The optimal growing temperature for cool-season grass species ranges from 18 to 23 °C, whereas air temperatures typically exceed 30 to 35 °C for daytime and 23 to 28 °C for nighttime during summer months in the transition zone (Kunkel et al., 2013). Drought stress is another major limiting factor for turfgrass growth, particularly during the summer months. The decline in TQ of fine fescues, which is commonly observed during the summer, is typically associated with heat, drought, or both and is referred as summer decline (Turgeon, 1996). Evaluating the stress-induced TQ decline caused by heat or drought and comparing responses across cultivars would offer a better understanding of the summer decline in fine fescues.
Healthy turfgrass stands are characterized by uniform and dense canopy, dark-green leaf color, and active growth (Beard, 1972). Extensive reports have shown that stress-related leaf senescence is associated with disruption or degradation of cellular membranes with downstream effects on photosynthetic carbohydrate synthesis (Huang et al., 2014; Wahid et al., 2007). Prolonged heat stress typically induces lipid peroxidation and membrane instability with subsequent effects on chlorophyll integrity and net photosynthetic rates in cool-season grass species, including creeping bentgrass (Agrostis stolonifera), kentucky bluegrass (Poa pratensis), and perennial ryegrass (Lolium perenne) (Jiang and Huang, 2001; Liu and Huang, 2000). Alternatively, drought stress caused by decreased rainfall or limited irrigation is another major problem leading to steady TQ decline of cool-season turfgrass stands during the summer months. Although drought stress similarly imposes negative effects on cellular membrane stability, photochemical efficiency, and chlorophyll integrity, it also induces significant decreases in leaf water potential in kentucky bluegrass (Abraham et al., 2004; Jiang and Huang, 2000). Similar effects of drought stress have been detected in other cool-season grasses including tall fescue (Festuca arundinacea), creeping bentgrass, and perennial ryegrass (Carrow and Duncan, 2003; Karcher et al., 2008; McCann and Huang, 2008; Wang and Bughrara, 2008). Given that drought and heat stress typically occur together under field conditions, it is important to determine which stress is more detrimental so that proper management can be taken to prevent or control summer decline in fine fescues.
The fine fescue family is comprised of several species and subspecies, including strong creeping red fescue, slender creeping red fescue, chewings fescue, hard fescue, and sheep fescue. Fine fescue species are cool-season grasses widely used in home lawns and golf courses throughout cool-temperate climates. They form attractive turf stands that are characterized by narrow and fine leaf textures (Christians and Engelke, 1994). They are well adapted to poor soil fertility, moderate shade, and acidic soil conditions; however, little is known regarding their tolerance to heat and drought stress (Turgeon, 2011). The objectives of this study were to 1) examine whether heat or drought stress (dry down by withholding irrigation) is more detrimental to fine fescues, 2) determine genotypic variations of heat and drought tolerance within fine fescues, and 3) identify physiological parameters that can be used as indicators for heat and drought tolerance in fine fescues.
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