REPRODUCTIVE DEVELOPMENT OF AN INDETERMINATE SOYBEAN AS AFFECTED BY MORPHOLOGICAL POSITION

The indeterminate growth habit of some soybean [Glycine max (L.) Merr.] cultivars results in sequential grain setting; thus, knowledge of the reproductive development of soybean is important for understanding yield variability. The objective of this study was to investigate the influence of the timing of flowering and the morphological position of reproductive organs on individual seed development and growth. 'Maple Arrow' plants, Maturity Group 00, were grown under field conditions in Guadeloupe (French West Indies, ferralic Fluvisol) for 2 yr and at Dijon (France, clayey Eutric Cambisol) for 1 yr, with various treatments including different sowing densities, a depodding treatment, and N supply during seed filling. Regardless of their morphological position, reproductive organs that flowered simultaneously also reached the beginning and termination of seed filling simultaneously. Thus, the lag-phase duration and the duration between flowering and physiological maturity were Linearly related to timing of flowering. Slopes for both relationships were significant for all treatments and locations. Conversely, timing of physiological maturity was variable among treatments. The onset of physiological maturity seemed to be determined by the time when N reserves of vegetative parts were exhausted. However, individual seed size seemed to determine physiological maturity when sources were greater than sinks. On the other hand, morphological position had a great effect on growth of individual seed, whereas timing of flowering was the main determinant of further seed development. Organization of reproductive development of the whole plant depends strongly on the timing of flowering of each reproductive organ. Conversely, the occurence of physiological maturity of the whole plant and individual seed growth probably depends on the amount of available assimilates during seed filling.