Regulation of vascular differentiation in leaf primordia during the rhythmic growth of Gnetum africanum

Abstract

The growth of the principal axis of Gnetum africanum Welw. is achieved by successive growth and rest periods. During the phase of growth arrest, the terminal bud produces a single pair of leaf primordia containing no vascular tissue whereas observation of the terminal bud of the vinelike stem reveals that the oldest of the leaf primordia do contain vascular tissue before emerging. The differentiation of vascular bundles in the leaf primordia of the principal axis begins only with the return of the growth period during which time new young leaves are formed. The rhythm of formation of the leaves and their vascularization can be accelerated by removal of the young leaves. A prolonged exogenous treatment with abscisic acid after removal of the young leaves reestablishes the inhibition and prevents initiation of vascularization. It seems, therefore, that in the principal axis young leaves inhibit vascular differentiation of the leaf primordia, which may partly explain the inhibition of growth.