Abscisic acid inhibits phloem loading of sucrose

The effect of abscisic acid (ABA) on the uptake of sucrose by discs from castor bean ( Ricinus communis L. cv. Zanzibarensis) cotyledons was investigated. Incubation on ABA solutions for one hour or longer significantly inhibited sucrose uptake. The effect was measurable at ABA concentrations as low as 0.1 μ M . The inhibition was due to a lowering of the apparent V_max of the sucrose-carrier system, the K_m being unaffected. Uptake of sucrose was coupled to proton uptake but ABA had no detectable effect on the latter process. It is suggested that ABA exerts an effect on the phloem loading of sucrose by enhancing the efflux of sucrose. In leaf discs from three other plant species ( Beta vulgaris L., Petunia hybrida L. and Phaseolus vulgaris L.) ABA exerted a similar effect. The results are discussed with respect to source-sink relationships.