6. Any time the stomata close during daylight hours, photorespiration can occur.
This happens because the supply of carbon dioxide is shut off. At the same time, oxygen levels increase as a result of continuing light-dependent reactions! When CO2 is in very short supply, the oxygen molecule may bond first to the enzyme Rubisco and then to the 5-carbon RuBP. This results in one PGA being made. This allows the light-independent reactions to continue, and thus photosynthesis still occurs. 12 full cycles of the light-dependent reactions must now occur to form one glucose (rather than 6), so photosynthesis occurs more slowly.
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