PHOTOPERIOD AND PLANTING DATE EFFECTS ON THE SPATIAL-DISTRIBUTION OF BRANCH DEVELOPMENT IN SOYBEAN

Reduced branch development in late-planted soybean [Glycine max (L.) Merr.] is a cause of yield reductions in the southeastern USA. To determine the effects of photoperiod and planting date on the spatial distribution of branches, photoperiod stimulations of an optimal (24 May) and a nonoptimal late planting date (24 June) at Baton Rouge, LA (30.degree. N Lat), were imposed on ''Centennial'' soybean in growth chambers. In a field study, ''Lee 74'' was planted at optimal (mid-May) and nonoptimal (early July) planting dates in 1984 and 1985 at Baton Rouge, LA [Louisiana, USA] on an Olivier silt loam (fine-silty, mixed thermic Aquic Fragiudalf) soil. Plants grown under the May stimulated photoperiod had greater total branch length and concentrated their branches in the lower and middle sections of the plant. Under the June simulation, there was little branch development below the sixth main axis node, with most branches occurring on the middle and upper sections of the plant. Total branch length and branch number were reduced in the July field planting as compared to the May planting. This was related to the failure of branches to form in the axils of the lower main axis nodes and by the lack of upper main stem nodes from which branching could occur. Main stem seed yield was greater in the July compared to May planting date. The photoperiod of a late nonoptimal planting date thus restricts branch development at the lower nodes through a direct influence on the formation of branches and at the upper nodes through indirectly reducing the potential sites for branch development by promotion of early flowering.