Photosynthetic adaptation to temperature in four species from the Colorado shortgrass steppe: a physiological model for coexistence.

Several aspects of photosynthetic adaptation to temperature were examined in four graminoid species from the Colorado shortgrass steppe. The experimental species were chosen to provide examples of a variety of in situ seasonal phenology patterns. The cool season grass, Agropyron smithii (C3), exhibited higher photosynthesis rates when grown in a cool temperature regime (20/15°C), and compared to warm grown plants (35/15°C). The warm season species, Bouteloua gracilis (C4) and Buchloe dactyloides (C4), exhibited higher photosynthetic capacities when grown in the warmer temperature regime. The sedge, Carex eleocharis (C3), which exhibits seasonal growth potential during the cool and warm portions of the growing season, exhibited a marked capacity for photosynthetic temperature acclimation. Differential effects of growth temperature on the intracellular conductance to CO2 appeared to have a greater regulatory role in these responses for the two C3 species, relative to stomatal conductance or photorespiration (O2 inhibition of photosynthesis). In the two C4 species decreases in the intracellular conductance in cool grown plants were correlated with the decreased photosynthetic capacity in normal air for B. gracilis, but not for B. dactyloides. Analysis of the Arrhenius relationship for CO2 saturated net photosynthesis at low leaf temperatures (4.5-17°C) indicated sharp breaks in the apparent energy of activation at 5.8-9.0°C in the warm season species B. gracilis and B. dactyloides. Leaves of A. smithii and C. eleocharis exhibited no significant low temperature limitations according to this analysis. The low temperature limitations in the warm season species were partially reflected in an inhibition of the quantum yield for CO2 uptake after 2 h at 5-6°C in the presence of high photon flux densities. Temperature dependent increases in the chlorophyll fluorescence yield at high temperatures revealed the lowest breakpoint values for A. smithii, and the highest values for B. gracilis. The differential patterns of temperature adaptation among the species further extend the proposal of Kemp and Williams (1980; Ecology 61:846-858) that seasonal temperature gradients in the shortgrass steppe have a regulatory role in maintaining offset patterns of resource utilization and decreasing interspecific competition.