Is axis position within tree architecture a determinant of axis morphology, branching, flowering and fruiting? An...

Growth and reproductive strategies of plants are often related to particular, although usually poorly characterized, spatial distributions of shoots within the plant's architecture. In this study it is therefore hypothesized that a close relationship exists between architectural position, axis morphology (length, diameter, leaf area), and functional behaviour (branching, flowering and fruiting). The study focused on the architectural position of mango growth units, defined here as being the relative position, apical or lateral, on the parent growth unit, i.e. growing from the apical or a lateral meristem, respectively.

Stem length and leaf characteristics (area, dry weight) were measured on apical and lateral growth units of four mango cultivars over two years. Branching, flowering and fruiting were assessed for both growth unit types using an exhaustive description of tree vegetative and reproductive growth over two years. The relationships between growth unit diameter and flowering and fruiting were assessed for one of the four cultivars.

A pronounced morphological dimorphism was observed for the four cultivars. Across cultivars, stem length was significantly 1.31-1.34 times longer and total leaf area was 2.54-3.47 times larger in apical compared to lateral growth units. Apical growth units tended to branch, flower and fruit more than lateral growth units. The relationship between growth unit diameter and flowering rate was quadratic and dependent on growth unit position. The relationship between growth unit diameter and fruiting rate was linear and independent of growth unit position.

Morphological traits of mango growth units were clearly involved in the determinism of flowering and fruiting, although in different ways. The results, however, showed that current hypotheses of flowering, such as carbohydrate availability and florigenic promoters, are not sufficient in themselves if they neglect the hierarchical relationships between axes, i.e. their relative position, apical or lateral.