Factors controlling cambial development in the hypocotyl of Ricinus communis L.

The development of the cambium in the interfascicular region of young castor bean hypocotyls was investigated under various experimental conditions. The cotyledons were found to be indispensable for the onset of cambial development. Young isolated hypocotyls developed a cambium when cultured in a simple medium lacking phytohormones but fortified with sucrose. Addition of indole-3-acetic acid to the medium did not enhance the rate of cambial development. When 2,3,5-triiodobenzoic acid was added, the typical acropetal extension of cambial development in the hypocotyl was maintained. Gibberellic acid (GA3) was found to have strong stimulatory properties, and, when combined with sucrose, to be capable of inducing a rate of cambial development comparable to that in the intact plant. Inhibitors of gibberellin biosynthesis applied to a hypocotyl-cotyledon system cultured in vitro or sprayed on the cotyledon of a decapitated seedling in vivo, depressed cambial development in the hypocotyl. Simultaneous application of gibberellic acid in the spray experiments restored the original level of cambial development. The significance of the present findings is discussed in relation to the prevailing concept of cambial stimulation.