Water and nutrient inputs are frequently applied to residential and commercial turfgrass landscapes to maintain growth and quality. The application of water to residential landscapes represents one of the major uses of potable water in many areas. For example, it was estimated that approximately two-thirds of average household water consumption in Central Florida was used for landscape irrigation (Haley et al., 2007). Thus, in many regions, this has led to water restrictions, especially during drought, that limit residential irrigation in an effort to conserve water. In addition, irrigation can contribute to leaching losses of applied N (Morton et al., 1988), which poses a significant environmental threat (Carpenter et al., 1998). Consequently, much attention has been focused on developing turfgrass germplasm and management practices that contribute to reduced water use and increased efficiency of water use by turfgrass systems. However, in many cases, we still lack a basic understanding of the physiological responses underlying effects of turfgrass genotypes and management practices on water use rates and particularly efficiencies of water use by turfgrass.
A number of factors have been shown to affect turfgrass ET rates. Lower mowing heights have been shown to reduce ET (Feldhake et al., 1983; Fry and Butler, 1989). In addition, increased fertilization generally results in increased ET from turfgrass (Barton et al., 2009; Ebdon et al., 1999; Feldhake et al., 1983). Decreased solar radiation and/or shade result in lower ET rates (Feldhake et al., 1983). Finally, species selection has been shown to influence ET, and in particular cool-season grasses generally have higher ET rates than warm-season grasses (Biran et al., 1981; Feldhake et al., 1983; Qian and Engelke, 1999). In addition to species differences, substantial variation among cultivars within a species has also been reported. (Ebdon and Petrovic, 1998; Shearman, 1989). Relatively lower ET rates have been associated with several plant traits, including horizontal leaf orientation, narrow leaf texture, high shoot density, and low leaf area (Ebdon and Petrovic, 1998; Kim and Beard, 1988). In fact, the factor that varied the most among high- and low-water use cultivars of kentucky bluegrass was horizontal leaf orientation, because low-water use cultivars had 17% more horizontal leaf orientation (Ebdon and Petrovic, 1998).
In addition to absolute water use by turfgrass (i.e., ET), the efficiency with which that water is used to produce turf biomass may also contribute to improved water conservation. Water use efficiency is a measure of carbon assimilated per unit of water transpired by the plant (Stanhill, 1986) and can be measured instantaneously using gas exchange approaches (e.g., Bunce, 2010) or integrated over time using cumulative biomass and transpiration measures or tissue carbon isotope composition (e.g., Ebdon et al., 1999). Although each of these methods has advantages and disadvantages, they all generally agree very well (Condon et al., 2004; Ebdon et al., 1999; Heitholt, 1989). Although there are numerous data on individual factors affecting turfgrass water use, our basic understanding of how these factors interact to affect water use and especially WUE is still limited. Ebdon et al. (1999) reported that ET was negatively related to WUE in kentucky bluegrass at low temperatures (i.e., low ET) indicating that water conservation and high WUE could be simultaneously achieved, but they did not find the same relationship at higher temperatures. Higher WUE has been reported for slow-release N fertilizers (Saha et al., 2005). Other studies have shown that N fertilization is positively related to WUE (Brueck, 2008; Heitholt, 1989), but it is not clear what the implications for growth/quality, ET, and water conservation might be with higher WUE associated with N fertilization (Barton et al., 2009; Blum, 2009).
It is becoming increasingly apparent that no single crop trait or cultural practice will be adequate for achieving water conservation in turfgrass systems. We must improve our understanding of the complexity of interactions and physiological responses to achieve water conservation across a wide range of environments. Therefore, the objectives of the current study were to evaluate the effects of N fertilization and light environment on relations among growth, carbon assimilation, water use, and WUE of two coarse-textured Zoysia japonica Steud. genotypes differing in canopy architecture. Zoysiagrasses provide a high-quality turf, have good shade tolerance, and are adapted to a variety of soils, but widespread adoption has been limited in part as a result of relatively high water use rates and poor drought resistance (Carrow, 1995; Qian and Engelke, 1999). Thus, we specifically wanted to test whether N fertilization would affect daily ET, daily ET per unit leaf area, carbon exchange rate, biomass, and WUE and whether these relations would differ by genotype and/or light environment.
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