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

In indeterminate soybean [Glycine max (L.) Merr.], yield is largely de pendent on seed number, so understanding of the dynamic of seed settin g is important. This study was conducted to describe the development o f the plant, taking into account the progression of the reproductive s tages of the seed along the stem. 'Maple Arrow' plants were grown unde r 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 deve lopment under contrasting environments. The stages of the seed after f lowering were determined by the seed water content (WC) relative to th e fresh weight. During the early phase, WC remained constant at about 0.85 g g-1. Then, WC decreased until physiological maturity, when WC r eached 0.60 g g-1. When WC began to decrease below 0.85 g g-1 at a nod e, the seed number at that node was fixed. This critical value corresp onds to the beginning of seed filling. The three reproductive stages, namely flowering, beginning of seed filling, and physiological maturit y, progressed linearly along the stem as a function of cumulative degr ee-days (base temperature 6-degrees-C). The rates of progression of th ese stages along the stem were constant under the two environments and between years. Conversely, durations of the lag-phase and seed-fillin g 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 wide ly among experiments. Seed WC appears to be a convenient criterion to determine the different periods of soybean seed development and confor ms to a general pattern of seed development in relation to their posit ion on the stem.