Termination of seed growth in relation to nitrogen content of vegetative parts in soybean plants

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Abstract Final seed weight can be analysed as the product of seed growth rate and 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 vegetative organs. Thus, an estimation of the nitrogen amount in vegetative parts could be useful to analyse termination of seed filling. The aim of this work was to estimate the nitrogen available for seeds during the seed filling period and to study the different conditions which lead to termination of seed filling. ‘Maple Arrow’ plants were sown in field experiments for 2 years. During the seed filling period, different treatments were applied to manipulate the source-sink ratio. Values from the literature were used to establish the amount of nitrogen that could not be remobilized, and thus the amount of nitrogen still available for remobilization in vegetative parts was estimated. In all cases, except for a de-podding treatment, seed filling ended when nitrogen available for remobilization was exhausted. However in the de-podding treatment, physiological maturity occurred when nitrogen was still available: seeds had reached their maximal size, a function of cell number in the seed. Consequently two mechanisms could lead to termination of seed growth, depending on source-sink ratio.