Interrelationship Between Plant Developmental Stage, Plant Growth Rate, Nitrate Utilization and Nitrogen Fixation in Hydroponically Grown Soybean

Abstract

The growth rate of the indeterminate soybean plant [Glycine max (L.)Merr.] slows as it proceeds from vegetative phase into reproductive growth. Yet, the well-nodulated plant acquires most of its nitrogen during reproductive growth. Thus, the interrelationship between plant developmental stage and nitrogen fixation was examined. It is shown that, regardless of the age of the hydroponically grown soybean plant at the time of its inoculation with Bradyrhizobium japonicum, the highest rate of nitrogen fixation occurs during the pod-filling stage (R5). Nevertheless, maximum total nitrogen fixation is generally achieved when inoculation occurs at the full-bloom stage (R2). It is shown, however, that flower buds and flowering are not responsible for the enhanced nodulation and nitrogen fixation. Rather, the data suggest that the onset of rapid nodulation occurs soon after the initiation of the developmentally programmed drop in foliar nitrate reductase activity. The ensuing increase in nitrogen fixation provides the plant with much of its needed nitrogen and hence stimulates plant mass accumulation during pod-fill. It is suggested that nitrogen fixation enhances growth of the soybean plant by increasing its net photosynthetic efficiency during reproductive growth and by providing the needed nitrogen at the appropriate time for maximum seed growth.