Sugar unloading in roots of Ricinus communis L.

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Abstract
The accumulation of exogenous D-fructose, D-glucose and sucrose into root tips of Ricinus communis was studied. Accumulation of sucrose is biphasic with respect to substrate concentration, both saturable and non-saturable phases being discerned. Accumulation is sensitive to pH, the exogenous solute potential, PCMBS, and other sugars. Sap representative of the apoplast was collected by centrifugation and contains approximately 40 mol m-3 hexose equivalents. Plants were labelled to the steady state with respect to cotyledonary sucrose derivatives; whole roots were then analysed by compartmental analysis and the results indicated similar apoplastic hexose concentrations. The vacuoles of roots grown in low salt solution contain most of the root soluble sugars, at a concentration of about 70 mol m-1 hexose equivalents. The sugar content of such roots decreases with availability of KCl to an extent which indicatesthat this involves a replacement of vacuolar hexose by alternative osmotica supporting the results from compartmental analysis.