Physiological responses to heat stress alone or in combination with drought: A comparison between tall fescue and perennial ryegrass

Heat and drought are two major factors limiting growth of cool-season grass es during summer. The objective of this study was to compare the effects of heat stress alone (H) or in combination with drought (H+D) on photosynthes is, water relations, and root growth of tall fescue (Festuca arundinacea L. ) vs. perennial ryegrass (Lolium perenne L.). Grasses were exposed to H (35 degreesC day/30 degreesC night) or H+D (induced by withholding irrigation) in growth chambers for 35 days. Soil water content declined under H+D for both grasses but to a greater extent for fescue than for ryegrass. Declines in canopy net photosynthetic rate (Pn), leaf photochemical efficiency (Fv/ Fm), and leaf relative water content (RWC) and the increase in electrolyte leakage (EL) were much more severe and occurred earlier for ryegrass than f escue subjected to both H and H+D and for both species than under H+D then H. Evapotranspiration (ET) rate increased to above the control level within 3 or 6 days of H and H+D for both species, but fescue had a higher ET rate than ryegrass at 3 and 6 days of H and 6 days of H+D. Root dry weight and viability in all soil layers decreased under H and H+D for both species. Ho wever, fescue had higher root dry weight and viability than ryegrass in the 20-40 cm layer under H and in both the 0-20 and 20-40 cm layers under H+D. The results indicated that maintenance of higher Pn, Fv/Fm, ET, RWC, and r oot growth and lower EL would help cool-season turfgrass survive summer str ess, and that their characteristics could be used for selecting stress tole rant species or cultivars.