Genotypic variation in heat tolerance for hard fescues

By Cathryn Chapman, Ryan Daddio, Henry Qu, and Bingru Huang; Rutgers University

Hard fescues are often utilized for their low input maintenance requirements, but are susceptible to heat stress. With annual temperatures rising, there is both a demand for more heat-tolerant hard fescue turfgrasses and a demand for effective strategies for assessing phenotypic traits to select or screen for heat-tolerant genotypes in hard fescue. Thus, the objectives of this study were to determine genotypic variations of heat tolerance within hard fescue plants, and to identify key phenotypic traits for determination of heat tolerance level.

Hard fescue plants with unique and different genotypes were propagated from established stock plants and maintained in a greenhouse in individual pots for 7 weeks until plants were fully established. Plants were then transferred into environmentally controlled growth chambers for an additional acclimation period of 7 d prior to initiation of heat stress treatment (for a total establishment period of 8 weeks). The 240 unique genotypes were then exposed to heat stress treatment (35/ 30 °C day/night) for 35 d. Each genotype was replicated 3 times, for a total of 720 plants. Throughout heat stress, visual and digital assessments were performed, including turf quality (TQ), normalized difference vegetation index (NDVI), and % canopy cover (as a measurement of canopy density). Photos were taken weekly under identical light conditions.

Throughout 35 d of heat stress, the TQ ratings varied among the different genotypes, with the largest separation observed between day 21 and 25 heat stress (Figure 1). The range of NDVI values also varied among the different genotypes, specifically on day 21 heat stress where the greatest separation was observed. There was further separation of genotypes based on % canopy cover during prolonged heat stress, which declined to be between 0 – 20% for most genotypes but was maintained between 40 – 50% for a select few of the genotypes at 35 d heat stress treatment.

Graph with days of heat stress on x-axis and number of plants on the y-axis
Figure 1. Variation of turf quality (TQ) among 240 hard fescue genotypes during 35 d of heat stress treatment.

Based on the visual TQ ratings, we selected 20 genotypes for each of three categories:  top, middle, and worst performing plants. Each genotype was placed into 3 categories of relative heat tolerance level, separated by Fisher’s protected LSD at the α = 0.05 level. Selection of heat tolerance level from TQ ratings generally correlated well with NDVI and percent canopy cover data, where the highest TQ, NDVI, and percent canopy cover represented the heat tolerant genotypes, and the lowest TQ, NDVI, and percent canopy cover represented the heat sensitive genotypes (Figure 2). We determined that percent canopy cover, TQ, and NDVI are key traits for classifying the level of heat tolerance for hard fescue genotypes.

2 rows of five plants. The top row has plants that are green and alive and the bottom row has plants that are yellow and dead
Figure 2. The 5 most heat tolerant and 5 most heat sensitive hard fescue genotypes at 21 d heat stress determined by multivariable analysis.

Hard fescue genotypes exhibited genetic variability in heat tolerance based on multiple phenotypic traits (TQ, NDVI, and percent canopy cover) and multivariable analysis. Normalized difference vegetation index and percent canopy cover obtained with imaging technology are effective indicators of determining heat tolerance level of hard fescue that support the subjective turf quality rating by visual evaluation. Future research endeavors involve identifying genes or molecular markers among the 60 genotypes that were selected as the top, middle, and worst performing plants, and their association with the selected traits (TQ, percent canopy cover, and NDVI). Such knowledge can be of use for improving breeding selections to help promote sustainable turfgrass growth during summer months in areas that utilize low input hard fescues under prolonged heat stress conditions.