Allometric Relationships Between Leaf and Inflorescence Mass in the Genus Protea (Proteaceae): An Analysis of the Exceptions to the Rule

Abstract

Recent studies have demonstrated that the sizes of plant parts ate allometrically related. In this study we investigated the relationship between leaf and inflorescence mass in 35 species and one interspecific hybrid in the morphologically and developmentally plastic genus Protea (Proteaceae). We show that the basic relationship is biomechanically based and identify four ways in which species have escaped this constraint: firstly, species with terminal inflorescences which delay the flowering of their inflorescence for a year after initiation have a high ratio of inflorescence to leaf mass; secondly, species with pendulous terminal inflorescences which have a similarly high ratio; and thirdly, species which have axillary inflorescences and thus escape the biomechanical constraint of supporting the mass of an inflorescence at the end of a branch. Fourthly, species with a prostrate habit have a low ratio of leaf to inflorescence mass, whether or not they belong to groups with terminal or axillary inflorescences. The allometric approach provides an objective method for identifying exceptions and how they may have arisen, and for formulating testable hypotheses about the adaptive value of traits. Key-words: Allometry, inflorescence mass, leaf mass, plant architecture, Protea