Effect of Abscisic Acid on Photosynthesis in Lemna minor L.

Summary Abscisic acid inhibited the rate of 14 CO 2 , fixation by whole plants of Lemna minor L. and the Hill activity in isolated chloroplasts of the same .plant, but increased the activities of RuDP 2 )-carboxylase, PEP-carboxylase, NAD- and NADP-malate dehydrogenase and malic enzyme. There was no change in the fixation pattern after growing the plants in ABAsolutions. As it was seen from electron micrographs, the reduction of the Hill reaction could not be written due to changes in chloroplast membrane structure. From the results it is deduced that ABA may have a direct effect on the electron transport of photosystem II and that the effect of ABA on the flow of 14 C into individual photosynthetic products, which we have reported earlier, comes mainly from variations in stomatal behaviour.