The basis for O2 sensitivity of C4 photosynthesis was evaluated using a C4-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruvate carboxylase-deficient mutant), and a C3-cycle-limited transformant of Flaveria bidentis (an antisense ribulose-1,5-bisphosphate carboxylase/oxygenase [Rubisco] small subunit transformant). Data obtained with the C4-cycle-limited mutant showed that atmospheric levels of O2 (20 kPa) caused increased inhibition of photosynthesis as a result of higher levels of photorespiration. The optimal O2 partial pressure for photosynthesis was reduced from approximately 5 kPa O2 to 1 to 2 kPa O2, becoming similar to that of C3 plants. Therefore, the higher O2 requirement for optimal C4 photosynthesis is specifically associated with the C4 function. With the Rubisco-limited F. bidentis, there was less inhibition of photosynthesis by supraoptimal levels of O2 than in the wild type. When CO2 fixation by Rubisco is limited, an increase in the CO2 concentration in bundle-sheath cells via the C4 cycle may further reduce the oxygenase activity of Rubisco and decrease the inhibition of photosynthesis by high partial pressures of O2 while increasing CO2 leakage and overcycling of the C4 pathway. These results indicate that in C4 plants the investment in the C3 and C4 cycles must be balanced for maximum efficiency. \ud \u
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