ABSTRACT
This chapter explores the modification of source metabolism with respect to photosynthetic capacity, partitioning of assimilates between starch and sucrose synthesis, and sucrose export. It discusses the possibilities to influence sink strength through adding or removing activities which hydrolyze sucrose or through manipulating starch biosynthesis. The chapter presents the genetic manipulation of carbohydrate metabolism. Prerequisite for the expression of foreign genes or the overexpression of endogenous genes is the ability to control the transcription of the respective gene in a desired way. The ectopic expression of proteins in transgenic plants creates novel genetic variability which cannot be generated by classical breeding techniques. In the transgenic potato plants in which AGPase has been down regulated under the control of the CaMV 35S promoter, tubers produced only marginal amounts of starch. Photoassimilates in the photosynthetically active leaf are essentially partitioned either into sucrose biosynthesis for export out of the leaf or into transitory starch formation.
