Development of sink capacity of the 'storage root' in a radish cultivar with a high ratio of 'storage root' to shoot

Abstract

The mechanisms that control sink capacity are poorly understood. In radish, a major sink is the 'storage root', which begins to thicken early in development, mainly as a result of thickening of the hypocotyl. We investigated changes in the accumulation of dry matter, sink activity (increase in dry weight of the hypocotyl per unit of dry weight present per unit of time), carbohydrate content, levels of metabolites, activities of enzymes related to the breakdown of sucrose, and the profile of soluble proteins, as well as changes in anatomy, using hypocotyls of a cultivar with a high ratio of 'storage root' to shoot. We found that sink activity was strongly related to the level and activity of sucrose synthase but not to the activity of invertase. We also found a significant correlation between sucrose content and the level and activity of sucrose synthase. Our results suggest that sucrose synthase, but not invertase, might be critical for the development of the sink activity of the radish hypocotyl and that the level of sucrose might regulate the expression of sucrose synthase. A discussion of sink capacity is presented that includes consideration of structural changes in the hypocotyl.