Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Peter W. Jordan


Effects of competition on growth and formation of shoot structure in sicklepod (Senna obtusifolia) were examined, with particular emphasis on the effect of nearest neighbors on branching and canopy formation. Sicklepod was established as (1) stands of 26 plants m$\sp{-2}$ added to soybean, (2) individuals widely enough spaced to produce only interspecific competition when added to soybean, or (3) regularly spaced monoculture stands with 15, 25, 35, and 50 cm interplant distances. Repeated surveys of individual shoots characterized sicklepod growth and shoot structure in terms of position of and growth at each node. Sicklepod leaf area, internode elongation, node production, and branching were affected by competition with soybean or other sicklepod, while shoot height was less affected. Branches originating at lower main-stem nodes were the primary means of expanding laterally and forming a closed canopy; these were strongly suppressed by increased competition. Thus, early-season competition with soybean substantially limited the number of leaves in the upper sicklepod canopy that shaded soybean later in the growing season. Decreased rate of branching and more vertical branch growth were detected even before effects of crowding altered shoot height or number of leaves. Branches rapidly filled available area in monocultures ranging over tenfold in stand density; this resulted in ninefold differences in individual shoot dry weight, but no difference in stand dry weight at harvest. Removal of neighboring plants, or planting adjacent to soybean rows, did not alter shoot form. Thus, while sicklepod growth reflected differences in distance to nearest neighbors, it did not respond with directional growth to lateral variation in density of the neighboring canopy. Stand density affected vertical distribution of leaf area before the canopies closed; but after canopy closure, vertical leaf distribution, leaf area index, and light interception characteristics were not different among stand densities. Models of light interception based on either leaf area index or number of leaf-bearing nodes per unit area were comparable, this suggested that the method of counting nodes provided a simple and rapid method of estimating shading of soybean by sicklepod.