THE KINETICS OF SUCROSE CONCENTRATION IN THE PHLOEM OF INDIVIDUAL VASCULAR BUNDLES OF THE RICINUS-COMMUNIS SEEDLING MEASURED BY NUCLEAR-MAGNETIC-RESONANCE MICROIMAGING

The sucrose concentration was measured at 70-min intervals in the phlo em of individual bundles of the hypocotyl of Ricinus seedlings by H-1 nuclear magnetic resonance (NMR) spectroscopic imaging. The sucrose co ncentration stayed fairly constant in all bundles for more than 7 h if the cotyledons were embedded in the endosperm or excised and incubate d in 100 mM sucrose. If, however, the sucrose solution was replaced by sucrose-free buffer solution, the sucrose levels in the phloem decrea sed with a kinetic depending on the seedling: in some cases there was a smooth decline, in some a decline followed by a slight recovery and in some cases a clear-cut oscillation. The sucrose concentration was o ften not identical in the phloem of the individual bundles. The oscill ations were larger in the phloem at the apex of the hypocotyl than in the phloem at the base of the hypocotyl. Cutting the petiole of one co tyledon led to a decrease in sucrose not only in the four bundles dire ctly connected to the severed petiole but in all eight bundles of the hypocotyl. Cutting the petiole and dividing the vascular ring at the c otyledonary node and at the root crown did not prevent the decline of sucrose in all eight bundles. Therefore, a functional equilibration of translocated solutes between the eight bundles may occur within the 1 -h measuring interval by radial diffusion through the parenchyma of th e hypocotyl.