR-23X, Campbell Scientific, Logan, UT) monitored weather variables and controlled the operation of the rainout shelter. Rainfall was detected using a heated grid sensor (Argus Control Systems Ltd., White Rock, BC, Canada) located 
3 m off the ground, which then initiated the process of moving the shelter over the experimental plot area during rain events. Powered by an electric motor and set on rails, the shelter automatically returned back to its original position after 30 min of a rain event. The shelter excluded unwanted rainfall from test plot areas and allowed quantitative control of soil moisture and irrigation while retaining the advantages of practical field conditions.
An additional on-site weather station located 130 m from the rainout shelter monitored temperature, relative humidity, wind speed, and solar radiation parameters in both 2002 and 2003 (Model CR-10X datalogger, Campbell Scientific, Logan, UT). Sensors for the different climatic variables were located 2 m off the ground.
Plots were arranged in a randomized split-plot design, with irrigation levels as the main plots (4.5 by 2.4 m) and species as the subplots (1.5 by 2.4 m). Each species and irrigation treatment was replicated four times for a total of 48 plots. Plots were irrigated three times per week, generally between 0800 and 1100 h, at four levels of irrigation quantity based on different percentages of the maximum daily water loss through ETa as measured using minilysimeters. The average daily loss in mass from the minilysimeters was converted on an area basis into the amount of irrigation water to replace onto individual plots. Plots were irrigated to replace (i) 100% ETa (control); (ii) 80% ETa; (iii) 60% ETa; and (iv) 40% ETa. Each subplot was individually irrigated using a hand-held hose with a fan-spray nozzle and water quantity was monitored using a digital flow meter attachment (Catalog number 4352K63, McMaster-Carr Supply Co., New Brunswick, NJ). The flow rate to dispense a given amount of water was determined for one irrigation pass to maintain even distribution and uniformity of irrigation application within and among plots. To prevent subsurface lateral water movement between different irrigation treatments, plots were separated using 40-cm-deep plastic edging. To avoid the accumulation of water on the turf and limit lateral surface water movement between plots, total irrigation amount was applied in separate quantities to be less than the infiltration rate.
Irrigation treatments were applied from July to November in 2002 and 2003, with two deficit irrigation treatment cycles (summer and fall) in each year. At the start of each of the deficit irrigation periods, volumetric soil water content of plots was determined to be at field capacity (30% v/v) using time-domain reflectometry (TDR) (Soil Moisture Equipment, Santa Barbara, CA). Treatments during the summer deficit irrigation period ceased when plots under the lowest irrigation regimes demonstrated significant drought injury (plant tissue brown and desiccated). Fall treatments ceased when temperatures dropped to below optimum for active turfgrass growth. In 2002, treatments ran from 1 July through 18 August (48 d), and then again from 13 September through 1 November (50 d). In 2003, treatments ran from 27 June through 8 September (73 d), and then again from 1 October to 1 November (31 d). In between the summer and fall irrigation treatments was a rewatering period to allow the turf in deficit irrigation plots to completely recover to prestress levels (dense, green turf canopy) before the initiation of the fall irrigation cycle. During the recovery period, all plots were watered three times per week to replace 100% ETa and maintain soil moisture at field capacity (30% volumetric soil moisture).
Measurements
Evapotranspiration rates of individual plots were measured using the gravimetric mass balance method with minilysimeters to represent ETa. Minilysimeters allow for direct calculation of mass changes due to plant water uptake and soil evaporation (Young et al., 1997) and have been utilized in several investigations on turfgrass water use (Feldhake et al., 1983; Aronson et al., 1987; Fu et al., 2004; Qian, 1996). Measurement of ETa based on weighing lysimeters is distinguished from ETp, where ET is estimated instead by means of an empirical model and based on climatic data (Kneebone et al., 1992). Approximately 1 mo before the initiation of irrigation treatments, cores including intact plants and soils (10-cm diam. and 20 cm deep) were removed from established field plots using a cup cutter and placed into polyvinyl chloride (PVC) tubes of the same size as the cores to form minilysimeters. Before the installation of minilysimeters, soil cores were randomly sampled from plots to estimate general root length characteristics of the bentgrass species. The height of the PVC minilysimeters was based on this information so that minilysimeters included a majority of the plant root system. Nylon mesh screen was taped to the bottom of each PVC tube to maintain the plant and soil column intact while allowing for water percolation out of the minilysimeters. Minilysimeters were installed in plots of the 100% ETa treatment for all thee species (one minilysimeter in each plot) in both 2002 and 2003, with new minilysimeters reinstalled in 2003. Minilysimeters received the same environmental and management conditions as the rest of the surrounding plot area. Minilysimeters were pulled out of the plots daily and weighed at 24-h intervals with a balance providing accuracy to the nearest gram. Daily ET was calculated based on the difference in the weight of minilysimeters at 24-h intervals.
Volumetric soil water content was measured weekly using TDR (Soil Moisture Equipment, Santa Barbara, CA) to monitor SWD. A pair of stainless steel 15-cm buriable probes was inserted vertically into the soil profile at two locations per plot, which were then averaged to determine soil water content for each plot. The SWD was calculated as the percentage decline in soil water content from 6 July to 23 July (1- to 20-d summer treatment), 23 July to 6 August (20- to 33-d summer treatment), and 30 September to 28 October (15- to 43-d fall treatment) in 2002, and 26 June to 29 July (1- to 34-d summer treatment), 29 July to 25 August (34- to 70-d summer treatment), and 29 September to 20 October (1- to 20-d fall treatment) in 2003.
Water use efficiency was calculated from gas-exchange measurements as the molar ratio in CO2 fixation (canopy photosynthetic rate) to H2O loss (canopy ET rate). Gas-exchange rates were measured using an infrared gas-exchange analyzer with a modified canopy chamber provided with 400 µL L–1 CO2 (LI-COR 6400, Li-COR, Inc., Lincoln, NE). The canopy chamber consisted of an acrylic cylinder (10-cm diam. and 8-cm height) which was pressed into the ground 3 cm to provide an adequate seal for canopy gas-exchange measurements. The canopy chamber was randomly placed within the plot area, and gas exchange parameters were allowed to equilibrate for 3 min before recording data. Gas exchange measurements were performed on days with minimal to no cloud cover, and at times of maximal solar radiation (approximately between 1100 and 1400 h).
Carbon isotope analysis was also performed to estimate WUE using CID. Plant tissue was sampled for carbon isotope analysis from plots at different treatment durations throughout the summer and fall experimental periods in 2002 and 2003. Leaf tissue was dried at 80°C and ground to a powder to pass through a 40-mesh screen. Ground plant material was sent to the Stable Isotope Ratio Facility for Environmental Research at the University of Utah, Salt Lake City for measurement of carbon isotopic composition. For a more detailed description on carbon isotope analysis and theory, see Smedley et al. (1991) and Ebdon et al. (1998). Carbon isotopic composition (
13C) was then converted into 13C isotope discrimination (
) and expressed on a per mil (
) basis according to Farquhar and Richards (1984):
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p was C isotopic composition of the plant sample, and a the C isotopic composition of the source air (assumed to be –8 for free atmospheric CO2) (Farquhar et al., 1989).
Statistical Analysis
Treatment effects were determined by ANOVA for a split-plot design according to the general linear model procedure of the Statistical Analysis System v. 8.2 (SAS Institute, Inc.). To remove the effect of year and sampling date, data were analyzed separately for each year and date for statistical analysis. Variation was partitioned into irrigation regime, grass species, and corresponding interactions. Differences between treatment means were separated by Fisher's protected least significance (LSD) test at the 0.05 probability level.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In general, SWD in 2003 was less than that observed in 2002 (Table 2). Additionally, based on climatic data, mean maximum air temperatures, vapor pressure deficit, and solar radiation were lower particularly during the summer treatment period in 2003 compared with corresponding months in 2002 (Table 3). During the initial dry down period in the summer of 2003 (26 June to 29 July, 1- to 34-d treatment), irrigating at 80% ETa caused no significant SWD for any of the species. Soil moisture declined by 51.2 and 23.4% for colonial bentgrass irrigated at 40 and 60% ETa, respectively, which was significantly greater than SWD for velvet bentgrass (30.0 and 9.7%, respectively). Creeping bentgrass SWD was not significantly different from either colonial or velvet irrigated at 60% ETa, but exhibited less SWD compared with colonial bentgrass at 40% ETa.
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Gas-Exchange Water Use Efficiency
Irrigation and species main effects were significant on six and four out of six evaluation dates, respectively, in 2002, and five and two out of six dates, respectively, in 2003 for gas-exchange WUE. There were no significant irrigation by species interactions in 2002, and three out of six significant irrigation by species interactions in 2003. Therefore, data are presented according to main effects, which contributed greater variation to differences observed. In 2002, irrigating at 40% ETa resulted in a 48% decline in WUE by 2 August (29-d summer treatment) and a 82% decline by 17 August (47-d summer treatment) compared with initial levels (Fig. 1A
). Irrigating at 60 and 80% ETa maintained the same level of WUE as the 100% ETa irrigation treatment throughout the summer treatment period. In the fall treatment period, WUE of plots irrigated at 40 and 60% ETa were comparable with those irrigated at 100% ETa.
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30% lower than that of velvet and creeping bentgrasses (Fig. 1B). In the fall, there was a general increase in WUE compared with summer months. However, WUE was not significantly different for colonial, creeping, and velvet bentgrasses. On 15 July 2003 (20-d summer treatment), plots irrigated at 40 and 80% ETa exhibited significantly lower WUE than plots at 60 and 100% ETa (Fig. 2A ). By 25 August (70-d summer treatment), WUE was almost 60% lower at 40% ETa than that at 60, 80, and 100% ETa. Similar to that observed in 2002, WUE increased in the fall months compared with the summer treatment period. On 16, 24, and 30 September, WUE of plots at 40% ETa were 37, 17, and 15% lower than plots irrigated at 80 and 100% ETa; however, WUE of plots irrigated at 60% ETa remained comparable with plots irrigated at 100% ETa on all fall dates in 2003.
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Carbon Isotope Discrimination
Species main effects for CID were highly significant on three out of four evaluation dates, and irrigation main effects were significant on two of the four evaluation dates. A significant irrigation by species interaction occurred on only one out of four sampling dates, corresponding to 8 Sept. 2003. Since species main effects contributed to greater variation than either irrigation effects or irrigation by species interaction, this indicated that CID values were relatively stable within a species regardless of deficit irrigation regime.
In 2002, velvet bentgrass had the lowest CID values on 23 July (18-d summer treatment) and 2 October (17-d fall treatment) in 2002 (Table 4). The CID for creeping bentgrass was higher than that of colonial bentgrass on 23 July; however, colonial bentgrass had higher CID on 2 October. In 2003, velvet bentgrass had significantly lower CID on 13 August (49-d summer treatment) (18.5) and 8 September (73-d summer treatment) (19.5) compared with both colonial (20.4 and 20.3, respectively) and creeping (20.4 and 20.0, respectively) bentgrasses. Colonial bentgrass exhibited either higher (8 September) or the same (13 August) CID compared with creeping bentgrass in 2003.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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To our knowledge, this is the first report of CID in bentgrasses. In general, velvet bentgrass exhibited lower CID than either colonial or creeping bentgrasses. The values for CID in the three bentgrasses were similar to those reported for Kentucky bluegrass (Ebdon et al., 1998) and tall fescue (Johnson, 1993). The CID values were relatively stable within a species regardless of deficit irrigation regime. In some other investigations with cool-season grasses, however, increasing levels of water stress resulted in a reduction in CID and increase in WUE (Jensen et al., 2002; Johnson et al., 2003). This type of association was only observed on one evaluation date (8 September 2003), and irrigation at 40% ETa was associated with the lowest CID, 60 and 80% ETa with intermediate CID, and 100% ETa with the highest CID (data not shown).
Carbon isotope discrimination has been negatively correlated with WUE in several C3 grass species, including crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.] (Read et al., 1991), wheat (Farquhar and Richards, 1984), Kentucky bluegrass (Ebdon et al., 1998), tall fescue (Johnson and Bassett, 1991; Johnson, 1993), and perennial ryegrass (Johnson and Bassett, 1991). Theoretically, CID is related to gas-exchange WUE, such that low CID is associated with a high carbon assimilation to transpiration ratio. In our study, relative species rankings according to CID were similar to the rankings based on water use and gas-exchange WUE, and lower CID corresponded to increased WUE in the three bentgrass species. However, gas-exchange WUE values may not always correlate to CID, probably due to the small fraction of time of this type of measurement vs. an integrated, longer-term measurement given by CID (Hall et al., 1992).
Kim and Beard (1988) proposed that inter- and intra-specific differences in water consumption could be related to morphological characteristics such as shoot density, leaf orientation, leaf width, and vertical leaf extension rate, which may affect canopy resistance to ET and leaf area. Ebdon and Petrovic (1998) reported that horizontal leaf orientation, high shoot density, slow vertical leaf extension rate, and fine leaf texture were characteristics associated with lower ET rates in Kentucky bluegrass cultivars under well-watered conditions. However, shoot density or leaf orientation were not correlated with ET in either bermudagrass (Beard et al., 1992) or zoysiagrass (Zoysia japonica Steud.) (Green et al., 1991). In the present study, these various morphological characteristics were not quantitatively evaluated. However, on the basis of our visual observations, colonial bentgrass exhibited less shoot density, greater vertical leaf extension, and greater vertical leaf orientation, whereas velvet bentgrass exhibited very high shoot density, fine leaf texture, and slow leaf extension rates. This suggests that differences in morphological characteristics may contribute to some of the variability in canopy resistance to ET, and thus differences in water use between the bentgrass species. Further evaluation is required to determine the direct correlation of morphological traits and water use in bentgrass species.
Deficit irrigation resulted in decreased water use or increased WUE for all three species, and the extent of reduction varied with irrigation regime and time of the year. Irrigating at 60 and 80% ETa were able to maintain the same level of gas-exchange WUE as the 100% ETa irrigation treatment in summer 2002 and fall 2003; however, irrigating at 40% ETa generally resulted in lower WUE. We previously reported that irrigation at 60 and 80% ETa maintained comparable turf quality, depending on species and evaporative conditions, to plots irrigated at 100% ETa for a majority of the summer deficit irrigation treatment; however, irrigating at 40% ETa for all three bentgrasses resulted in a significant decline in turfgrass quality (DaCosta and Huang, 2006). In the fall treatment period, irrigating the bentgrasses at 40% ETa was sufficient to maintain minimal acceptable turf quality (DaCosta and Huang, 2006), and this could be associated with low soil moisture depletion and high WUE. The results in this study in combination with previously reported turf performance data suggest that irrigating colonial, creeping, and velvet bentgrasses at 60 to 80% ETa could be practiced to increase WUE during summer and 40% ETa irrigation could be utilized to reduce water use in fall.
Water use rate also varied depending on environmental conditions between seasons. Water use significantly declined from July to October in both years. In 2002 ET rates averaged across species were 6.2, 4.8, 5.7, and 2.2 mm d–1 for plots irrigated at 100% ETa in July, August, September, and October, respectively. In 2003 ET rates averaged across species were 6.4, 5.3, and 2.7 mm d–1 for plots irrigated at 100% ETa in July, August, and October, respectively. This corresponded to a 60% decrease in water use from July to October for both years. Under nonlimiting water conditions, ET is primarily governed by climatic variables and extent of vegetative cover (Doss et al., 1964; Aronson et al., 1987). On the basis of meteorological conditions, there was a general decrease in mean maximum air temperatures, vapor pressure deficit, and solar radiation from July to October, which could account for the significant decrease in evaporative demand and water use. Changes in ET rates based on climatic factors have been observed for other cool-season grasses (Aronson et al., 1987).
On the basis of the present study and that of DaCosta and Huang (2006), colonial bentgrass exhibited lower drought resistance, higher water use, and lower WUE compared with velvet bentgrass, which exhibited higher drought resistance, decreased water consumption, and higher WUE. However, water use rates are not necessarily correlated with increased drought tolerance, such that species with low consumptive water use are not necessarily more drought tolerant than a species with higher consumptive water use. Fernandez and Love (1993) evaluated 25 commercially available cool-season turfgrass cultivars and found that some tall fescue cultivars had higher water use, but still ranked higher in quality under progressive water stress compared with some perennial ryegrass cultivars with lower water use rates. Kopec et al. (1988) reported that tall fescue cultivars with decreased water use wilted sooner than those with higher water use rates. Superior performance of drought stress-tolerant Kentucky bluegrass cultivars was associated with higher water use compared with less tolerant cultivars that adapted to drought by decreasing water use (Bonos and Murphy, 1999). This demonstrates the importance of the integration of whole-plant morphological and physiological responses for evaluation of plants with improved drought and water use characteristics. Water consumption is only one factor to be considered when evaluating the overall drought resistance ranking of a particular species and/or cultivar.
In summary, water use was a function of species, irrigation regime, and climatic conditions. Deficit irrigation resulted in increased WUE in three bentgrass species. Lower soil moisture depletion rate and higher WUE could have contributed to lower water use, particularly for velvet bentgrass. Considering the effects on turf performance (DaCosta and Huang, 2006) and water use, irrigation at 60–80% ETa during conditions of high evaporative demand (e.g., summer months) and 40% ETa irrigation under low evaporative demand (e.g., fall months) could be practiced for water conservation in bentgrass management.
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ACKNOWLEDGMENTS |
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Received for publication March 7, 2006.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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0.05) indicating statistically significant differences for irrigation and species treatment comparisons at a given day of treatment.