Effect of varying environments on photosynthetic parameters of C, C-C and C species of Panicum.

CO exchange characteristics and the activity of the carboxylating enzymes phosphoenolpyruvate carboxylase (PEP-C, E.C. 4.1.1.31) and ribulose 1,5-bisphosphate carboxylase (RuBP-C, E.C. 4.1.1.39) during one year in the greenhouse and at two levels of light and temperature in growth chambers were determined in the C-C intermediate species P. milioides Nees ex. Trin. These results were compared with those of P. bisulcatum Thumb. (C) and P. maximum Jacq. (C). Under all tested conditions, and even when the influence of leaf surface temperature on photosynthetic rates and CO compensation points were measured, the biochemical and physiological behaviour of the C-C intermediate was more similar to that of the C plant than the C species. The C plant P. maximum, however, responded positively, mainly in terms of PEP-C activity and photosynthetic rate, to the regime of high light and temperature. The results presented indicate that in the C-C Panicum grown in high light and temperature no direct relationships between a low CO compesation point and superior growth are evident. It has still to be clarified why in nature a photosynthetic-photorespiratory pathway leading to an intermediate CO compensation value has evolved in P. milioides.