Periodic Shading and the Location and Timing of Branches in Soybean1

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AbstractAn understanding of the environmental influences on branch initiation and growth is essential for developing mechanistic models that simulate crop behavior. In this study shading was used to determine how limited C availability affects the timing and location of branches in soybean [Glycine max (L.) Merr.] plants grown outdoors. Carbon availability was limited by covering the plants with 66% shade cloth (34% light transmittance) for up to 3 weeks at various times during the seedling stage. Treatments were designated as 000, 100, 110, 111, 011, and 001, where 0 indicates the absence and 1 the presence of shade during each of the 3 weeks from the VC to the V6 stages. The number of branches and the number of nodes on branches were counted at each mainstem node on three observation dates. The total number of nodes on the branches was inversely proportional to the number of weeks in shade. In addition, both timing and location of branch growth on the plant were affected by when the shade treatments were imposed. The cotyledonary node was the preferred site for branch initiation prior to the V2 stage. The unifoliolate node was the preferred site between the V2 and the V6 stages. The number of nodes on trifoliolate branches was affected by the number of branch nodes already present at the cotyledonary and unifoliolate sites. Active branch growth at the lower nodes tended to suppress branch growth at the trifoliolate nodes.