The effect of oxygen on CO2 fixation by mesophyll protoplast extracts of C3 and C4 plants.

Oxygen inhibits CO₂ fixation by mesophyll protoplast extracts of the C₃ plants, $\text{Hordeum vulgare}$ and $\text{Triticum aestivum}$, but stimulates the pyruvate induced CO₂ fixation by mesophyll protoplast extracts of the C₄ plants $\text{Digitaria sanguinalis}$ and $\text{Urochloa panicoides}$. The former is reversed by increased levels of bicarbonate, whereas the latter effect is independent of bicarbonate concentration. The results are consistent with the proposal that oxygen inhibits C₃ photosynthesis by competing with CO₂ in the RuDP carboxylase/oxygenase system. The oxygen enhancement of C₄ mesophyll photosynthesis is proposed to be due to pseudocyclic electron flow supplying additional ATP for the CO₂ fixation process.