Inhibition of photosynthesis of sunflower leaves by an endogenous solute and interdependence of different photosynthetic reactions.

Photosynthesis of Helianthus annuus L. leaves was transiently inhibited and respiration was stimulated when a leaf was detached from the plant by cutting the petiole under water. These effects were caused by a solute which was released by cutting and was transported by the transpiration stream to the leaf blade. This endogenous solute decreased the quantum efficiency of photosynthesis and inhibited reactions of the Calvin cycle. It exerts its effects by uncoupling ATP synthesis from electron transport, thus stimulating respiration and inhibiting photosynthesis. The observation that not only the ATP-dependent reactions of photosynthesis, but also the light-regulated enzymes such as fructose bisphosphatase and ribulose bisphosphate carboxylase were inhibited in the presence of the solute illustrates the complex dependence of Calvin-cycle enzymes on the energization and the redox state of the thylakoid system. Since electron pressure increased during the inhibition of photosynthesis, deactivation of fructose bisphosphatase cannot be explained by effects on the thioredoxin system which is responsible for the light activation of this enzyme.