Structural differentiation among annual shoots as related to growth dynamics in Luma apiculata trees (Myrtaceae)

Abstract

A close examination of axis differentiation is key to the understanding of plant architecture and its ecological consequences. The present study was aimed at evaluating the relationship between shoot size/structure and shoot growth dynamics in Luma apiculata (Myrtaceae), a widely distributed and cultivated but little investigated Andean-Patagonian woody species. We assessed (a) organ preformation in apical buds, (b) shoot growth through repeated observations of labeled axes, (c) axillary production, leaf area, stem volume, dry mass of leaves, stem and reproductive structures, and allometric relationships for annual shoots of different sizes. The number of preformed organs in apical buds of L. apiculata averages seven nodes and exhibits little variability, in contrast with the highly variable growth period (from few weeks up to more than six months) and the size reached by annual shoots. The leaf area/stem volume ratio increased more sharply during shoot extension in short than in long shoots. Stem thickness was proportionally higher than other shoot-size measures in short shoots and long shoots as compared to intermediate-size shoots. Both the presence and the position of branches along shoots varied notably with shoot size. Proportional mass and meristem allocation to fruit production decreased with shoot size. Unlike many other broadleaved temperate and temperate-cold tree species so far studied, the size reached by L. apiculata shoots developed in the same year varies notably among axes in the same topological position, and seems to depend largely on the expansion of non-preformed organs. A low dependency of shoot growth on organ preformation may increase plasticity in axis structure, at the cost of a less hierarchical (more shrubby) architecture. Simultaneous organ differentiation and expansion may allow plants to fine-tune shoot structure to the prevailing conditions during the growing season, and represent an advantage whenever conditions favourable for growth exhibit high between-year variations.