ABSTRACT
This chapter considers the producer-consumer relationship between source and sink tissues and regulation of the energy balance that exists between them. The molecular mechanisms whereby these complex interactions are achieved are far from understood, but many lines of evidence suggest that there is metabolic coordination between source and sink reactions that involves many steps of reciprocal control. Maximal sink strength and maximal values for net carbon import occur at a stage of about 10% final leaf area. Large changes in C and N partitioning were observed in the plants expressing very low levels of NR activity, where the ability to assimilate N clearly restricts growth and photosynthesis. The mechanisms whereby photosynthetic electron transport activity is integrated with the metabolic activity of the chloroplast are complex and relatively poorly understood. Fruit development and in seedling growth the transition between sink and source in leaves is accompanied by a dramatic change in the relative activities of the enzymes involved in sucrose metabolism.
