Role of cytokinin in vessel regeneration in wounded Coleus internodes

Cytokinin was found to be a controlling or limiting factor in vessel regeneration around a wound in internodes of Coleus blumei Benth. in which the endogenous cytokinin level was minimized. The cytokinin was applied in aqueous solution to the base of excised, mature internodes that had an active vascular cambium. Each internode also received IAA in lanolin at its apical end. Under low (0.1 %, w/w) or high (10%, w/w) auxin concentrations, the control internodes (without exogenous cytokinin) exhibited small amounts of vessel regeneration. At appropriate concentrations zeatin, kinetin and 6-benzylamino-purine (BAP) induced a significant increase in vessel regeneration around the wound. The three cytokinins also induced novel patterns of supplementary regeneration further from the wound surface. Kinetin and BAP showed the strongest promoting effect at 5 and 10 μg ml^-1, while zeatin was most effective at 20 μg ml^-1. At a low (0.1 %) auxin level zeatin was the most effective cytokinin, whereas kinetin was the most effective one at high (1 %) auxin. An inhibitory effect on vessel regeneration was observed at the highest kinetin concentration tested (50 μg ml^-1). The regeneration of vessels induced by cytokinin was very polar. Many more regenerated vessel members differentiated below the wound than above it, and the regeneration process proceeded acropetally from the base of the internode to its upper parts. Our results imply a basipetal polar increase in cambium responsiveness along the stem axis from internode 5 to 7. The possible significance of such a basipetal increase in cambium sensitivity in wood formation in trees is discussed.