Heat Tolearance 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 programs. Facilitative screening tests have been devised, however, in the few studies that have been performed only immature and greenhouse 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 bluegrass (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⁻¹) 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. Pennfine 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 discerned. The results from the screening of field grown material followed 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 accurately and rapidly heat tolerant cultivars of Kentucky bluegrass and possibly perennial ryegrass. The overall heat tolerance of the cultivars on each sampling date correlated with the amount of precipitation (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.