Cool-season turfgrasses exhibit a bimodal growth pattern with most of their growth and development in the spring and fall months and a drastic decline in growth and overall turf quality during the summer months (Turgeon, 2005). The decline in overall turf quality of cool-season grasses during summer months is commonly referred to as summer stress. Summer stress can be broken down into two major components, heat stress and drought stress (Huang et al., 1998a; Jiang and Huang, 2000, 2001b). Although these two stresses often occur simultaneously, that is not always the case. Quantification and comparison of the stress-induced decline caused by each will help better combat the summer decline of cool-season turfgrasses both through best management practices and breeding of improved varieties. The occurrence of dormancy during periods of summer stress has been shown to be a desirable trait that can aid in the long-term survival of turfgrasses (Hopkins and Bhamidimarri, 2009; Malinowski et al., 2005). However, for many turfgrass managers, dormant turf is not an option because of the loss of turf quality and function. Therefore, in this study, dormancy will be considered a negative attribute.
Global focus on environmental sustainability continues to spread and intensify (Jiang and Huang, 2001b). From 1960 to 2000, there have been increases of 30%, 28%, 24%, and 26% for each decade, respectively, in global water consumption (Kirda and Kanber, 1999). These steady increases have caused the worldwide consumption of freshwater in 2000 to be up more than 160% from 1960 levels (Kirda and Kanber, 1999). If demand for fresh water continues to increase, it is likely that its use for irrigation will become limited to food crops, leaving turf managers with less available water.
Tall fescue [Lolium arundinacea (Schreb.) Darbysh.] has long been considered one of the best-adapted cool-season grasses for hot and dry conditions (Turgeon, 1980). Tall fescue’s superior performance during hot dry summer months is not a result of it having a low water use rate like hard fescue (Festuca brevilipa R. Tracey), another drought-tolerant species (Brar and Palazzo, 1995). Conversely, tall fescue has significantly higher water use than many other cool-season turfgrasses and higher water use than several warm-season turfgrasses (Beard, 1989; Biran et al., 1981; Githinji et al., 2009). Tall fescue facilitates its high water use, even during times of drought, through a deep and expansive root system (Carrow, 1996). Its ability to avoid drought conditions by continually having access to water even during prolonged dry periods allows cells to remain turgid and actively growing. In addition, its steady high use of water allows tall fescue to actively cool itself though transpiration cooling. Jiang and Huang (2001b) compared tall fescue with perennial ryegrass (Lolium perenne L.), and found that tall fescue had higher leaf water content while experiencing heat and drought stress compared with perennial ryegrass. They also suggested that tall fescue was cooling itself through transpiration cooling and that this was one of the main factors leading to its superior performance during heat and drought stress. Tall fescue’s ability to maintain lower temperatures in its leaf tissue leads to increased membrane stability as well as increased photosynthetic efficiency. Membrane stability and plant photosynthetic efficiency are highly sensitive to elevated cell temperatures (Taiz and Zeiger, 2002).
Cui et al. (2006) compared the photosynthetic performance of two tall fescue genotypes known to have differing levels of heat tolerance. In this study, ‘Jaguar 3’ (heat-tolerant) and ‘TF 66’ (heat-sensitive) were heat-stressed in growth chambers with day/night temperatures of 35/30 °C for 20 d. They found that maintenance of photosynthesis, more specifically the maintenance of photosystem II and chlorophyll content, was very important in tall fescue’s ability to survive during high temperatures. They also found that prevention of cell membrane damage through antioxidant production was strongly correlated with heat tolerance. Wang et al. (2009) similarly subjected ‘Jaguar 3’ and ‘TF 66’ to heat stress in growth chambers for 20 d with day/night temperatures of 35/30 °C. The relative growth rate of ‘Jaguar 3’ decreased by 10% at Day 20, whereas the relative growth rate at Day 20 for ‘TF 66’ decreased by 93.7%. ‘Jaguar 3’ did not suffer the significant reduction in root-to-shoot ratio compared with ‘TF 66’. Leaf and root electrolyte leakage were also significantly less for ‘Jaguar 3’ (Wang et al., 2009).
Understanding what environmental factors are contributing to the overall decline exhibited by cool-season turfgrasses during summer months would be valuable for proper management of these turfgrass ecosystems. Differentiating heat and drought as components causing this overall decline will allow for management practices tailored to specifically alleviating the particular cause of the decline in summer turf quality. The use of improved varieties capable of maintaining acceptable quality during periods of intense summer stress would be an important tool. With this in mind, this experiment was designed to 1) evaluate diverse tall fescues under heat and drought stress alone and in combination; and 2) to select plants with a high level of summer stress tolerance to be used in future breeding projects.
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