The influence of shoots, roots, and hormones on sucrose distribution

The effects of hormones on sucrose distribution were studied in relation to the control of the relative growth of the shoot and the root. The roots and the shoots above the cotyledons of bean seedlings were removed and the remaining explants allowed to regenerate so that they consisted of growing buds, a segment of the hypocotyl, and short growing roots. [¹⁴C]Sucrose was applied to the middle of the hypocotyl and its distribution was measured in relation to the presence of the shoots and roots and to their replacement by auxin and a cytokinin. It was found that radioactivity moved towards both the shoot and the root, and that removing one growing region greatly reduced the transport in its direction and increased the relative transport in the opposite direction. Both hormones restored a large part of the movement of radioactive sucrose when they replaced any one of the growing regions. However, there was some preferential action of benzyladenine when it replaced the buds and of auxin when it replaced the roots. The results indicate that in the intact plant the development of the shoot and the root is correlated by a positive feedback involving auxin and cytokinins, and the movement of sucrose is controlled indirectly by the sink activity of these growing regions. This control of the movement of sucrose, and presumably other metabolites,accounts for the short-term competitive relations between the shoot and the root.