Effect of Temperature on Carboxylase Activity and Stability in Some Calvin Cycle Grasses from the Arctic

Abstract

The thermal stabilities and activities of carboxylating enzymes of three arctic grasses were studied. All grasses in the vicinity of Barrow, Alaska, are C3 grasses and photosynthesize with ribulose-,5-diphosphate as the principal carboxylating enzyme although low levels of phosphoenolpyruvate carboxylase are present. In contrast to other published results, this study indicates an in vivo thermal stability to temperatures as high as 40°C for both ribulose-1, 5-diphosphate carboxylase and phosphoenolpyruvate carboxylase. Therefore, the low temperature optima characteristic of whole leaf photosynthesis in these grasses and the decrease in whole leaf photosynthesis at temperatures greater than 15°C cannot be explained by a thermal inactivation of carboxylating enzymes. In the extracted form, thermostability was much lower for both enzymes from all species. Thus, temperature response data must be considered tentative. However, in our system the optimum for RuDP carboxylase activity in all species was 50°C and 40°C for PEP carboxylase. Similar stabilities and activities were found in the few C4 plants tested.