Acclimation of potato plants to polyethylene glycol-induced water deficit I. Photosynthesis and metabolism

The acclimation of photosynthesis and metabolism in response to waterdeficit is characterized using hydroponically grown potato plants(Solanum tuberosum cv. Désirée).Plants were subjected to a reduced water potential of the nutrient solutionby adding 10% (w/v) PEG 6000. PEG-treated plants were retarded in growth.Leaves which had been fully developed before the PEG treatment and leavesgrown during the PEG treatment showed different phenotypes and biochemicaland physiological properties. Photosynthesis of all leaves decreased duringthe whole treatment. However, the decrease of photosynthesis in the twotypes of leaves had different causes indicated by differences in theirmetabolism. Leaves which were fully developed at the beginning of the PEGtreatment began to wilt starting from the leaf rim. The apoplastic ABAcontent increased, coinciding with a decreased stomatal conductance.Increased energy charge of the cells indicated impaired chloroplasticmetabolism, accompanied by a decrease of amounts of chloroplastic enzymes.The apoplastic and the symplastic ABA content were increased during waterdeficit and because ABA was concentrated in the cytosolic compartment it issuggested that ABA is involved in decreasing photosynthetic enzyme contentsin old leaves. Young leaves, grown after the imposition of water deficit,were smaller than control leaves and had a curly surface. In young leavesapoplastic and cytosolic ABA contents were identical with control values.Carboxylation efficiency of photosynthesis was decreased, but the water useefficiency remained unchanged. Metabolic data of the photosyntheticpathways indicate a down-regulation of chloroplastic metabolism. It isconcluded that in young leaves photosynthesis was non-stomatally limited.This limitation was not caused by ABA.