Transgenic rice plants expressing the maize phosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) exhibit a higher photosynthetic capacity (up to 35%) than untransformed plants. The increased photosynthetic capacity in these plants is mainly associated with an enhanced stomatal conductance and a higher internal CO₂ concentration. Plants simultaneously expressing high levels of both enzymes also have a higher photosynthetic capacity. The results suggest that both PEPC and PPDK play a key role in organic acid metabolism in the guard cells to regulate stomatal opening. Under photoinhibitory and photooxidative conditions, PEPC transgenic rice plants are capable of maintaining a higher photosynthetic rate, a higher photosynthetic quantum yield by PSII and a higher capacity to dissipate excess energy photochemically and non-photochemically than untransformed plants. Preliminary data from field trials show that relative to untransformed plants, the grain yield is about 10–20% higher in selected PEPC and 30–35% higher in PPDK transgenic rice plants, due to increased tiller number. Taken together, these results suggest that introduction of C4 photosynthesis enzymes into rice has a good potential to enhance its tolerance to stress, photosynthetic capacity and yield.