Oxygen Requirement and Inhibition of C(4) Photosynthesis

The basis for O(2) sensitivity of C(4) photosynthesis was evaluated using a C(4)-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruvate carboxylase-deficient mutant), and a C(3)-cycle-limited transformant of Flaveria bidentis (an antisense ribulose-1,5-bisphosphate carboxylase/oxygenase [Rubisco] small subunit transformant). Data obtained with the C(4)-cycle-limited mutant showed that atmospheric levels of O(2) (20 kPa) caused increased inhibition of photosynthesis as a result of higher levels of photorespiration. The optimal O(2) partial pressure for photosynthesis was reduced from approximately 5 kPa O(2) to 1 to 2 kPa O(2), becoming similar to that of C(3) plants. Therefore, the higher O(2) requirement for optimal C(4) photosynthesis is specifically associated with the C(4) function. With the Rubisco-limited F. bidentis, there was less inhibition of photosynthesis by supraoptimal levels of O(2) than in the wild type. When CO(2) fixation by Rubisco is limited, an increase in the CO(2) concentration in bundle-sheath cells via the C(4) cycle may further reduce the oxygenase activity of Rubisco and decrease the inhibition of photosynthesis by high partial pressures of O(2) while increasing CO(2) leakage and overcycling of the C(4) pathway. These results indicate that in C(4) plants the investment in the C(3) and C(4) cycles must be balanced for maximum efficiency.