TERMINATION OF SEED GROWTH IN RELATION TO NITROGEN-CONTENT OF VEGETATIVE PARTS IN SOYBEAN PLANTS

Final seed weight can be analysed as the product of seed growth rate a nd duration of seed filling, both of which can vary with environment. In indeterminate soybean (Glycine max L. Merr.), seed filling is often considered to be limited by the translocation of N compounds from veg etative organs. Thus, an estimation of the nitrogen amount in vegetati ve parts could be useful to analyse termination of seed filling. The a im of this work was to estimate the nitrogen available for seeds durin g the seed filling period and to study the different conditions which lead to termination of seed filling. 'Maple Arrow' plants were sown in held experiments for 2 years. During the seed filling period, differe nt treatments were applied to manipulate the source-sink ratio. Values from the literature were used to establish the amount of nitrogen tha t could not be remobilized, and thus the amount of nitrogen still avai lable for remobilization in vegetative parts was estimated. In all cas es, except for a de-podding treatment, seed filling ended when nitroge n available for remobilization was exhausted. However in the de-poddin g treatment, physiological maturity occurred when nitrogen was still a vailable: seeds had reached their maximal size, a function of cell num ber in the seed. Consequently two mechanisms could lead to termination of seed growth, depending on source-sink ratio.