Stand density effects on branching in an annual legume (Senna obtusifolia)

Effects of proximity to nearest neighbours on shoot morphology and branch growth were examined for Senna obtusifolia (L.) I. & B., an annual legume species that displays wide variation in branching. Periodic surveys described location and type of growth at all nodes on plants in regularly spaced monocultures. Stands with interplant distances of 15-50 cm (51-5 plants m(-2)) formed closed canopies with similar amounts of leaf area and biomass. Number of lower primary branches, their degree of curvature, and location of branch apices relative to the main stem responded to interplant distance relatively early in shoot growth, before canopy closure. The final effects of ten-fold differences in planting density on the formation of these ascending lower branches were only two-fold differences in number of vertical shoot axes within the stand. An additional study examined response of lower branches to an adjacent gap in the stand. Initial number and direction of branch growth were not affected by location of the gap. The first effect of crowding by neighbours was on number of branches initiated by developing shoots. Differences in branching subsequently increased through differential survival and direction of branch growth. Such alterations principally affected lateral expansion of individual shoots, with little effect on radial symmetry or vertical distribution of leaves.