OXYGEN REQUIREMENT AND INHIBITION OF C-4 PHOTOSYNTHESIS - AN ANALYSISOF C-4 PLANTS DEFICIENT IN THE C-3 AND C-4 CYCLES

The basis for O-2 sensitivity of C-4 photosynthesis was evaluated usin g a C-4-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruva te carboxylase-deficient mutant), and a C-3-cycle-limited transformant of Flaveria bidentis (an antisense ribulose-1,5-bisphosphate carboxyl ase/oxygenase [Rubisco] small subunit transformant). Data obtained wit h the C-4-cycle-limited mutant showed that atmospheric levels of O-2 ( 20 kPa) caused increased inhibition of photosynthesis as a result of h igher 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 associate d with the C-4 function. With the Rubisco-limited F. bidentis, there w as less inhibition of photosynthesis by supraoptimal levels of O-2 tha n in the wild type. When CO2 fixation by Rubisco is limited, an increa se in the CO2 concentration in bundle-sheath cells via the C-4 cycle m ay further reduce the oxygenase activity of Rubisco and decrease the i nhibition of photosynthesis by high partial pressures of O-2 while inc reasing 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 m ust be balanced for maximum efficiency.