The light reaction of photosynthesis regulates the Calvin cycle by producing the reactants required for it. If there is less light, the quantity of ATP and NADPH produced by the light reactions will be reduced, and so the Calvin cycle will not be able to go as fast as it would under high light intensities.
Thioredoxin is a protein that detects the redox potential change caused by the light reaction of photosynthesis. This allows it to control the activity of some enzymes in the Calvin cycle, allowing control of rate. This prevents the exhaustion of ATP and NADPH supplies in the cell, allowing other metabolic reactions to still continue. The activity of thioredoxin is regulated by ferredoxin-thioredoxin reductase.