PHOTOSYNTHETIC SENSITIVITY TO TEMPERATURE IN POPULATIONS OF 2 C4 BOUTELOUA (POACEAE) SPECIES NATIVE TO DIFFERENT ALTITUDES

The relationships between photosynthesis, flowering, and growth temper atures were examined experimentally in four populations of the C4 gras s genus Bouteloua. Field-collected plants were grown under two tempera ture regimes, cool (20 C day/6 C night) and warm (30/16), representati ve of the extreme populations. Populations collected from the warm cli mates had significantly lower photosynthetic capacity when grown in th e cool chamber relative to the warm chamber, while photosynthetic capa city in the cool climate populations did not differ between the growth conditions. Additionally, exposure to a 2-day cold temperature treatm ent (10/-2), representative of late-season frosts in high altitude sit es, resulted in further reductions in photosynthesis in the warm clima te plants, but not in the cool climate plants. This effect was greater for plants grown in the cool growth chamber. Flowering was reduced by 70% in the warm climate plants grown in the cool chamber, and was cor related with photosynthetic inhibition following the short-term cold t emperature treatment. These results indicate that genetic differentiat ion for photosynthetic temperature sensitivity has occurred in the coo l climate populations, and that long-term exposure to cool temperature s coupled with short-term relatively extreme low temperatures results in greater photosynthetic inhibition in nontolerant populations.