Temporal Requirement for Phytohormone Balance in the Control of Organogenesis in Vitro

Leaf explants of Convolvulus arvensis produce roots or shoots when cultured irt uitro on media of appropriate phytohormone balance. Each of these processes of organogenesis can be divided into three parts: phase 1, the acquisition of competence for induction, phase 2, induction per se, and phase 3, morphological differentiation and growth. Control of the type of organogenesis by the balance of exogenous phytohormones resides in phase 2; phases 1 and 3 occur over :i vvide range of hormone balances. The competence for induction acquired in phase 1 has a positional component. Root-, shoot-, and callus-inducing media. RIM, SIM, or CIM. reslwctively, all can product competence for both root and shoot induction; however, roots arise from cells in the upper part of the callus, and shoots, from cells in contact with the medium. Some genotypes of Conwlnulus do not make roots on RIM, others, make no shoots on SIM. Because explants of some of these genotypes can be made to regenerate organ types by short precultures on seemingly inappropriate media, we conclude that these genotypes are blocked in the acquisition of competence.