PHYSIOLOGICAL ASPECTS OF VARIABLE HIGH-TEMPERATURE TOLERANCE AMONG KENTUCKY BLUEGRASS GENOTYPES (HEAT, STRESS)

An investigation was conducted to examine various physiological parameters which could contribute to variability in the high-temperature tolerance among Kentucky bluegrass genotypes. Exposure of intolerant genotypes to 30C resulted in greatly reduced growth rates, while after a brief acclimation period, high growth rates were observed in tolerant genotypes. At 10C, growth rates were similiar among all genotypes. Differences did not exist in the distribution of dry weight or water-soluble carbohydrate among plant parts at either 10 or 30C. Although all plants stored much more fructosan when grown at 10C compared to 30C, genotypic differences were not observed in the amounts of fructosan, starch, glucose, fructose or sucrose when grown at either temperature. Differences in nitrogen uptake and metabolism did not occur, as no genotypic differences in the nitrate, amino, protein or total nitrogen fractions were observed at either temperature. In vivo nitrate reductase activity at 20C was inversely related to the high-temperature tolerance of genotypes, and such assays may serve as an excellent germplasm screening technique. Although shifts in the temperature optima for net photosynthesis occurred upon acclimation to 10, 20 or 30C, no genotypic differences were observed. Thus, differential high-temperature tolerance was not associated with the temperature stability of the photosynthetic apparatus.;Exposure to high irradiance (2000 uE/m('2)/sec PAR) caused much more severe inhibition of photosynthesis in intolerant than in tolerant genotypes. When grown at moderate irradiance (300 uE/m('2)/sec PAR), the leaf blades of intolerant genotypes folded at the midrib, presumably as an avoidance mechanism for photoinhibiton. This folding would result in even lower net photosynthesis under nonsaturating light due to lower light interception. It is concluded that higher photosynthetic rates, and subsequently higher growth rates, in high-temperature-tolerant as compared to -intolerant Kentucky bluegrass genotypes reflect their superior capacity to utilize light energy via the photosynthetic process. Hence, they are less susceptible to photoinhibition at high irradiance.;Net photosynthetic rates at both 2% and 21% O(,2) were related to high-temperature tolerance. Tolerant genotypes exhibited substantially greater net rates than intolerant genotypes under both saturating and nonsaturating irradiances. Such trends also occured under CO(,2)-enriched conditions, thus, differences are probably not attributed to differing diffusive resistances. These photosynthetic differences likely resulted in the higher growth rates observed at high temperature in heat-tolerance genotypes.