PHOTOSYNTHETIC RESPONSES TO TEMPERATURE OF PHOSPHOENOLPYRUVATE CARBOXYKINASE TYPE C-4 SPECIES DIFFERING IN COLD SENSITIVITY

Photosynthetic rates, the activities of key enzymes associated with th e C-4 cycle and ribulose-1,5-bisphosphate carboxylase (RuBPCase), and the levels of metabolites involved in the C-4 cycle were compared betw een the two phosphoenolpyruvate carboxykinase (PCK) type C-4 species S partina anglica, which is cold-tolerant, and Zoysia japonica, which is cold-sensitive, during exposure to low temperature. Plants of both sp ecies growth outside in summer were placed in a growth chamber at 27/2 0 degrees C day/night temperatures. After 1 week, plants were exposed to 20/17 degrees C for 1 week and then to 10/7 degrees C for 2 weeks, Photosynthetic rates in Z. japonica decreased progressively to about 2 5 % during the chilling treatments. In contrast, S. anglica exhibited a 43% increase in photosynthetic rates after exposure to 20 degrees C for 1 week, which remained relatively constant thereafter, Consistent with these observations, most of the C-4 enzymes and RuBPCase in Z. ja ponica declined. Phosphoenolpyruvate carboxylase (PEPC) and PCK activi ties declined particularly drastically during the treatments. However, the activities of these enzymes in S. anglica showed either a slight increase or decrease upon a mild cold treatment, and remained relative ly constant during further chilling treatments. There was a sharp decl ine in phosphoenolpyruvate in Z. japonica after exposure to 10 degrees C. On the other hand, metabolite levels in S. anglica were largely un affected by the chilling treatments. These results suggest that the dr astic declines of both PEPC and PCK activities may be important limiti ng factors responsible for cold sensitivity in C-4 photosynthesis of Z . japonica.