The Intermountain West (IMW) of North America is considered a high desert environment and is experiencing substantial population growth that means increasing demand for water, particularly as a result of irrigated urban landscapes (Kjelgren et al., 2000). Hot, dry summers are characteristic of the IMW, where urban turfgrass requires irrigation to survive and thereby driving demand for water. However, the IMW has very limited water supplies; thus, water conservation in irrigated urban landscapes has become an important policy to moderate consumption (Hilaire et al., 2008). Hydrological drought resulting from low winter snowpack is very common in the IMW and often leads to water conservation measures that can result in water stress of landscape plants, particularly turfgrass. Additionally, high temperatures and low humidity can increase drought stress on plants when irrigation is insufficient.
Drought tolerance refers to the ability to experience and undergo drought stress but survive (Fry and Huang, 2004). Plants adapted to water-limiting environments such as the IMW use a variety of adaptive mechanisms (McCann and Huang, 2007). Stomata control the exchange of water vapor and CO2 between the interior of the leaf and the atmosphere, which contributes to control of the plant's internal water status and to gaining carbon for photosynthesis (Hetherington and Woodward, 2003). Plants prevent water loss by closing stomata to reduce transpiration, but at the cost of reducing evaporative cooling, increasing leaf temperature, and decreasing photosynthesis and growth (Fry and Huang, 2004).
Stomatal closure with increased vapor pressure deficit of ambient air (VPD) is common in many plants to moderate transpiration under high evaporative demand (Bates and Hall, 1981; Monteith, 1995; Turner et al., 1984). More specifically, stomatal sensitivity is driven by leaf-to-air vapor pressure difference (LAVPD) for species in dense canopies (like turf) or with large leaves with boundary layers that limit convective heat loss (Landsberg and Butler, 1980; Montague et al., 2000; Turner, 1991). For such species, drought can trigger a feed-forward loop of progressively increasing LAVPD and stomatal closure until transpirational evaporative cooling is balanced by convective cooling (Jones, 1999). Progressive drought stress hastens this loop and increases stomatal sensitivity to LAVPD and correlates with decreased evapotranspiration rates (Al-Faraj et al., 2001).
Turfgrass species used in the IMW avoid drought but differ in mechanisms. TF avoids drought because it maintains normal physiological function in water-limiting conditions by developing an extensive, deep root system to extract more water from a deeper and greater volume of soil (Huang and Gao, 2001). This postpones tissue dehydration (Sheffer et al., 1987). TF appears to also reduce water loss from transpiring leaves by rolling its leaves as soil water content declines (Qian and Fry, 1997). By contrast, KBG avoids drought by entering summer dormancy (Ervin and Koski, 1998), sometimes referred to as quiescence. However, once adequate moisture is again available, plants will resume active growth (Laude, 1953).
Osmotic adjustment is another drought tolerance mechanism that grasses use to maintain cellular turgor and allow them to take up water at lower soil water potentials (Perdomo et al., 1996; White et al., 1992). Osmotic adjustment under stress conditions has been reported to occur in both TF (Qian and Fry, 1997; West et al., 1990; White et al., 1992) and KBG (Jiang and Huang, 2001; Perdomo et al., 1996).
Grass responses during prolonged summer drought have long been studied. Most research has been conducted in greenhouses under controlled conditions (Aronson et al., 1987; Brown et al., 2004; Qian and Fry, 1997), whereas some were field investigations (Carrow, 1996; Laude, 1953; Richardson et al., 2008). Traits used to measure drought response have more commonly included morphological responses such as growth reduction, turfgrass quality rating, and root density (Ervin and Koski, 1998; Qian and Fry, 1997; Sheffer et al., 1987). Less often but more recently, physiological responses such as water relations, gS, photosynthesis, and hormone (abscisic acid) concentration have been measured (Jiang and Huang, 2000; Perdomo et al., 1996; Volaire et al., 2009; West et al., 1990).
Because the mechanisms that KBG and TF use to cope with drought are quite different, a comparison under common field conditions, with detailed measurements, will help us understand the distinct drought tolerance or avoidance mechanisms used by these grasses. The objective of this work was to compare the physiological responses of KBG and TF, which differ in drought-coping mechanisms, that might contribute to persistence of field-grown grasses during a prolonged drought in the IMW.
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