Plant Physiology 70:335-339 (1982)
© 1982 American Society of Plant Biologists
Articles
Sorbitol Metabolism and Sink-Source Interconversions in Developing Apple Leaves 1
Wayne H. Loescher,
Gary C. Marlow and
Robert A. Kennedy
Department of Horticulture and Landscape Architecture, Washington State University, Pullman, Washington 99164
In apple (Malus domestica Borkh.) sorbitol is the primary product of photosynthesis, the major translocated form of carbon, and a common fruit constituent and storage compound. Previous work on sorbitol metabolism has revealed a NADPH-dependent aldose 6-phosphate reductase (A6PR) in green tissues, and a NAD-dependent sorbitol dehydrogenase in nongreen tissues. Results here show a decrease in sorbitol dehydrogenase activity and an increase in A6PR activity as leaves developing in the spring undergo the transition from sink to source. Sorbitol dehydrogenase activity reached a minimum as A6PR peaked. These changes were related to increases in leaf carbohydrate levels, especially sorbitol, and to increases in rates of net photosynthesis. Studies conducted in the autumn on senescing leaves also showed changes in enzyme activites, leaf carbohydrate levels, and photosynthesis. At this time, however, sorbitol dehydrogenase increased in specific activity, whereas A6PR activity, leaf carbohydrates, and photosynthetic rates all decreased substantially. Other experiments showed differences in the ability of young and mature leaves to metabolize sorbitol and in the distribution of sorbitol enzymes in leaves at transitional developmental stages. The results suggest that sorbitol metabolism in apple is tightly controlled and may be related to mechanisms regulating partitioning or source and sink activity.
1 Supported in part by the Columbia River Orchard Foundation, the Washington State Tree Fruit Research Commission, National Science Foundation Grant DAR-80-20160, and United States Department of Agriculture, Science and Education Administration Grant 59-2531-1-2-008-0. This is scientific paper 6178, College of Agriculture Research Center, Washington State University.
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