Oxygen requirement and inhibition of C4 photosynthesis -: An analysis of C4 plants deficient in the C3 and C4 cycles

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 CO2 fixation by Rubisco is limited, an increase in the CO2 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 CO2 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.