Pasture & Range: March 2003

One major objective of the Forage Biotechnology Group (FBG) at the Noble Foundation is to develop persistent cool-season forage grass cultivars for the southern Great Plains (SGP). Perennial cool-season grasses are of interest to FBG because they have the potential to increase forage production during the spring and fall of the year when warm-season grasses are either dormant or slow growing and are not producing much forage. Unlike the annual forage grasses (e.g., winter wheat and rye), the perennial grasses do not need to be planted on a yearly basis. Thus, perennial grasses reduce the cost of production as well as minimize the erosion of valuable topsoil when compared to the annual grasses.

Most cool-season perennial grasses lack persistence through the hot and dry summers of the SGP. Based on multiple years of field evaluations under grazing, Dr. Andrew Hopkins (grass breeder, FBG) has identified a number of tall fescue (Festuca arundinacea Schreb.) lines that showed a high level of persistence in the SGP. Heat tolerance may be a possible mechanism of persistence of tall fescue in the SGP. Fungal endophytes (Neotyphodium coenophialum) can also contribute to persistence of tall fescue. We have evaluated the heat tolerance of nine of the most persistent tall fescue genotypes against nine of the least persistent genotypes in environmentally controlled experiments.

The 18 genotypes were clonally replicated and were grown in 6-inch pots in a greenhouse for about three weeks under good growing conditions. Four replicates of each genotype were subjected to high-temperature stress in a growth chamber by gradually raising the temperatures from 75 degrees F to 108 F over a period of 20 days. Then the plants were grown under the extreme heat stress (108 F day and 90 F night) for seven days before gradually lowering the temperatures back to 75 F. The plants were then grown at 75 F for 18 days before harvest. The experiment was repeated to confirm the findings of the first experiment.

Figure 1. The two heat-tolerant genotypes (PI423078 and PI297901) demonstrate normal growth, Kentucky 31 had a very poor growth and the sensitive genotype (PI283316) died.

Immediately following the completion of heat stress at 108 F, the percent of green versus dead tissue for each plant was visually estimated and the live tillers were counted. After harvest, the number of live tillers and shoot biomass were recorded. Based on the results of experiments 1 and 2, the two most tolerant and the two least tolerant lines were selected for further evaluation by conducting a third experiment with 12 replicates under the same conditions. The popular tall fescue cultivar Kentucky 31 was included in all these experiments as a reference genotype.

The plants were supplied with enough water throughout the experiments in order to avoid drought stress.

The two heat-tolerant tall fescue genotypes (PI423078 and PI297901) were able to survive well through the heat stress at 108 F and grew well after decreasing the temperature in the chamber (Fig. 1). However, cultivar Kentucky 31 barely survived the stress and a heat-sensitive genotype (PI283316) died (Fig. 1). More than 80 percent of the shoots remained green in the tolerant genotypes while less than 15 percent of the shoot tissues were green in the sensitive genotypes immediately after the completion of heat stress at 108 F (Fig. 2).

The tolerant genotypes had higher numbers of live tillers after heat stress and greater tiller regeneration during the recovery period (Fig. 3). Similarly, the shoot biomass production was also higher for the tolerant plants when compared to the sensitive plants (Fig. 4). PI423078 scored high in field persistence and it also appears to be endophyte free.

We have identified two heat tolerant tall fescue genotypes (PI423078 and PI297901) that may be useful in development of persistent tall fescue cultivars for the Southern Great Plains.