ABSTRACT
This chapter explores the information available on the three key enzymes in starch synthesis in sinks and sources. Starch is a plant reserve polysaccharide, an end-product of carbon fixation by photosynthesis, in which D-glucosyl units are linked predominantly by α-(1, 4) bonds. Starch seems to have an important function in the operation of stomatal guard cells, where it is broken down during the day while stomata are open and is resynthesized in the late afternoon or evening. Information regarding the amyloplasts can be obtained in a number of ways, for example localization of the starch biosynthetic enzymes using immunocyto-chemical studies, measurement of enzyme activity in isolated amyloplasts, and measurement of uptake of labeled metabolites by isolated plastids. The biosynthesis and degradation of starch in the leaf are, therefore, more dynamic than the metabolism in reserve tissues. Starch granules in storage tissues vary in size, shape, composition, and properties.
