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

The effect of foliar application of K(+) on processes associated with phloem loading was investigated in source leaves of sugar beet (Beta vulgaris L.). KCI was supplied exogenously at concentrations of up to 100 millimolar in the solution bathing the abraded upper epidermis of source leaves. K(+) added at concentrations below 30 millimolar generally promoted the rate of export of material derived from (14)CO(2) but not from exogenously applied [(14)C]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 of K(+) to the free space. Net photosynthetic rate was not affected. K(+) at 5 and 15 millimolar concentrations did not stimulate uptake of [(14)C]sucrose into source leaf discs.The data suggest that the promotion of export rate by K(+) results 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.