RESPIRATION DURING PHOTOSYNTHESIS

The respiratory activity of plants in the light, measured as CO:! release from the TCA cycle or O-2 consumption by the respiratory chain, varies between 25 and 100% of the dark respiratory activity. This has been interpreted as evidence for an inhibition of respiration during photosynthesis. However, studies with specific respiratory inhibitiors have shown that oxidative phosphorylation occurs in the Light and provides the cytosol with ATP, which is required for sucrose synthesis. Respiratory activity in the light might also be required to sustain a high photosynthetic capacity and might even prevent photoinhibition. Sources of redox equivalents for oxidative phosphorylation can be photosynthetically generated, externally oxidized NADPH; photorespiratory oxidation of glycine to serine; or a partial activity of the TCA cycle. Fifty to seventy-five percent of the redox equivalents produced in the mitochondria remain in the matrix and can function in ATP syntheses. The other 25-50% are exported via the mitochondrial malate-oxaloacetate shuttle and can function in nitrate reduction in the cytosol or hydroxypyruvate reduction in the peroxisomes.