Phloem unloading in roots of Ricinus communis L.

Processes relating to the movement of translocated and exogenously supplied sucrose into extravascular tissues of the primary root of Ricinus communis have been examined, particularly those concerned with sucrose metabolism and compartmentation. Sucrose synthetase activity increases during root development, levels in the stele being greater than those in the cortex. Activity in leaf homogenates was low, but developing leaves appear to contain greater sucrose synthetase activity than do fully-developed leaf tissues. Acid invertase activity is associated with soluble and insoluble fractions of crude homogenates from a variety of tissues. Soluble acid invertase activity is greatest in rapidly developing tissues of the root and shoot, while low activity is associated with root vascular tissue, leaf main veins and phloem-enriched tissue of the stem. Less than 20% of the total acid invertase activity was associated with pelletable fractions of crude homogenates, the exception being extrafloral nectaries sited on the petioles where 60-82% of the total acid invertase activity was insoluble. Insoluble activity of root and leaf tissues cannot be made soluble by chemical treatment and in the root a proportion of this activity appears to be present in the apoplast. The accumulation of exogenous sugar into excised root tips is optimal at a neutral pH and high solute potential. Both acid invertase activity and sucrose accumulation exhibit biphasic kinetics with respect to substrate concentration, and are inhibited by p-chloromercuribenzene sulphonic acid, glucose and fructose. Accumulation of hexose sugars occurs at rates greater than those of sucroseaccumulation, and hexose uptake appears to be preferential to that of sucrose. The extrastelar apopolast of the root can contain significant apoplastic pools, yet very little translocate can be washed from the apoplast in the presence of a variety of sugar accumulation inhibitors. It therefore appears that translocate fluxes within extrastelar tissues of the root are predominantly symplastic, whilst small pools of soluble sugar are maintained in the root apoplast. (D74412/87)