Effect of Exogenously Supplied Foliar Potassium on Phloem Loading in Beta vulgaris L.

Abstract

The effect of foliar application of K+ on processes associated with phloem loading was investigated in source leaves of sugar beet (B. vulgaris L.). KCl was supplied exogenously at concentrations of up to 100 mM in the solution bathing the abraded upper epidermis of source leaves. K+ added at concentrations below 30 mM generally promoted the rate of export of material derived from 14CO2 but not from exogenously applied [14C]sucrose. Paralleling promotion of export, the level of material derived from photosynthesis, which was released into the bathing solution, also increased in response to addition to K+ to the free space. Net photosynthetic rate was not affected. K+ at 5 and 15 mM concentrations did not stimulate uptake of [14C]sucrose into source leaf discs. The promotion of export rate by K+ may result primarily from an effect on the site regulating efflux of sucrose into the apoplast prior to loading into the minor veins, rather than on the loading site itself. A change in the level of sucrose in the free space appears to precede a change in export rate.