VARIABLE HIGH-TEMPERATURE TOLERANCE AMONG KENTUCKY BLUEGRASS CULTIVARS

The tolerance to chronic high-temperature stress varies greatly among Kentucky bluegrass (Poa pratensis L.) cultivars. This study of photosynthesis characteristics was conducted to determine whether such traits differ among cultivars and contribute to variable high-temperature tolerance. High-temperature-tolerant cultivars exhibited superior growth rates at 30-degrees-C but growth rates were similar among cultivars at 10-degrees-C. At 10-degrees-C, fructosan comprised an approximately two-fold larger portion of the plant dry weight than at 30-degrees-C, however nonstructural carbohydrate fractions did not differ among cultivars. Under each experimental temperature, the sum of glucose, fructose, and sucrose fractions was approximately the same, though the relative concentrations were temperature dependent. Apparently fructosan served as a buffer fraction that allowed the sum of the other nonstructural fractions to remain constant. Photosynthetic response to increased leaf temperature was similar among cultivars but differed among acclimation temperature regimes. Net photosynthetic rates varied by as much as one-third among cultivars, however, with a positive relationship between net photosynthetic rates and high-temperature tolerance. These differences were not attributable to differential CO2 diffusive resistances. This evidence indicate that high-temperature-tolerant cultivars were capable of sustaining superior net photosynthetic rates under high but not low temperature conditions. It is suspected that cultivar differences in photosynthetic electron transport capacity may have caused this trend.