Heat Tolerance Screening of Field-Grown Cultivars of Kentucky Bluegrass and Perennial Ryegrass

The quality of cool-season turfgrasses frequently declines during periods of high temperature stress. Simple tests are needed to rapidly identify heat tolerant germplasm for incorporation into breeding pro­ grams. Facilitative screening tests have been devised, however, in the few studies that have been performed only immature and green­ house or growth chamber-grown plants have been evaluated. To be of practical value, results of screening tests, employing plants grown under artificial conditions, should correlate closely with results of tests involving field grown plants. The objective of this research was to evaluate the heat tolerance of several cultivars of Kentucky blue­ grass (Poa pratensis L.) and perennial ryegrass (Lolium perenne L.) grown in the field under four different regimes of N fertilization (0, 98,148, or 196 kg ha-1 yr-l) in a Typic Hapludults, fine silty, mixed mesic soil for comparison with published results in which greenhouse and growth chamber-grown material was used. On six sampling dates, plants representing all cultivar and N combinations were exposed to 42, 44, and 46°C by immersion in a water bath. Heat tolerance of the cultivars was compared using the mean percent recovery weight for the three temperatures. The Kentucky bluegrass cvs. Sydsport, Vantage, and Pennstar were more heat tolerant than the perennial ryegrass cvs. Pennfine, Citation, and Caravelle. When data were averaged over 2 years, it was shown that Sydsport was significantly more heat tolerant than all other genera and cultivars tested. Penn­ fine had higher recovery weights than the other two ryegrasses on four of six sampling dates. When data were averaged, however, no significant heat tolerance differences among the ryegrasses were dis­ cerned. The results from the screening of field grown material fol­ lowed the same trends as published results using greenhouse or growth chamber-grown samples. This investigation therefore provides strong evidence that laboratory screening tests may be used to identify ac­ curately and rapidly heat tolerant cultivars of Kentucky bluegrass and possibly perennial ryegrass. The overall heat to)erance of the cultivars on each sampling date correlated with the amount of pre­ cipitation (r= -0.91) and the average high temperature (r=0.93) for the period just prior to and during sampling. The moderate N fertility regimes imposed had little effect on the heat tolerance of the grasses. Additional index words: Environmental stress, Turfgrasses, Poa pratensis L., Lolium perenne L. -=-------­ K bluegrass (Paa pratensis L.) and per­ ennial ryegrass (Latium perenne L.) are widely used in the Mid-Atlantic region as turfgrasses although their quality declines during periods of high temper­ ature stress. Successful breeding programs to improve the heat tolerance of cool-season grasses depend upon the identification of heat tolerant germplasm. The use of screening tests to identify heat tolerant material can speed the selection process. To be useful, the results from a screening test must parallel the results of field performance trials. The results of these tests should correlate with the field performance of cultivars in lo­ cations where heat stress is encountered. This corre­ lation may be difficult to obtain since laboratory heat stress testing isolates the plants from the effects ofother stresses. Field cultivar evaluations, however, are in­ fluenced by other factors acting on the plant concom· mitantly with heat stress. The optimum temperature for growth ofcool-season turfgrasses is in the range of15 to 24°C (Beard, 1973). Above 24°C, growth declines and at very high tem­ peratures, severe injury or death can occur. In con­ trolled-environment pot experiments, Kentucky blue­ grass produced maximum dry weight of top growth at 21.6 °C, and growth declined as temperature was in­ creased to 24.9 °C (Baker and Jung, 1968). Plants grown at 34.8 °C produced less than half the top growth of those at 21.6 0c. Julander (1945) found that Kentucky bluegrass plants were killed when exposed to 48°C for 16 h. Wehner and Watschke (1981) evaluated the heat tol­ erance of several cool-season turfgrass species by ex­ posing 10-week-old growth chamber or greenhouse grown plants for 30 min to temperatures in the range of 41 to 49°C. Plants, sealed in plastic bags, were heat stressed by immersion into a hot water bath. They found that Kentucky bluegrass was more heat tolerant than perennial ryegrass and annual bluegrass (Paa an­ nua L.). The Kentucky bluegrass cultivars tested were similar in heat tolerance; whereas, among the rye­ grasses, 'Loretta' was less heat tolerant than 'Pennfine', 'Diplomat', and 'Citation'. Nitrogen fertilization has been shown to influence heat tolerance in turf. Carroll (l943) fertilized field 1 Contribution from the Agronomy Dep., Univ. of Maryland as Scientific Paper No. A-3288 and ContributIOn No. 6360 ofthe Mary­ land Agric. Exp. Stn., College Park, MD 20742. Received 27 Aug. 1982. 2 Former graduate student, Agronomy Dep., Univ. of Maryland; assistant professor, Agronomy Dep., Univ. of Maryland; assistant professor, Horticulture Dep., Univ. of l1linois; and assistant pro­ fessor, Agronomy Dep., Univ. of Maryland, respectively. 773 MINNER ET AL.: HEAT TOLERANCE SCREENING OF KENTUCKY BLUEGRASS Table 1. Nitrogen application schedule in 1979 and 1980. Nitrogen Spring Summer Fall Spring Summer Total N regimes 4·11·79 6·13-79 9·15-79 9·30·79 4-18'80 6·20·80 per year