Heat stress is a major problem throughout the Southwest in summer months, particularly with cool season grasses such as creeping bentgrass. Currently, it takes years for plant breeders to select and develop new heat tolerant turfgrass cultivars. Cellular electrolyte leakage (a laboratory technique) was used in attempt to predict relative heat tolerances of 6 Kentucky bluegrass cultivars. To determine actual relative heat tolerance, grasses were grown under controlled high temperature conditions in a growth chamber. Actual relative heat tolerance decreased in this order: BM-3, Ryss, Nugget, Midnight, and Lavang. Electrolyte leakage was able to predict the most heat sensitive (Lavang) and most heat tolerant (BM-3) cultivars only.

Heat stress, a major problem is cool season grasses in the Southwest, is difficult to determine. Currently, plant breeders require years to select for heat tolerance, as they must observe accessions in the field. A laboratory technique, cellular electrolyte leakage, was used to quantify heat stress of 6 Kentucky bluegrass cultivars in attempt to predict relative heat tolerance.

Six Kentucky bluegrass (Poa pratensis L.) cultivars were used, representing extremes in climactic adaptation: 'BM-3' (Southern Italy), 'Lavang' (Norway), 'Midnight' (Alaska), 'Nugget' (U.S.), and 'Ryss' (Norway). Grasses were grown in sterile 1 gallon nursery pots in sterile potting media fertilized with slow release fertilizer with micronutrients. They were grown in a growth chamber at 500 uEinsteins light, 80% relative humidity. Temperature was gradually raised to a final level of 40C (104F), and held for 4 weeks. Grasses were clipped weekly at 3", and the clippings dried and weighed. At the end of 4 weeks, the grasses were rated for percent leaf firing, an indication of tissue injury due to heat stress. For the electrolyte leakage test, 2 cm leaf segments were placed in test tubes (10 segments per tube) and rinsed with distilled water. Tubes were sealed and placed in a hot (50C) water bath for varying periods of time. Four tubes of each grass were taken out of the bath at 10 minute intervals over a period of 80 minutes. Tubes were immediately flooded with 10 ml distilled water, and electrical conductance measured after incubation in a refrigerator overnight. The amount of conductance indicates the amount of solute leakage from cells (or the leakiness of the cells), which is an indication of the amount of cellular damage due to heat stress. The amount of time required in the hot water bath necessary to reach 50% of total cell leakage was used as the indicator of relative heat tolerance.
 
 

After 4 weeks of 40C exposure, % leaf firing was used to indicate heat injury. Percent leaf firing increased (and therefore heat tolerance decreased) in this order: BM-3, Ryss, Nugget, Midnight, and Lavang (Table 1). Total leaf clipping dry weights were measured over the 4 week heat stress period. Leaf clippings decreased in this order: Ryss, BM-3, Nugget, Lavang, and Midnight (Table 1). As you can see, leaf firing and clipping weights are in fairly close agreement in predicting heat tolerance. The time of exposure (50C) necessary to reach 50% of total cellular electrolyte leakage decreased in this order (as time of exposure decreases, cells are less heat tolerant): BM-3, Midnight, Nugget, Ryss, Lavang (Table 2). Electrolyte leakage was able to predict the most heat tolerant and least heat tolerant cultivar (from leaf firing). Nugget was also in the middle. However, Midnight and Ryss reversed orders. It appears that electrolyte leakage can predict major differences in heat tolerance among cultivars, and may be used as a breeding tool for new cultivar development. More studies will be done to determine how sensitive the technique is in predicting heat tolerance.

Table 1. Percent leaf firing of 5 Kentucky bluegrass cultivars after 4 weeks of heat stress, and total leaf clipping dry weights during 4 weeks of heat stress (40C).

Ryss 63b 0.213a

Nugget 68b 0.176a

Midnight 83c 0.035b

Lavang 95d 0.036b

Table 2. Time necessary for 50% of total cellular electrolyte leakage to occur in grass leaves exposed to 50C. Longer times indicate greater cellular heat tolerance.

Ryss 54

Nugget 59

Midnight 63

BM-3 80