Proper irrigation management is critical to growing quality turfgrass with limited water in arid and semiarid regions. Deficit irrigation, defined as applying water in amounts less than the reference evapotranspiration rate, is an irrigation management practice that could result in water savings (Fu et al., 2007). The ETo, measured as amount of daily water loss from a canopy under non-water-limiting conditions, often is used to estimate water requirements in turfgrass irrigation. Reduction in irrigation application not only can reduce costs associated with water consumption and can improve environmental stress tolerance, but it also prevents turfgrass from the injury of mechanical stresses, cyanobacteria, and diseases (Beard, 1973; Dernoeden, 2002; Turgeon, 2008).
Some researchers have found that turfgrass is able to tolerate moderate drought (DaCosta and Huang, 2006; Fu et al., 2004; Gilbeault et al., 1985). Tall fescue, bermudagrass (Cynodon dactylon), and zoysiagrass (Zoysia japonica) irrigated to 60% or 80% of ETo exhibited similar turfgrass quality when compared with well-watered turfgrass (Fu et al., 2004). Gilbeault et al. (1985) also observed that tall fescue, kentucky bluegrass (Poa pratensis), and perennial ryegrass (Lolium perenne) had only a slightly lower level of quality when irrigated at 80% ETo relative to 100% ETo.
Availability of total nonstructural carbohydrate (TNC) has been widely used as a physiological measure of stress tolerance, because carbohydrates provide energy and solutes for osmotic adjustment. Sucrose, an important component of TNC, is the dominant form of carbohydrate transported to developing plant organs and is one of the sugars stored in higher plants (Khayat and Zieslin, 1987). Sucrose also serves as an osmotic solute (Premachandra et al., 1992; Rekika et al., 1998; Tan et al., 1992; Zhang and Archbold, 1993). The effect of water deficits on sucrose levels has been reported in some plants. For example, improved responses of bean (Phaseolus vulgaris) to water deficits were associated with sucrose metabolism (Castrillo, 1992; Vassey et al., 1991). McManus et al. (2000) found that after white clover (Trifolium repens) was exposed to a period of moderate drought stress, leaf sucrose content increased significantly. Leaf sucrose level increased by 300% at the end of an 8-d-long drought period in sugarbeet (Beta vulgaris) (Harn and Daie, 1992).
Because sucrose may serve an important role in drought tolerance, understanding the enzyme activity affecting sucrose metabolism is critical. Sucrose synthesis can be regulated by rapid changes in the activity of sucrose phosphate synthase, sucrose synthase, and acid invertase (Castrillo, 1992; Hawker, 1985; Huber and Huber, 1996). Sucrose phosphate synthase catalyzes the synthesis of sucrose–phosphate from uracil–diphosphate (UDP)–glucose and fructose-6-phosphate, and this reaction occurs predominantly in the cytosol of sucrose-source leaf tissue. Leaf SPS activity is often correlated with the rate of sucrose synthesis and export (Huber and Israel, 1982; Rocher et al., 1989; Stitt et al., 1988). Some researchers found that water deficits lead to an increase in SPS activity in potato tubers [Solanum tuberosum (Geigenberger et al., 1997)], soybean leaves [Glycine max (Cheikh and Brenner, 1992)], and pigeonpea leaves [Cajanus cajan (Keller and Ludlow, 1993)]. However, Castrillo (1992) reported that the values of total (substrate-saturating conditions) and Pi-insensitive (substrate limiting conditions plus inorganic phosphate) SPS activity in P. vulgaris were reduced by drought stress. These conflicting results suggest that effects of drought stress on SPS activity depend on experimental conditions. Sucrose synthase catalyzes both the synthesis and cleavage of sucrose. Yang et al. (2001) reported that water deficits enhanced SS activity in the cleavage direction, but the activity of SS in the synthesis direction was not measured. Castrillo (1992) reported that the synthesis activity of SS was increased by water deficit. In contrast, the effects of drought on AI appear to be negative. AI catalyzes the hydrolysis of sucrose into glucose and fructose. Dorion et al. (1996) observed that the activity of soluble AI in wheat (Triticum aestivum) declined fourfold during a drought stress period and never recovered.
Effects of drought stress on sucrose metabolism have been examined intensively, mainly in annual crops, as discussed previously. However, how different levels of deficit irrigation influence sucrose metabolism and associated enzymes are not well understood, because plants are often subjected to different levels of irrigation regimes or levels of drought stress. The objectives of this study were to address the question by examining the influence of deficit irrigation on leaf ψS, sucrose level, and sucrose metabolic enzymes in a cool-season grass species, tall fescue. This species is a widely used species as turfgrass, which has superior drought resistance to many other cool-season grass species (Fry and Huang, 2004). Tall fescue is widely used in temperate climates, because it tolerates heat and drought well compared with other cool-season turfgrasses (Fry and Huang, 2004). Greater knowledge of these responses might provide insights into drought resistance mechanisms of cool-season grass species in areas with varying irrigation availability.
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