Sucrose leakage from the maize scutellum: Evidence for the participation of the phloem

Abstract

Sucrose leaked from maize scutellum slices when they were incubated in fructose. The rate of leakage declined with time, but the initial rate could be maintained in the presence of HCl, ATP or certain buffers. The leakage process was a labile one and the ability of cells to leak (but not produce) sucrose declined during incubation of the slices in water at 30°. The leakage process was protected, however, during incubation in ice water. Sucrose leakage from whole scutella had properties similar to leakage from slices but had only about one-third the rate. The whole scutellum leaked appreciable sucrose only after the root—shoot axis was removed. The compounds which increased leakage also have the ability to displace or to form complexes with cations, and it appears that their effect on leakage is due to the removal of Mg 2+ (and possibly Ca 2+) from the slices. Evidence is presented that there is a pool of leakable sucrose which, when the leakage process is functioning, can be emptied into the bathing solution. The pool level is maintained, however, when fructose is present as a source of sucrose. The results presented are consistent with the idea that sucrose leakage originates in the sieve tubes of the phloem and is the end result of a series of events which includes intercellular sucrose transport, vein loading and phloem transport.