A chemical contact pressure model for phyllotaxis

Abstract

This paper follows up an earlier suggestion that the concept of chemical contact pressure might be combined with a modified version of Adler's contact pressure model to provide a complete theory of phyllotaxis. It is proposed that the initiation and subsequent development of leaf (or other organ) primordia at the shoot apex proceeds as follows. Firstly, primordia are initiated at points on the apex where the nutrient level exceeds a certain threshold. The apical tip and existing primordia compete for nutrients, which move vertically more readily than tangentially. After they have been initiated, the primordia grow by spreading preferentially into areas of high nutrient density, so that the relative positions of their mid-points, and hence the divergence angles between successive primordia, are altered as a result of chemical contact pressure. This mechanism, coupled with the assumption of a decreasing ratio of the vertical spacing (corrected to account for the greater vertical mobility of nutrients) to the apical girth, leads to Fibonacci phyllotaxis, provided the primordia are initiated singly. Fujita's observations of divergence angle frequency are shown to be in good agreement with the proposed theory.