SEQUENTIAL DEVELOPMENT OF FLOWERS AND SEEDS ON THE MAINSTEM OF AN INDETERMINATE SOYBEAN

In indeterminate soybean [Glycine max (L.) Merr.], yield is largely dependent on seed number, so understanding of the dynamic of seed setting is important. This study was conducted to describe the development of the plant, taking into account the progression of the reproductive stages of the seed along the stem. 'Maple Arrow' plants were grown under field conditions in Guadeloupe (French West Indies, tropical climate, ferralic Fluvisol) and at Dijon (France, temperature climate, clayey eutric Cambisol) for 2 yr in order to investigate the pattern of development under contrasting environments. The stages of the seed after flowering were determined by the seed water content (WC) relative to the fresh weight. During the early phase, WC remained constant at about 0.85 g g-1. Then, WC decreased until physiological maturity, when WC reached 0.60 g g-1. When WC began to decrease below 0.85 g g-1 at a node, the seed number at that node was fixed. This critical value corresponds to the beginning of seed filling. The three reproductive stages, namely flowering, beginning of seed filling, and physiological maturity, progressed linearly along the stem as a function of cumulative degree-days (base temperature 6-degrees-C). The rates of progression of these stages along the stem were constant under the two environments and between years. Conversely, durations of the lag-phase and seed-filling periods at the first node, calculated respectively between flowering and WC = 0.85 g g-1 and between WC = 0.85 and 0.60 g g-1, varied widely among experiments. Seed WC appears to be a convenient criterion to determine the different periods of soybean seed development and conforms to a general pattern of seed development in relation to their position on the stem.