Assimilation of Nitrate into Asparagine for Transport in Soybeans

Abstract

In this study, the systematic analysis of nitrate assimilation and transport in soybean roots was further improved by analyzing the concentrations of nitrate assimilates, asparagine and glutamine, in soybean roots and the related enzyme activities. This provided a theoretical basis for the efficient utilization of nitrogen fertilizer in soybean farming. A dual-root soybean system with both sides being nodulated was used to provide nitrate withdrawal and resupply in three phases on one side, while the other side received nutrients without nitrogen under sand culture conditions. Measurements were taken of the root’s nitrate reductase and glutamine synthetase activities, as well as the concentrations of asparagine. Measurements were also taken of the asparagine concentration in the basal root peeled skin (where the primary transport pathway is the phloem) and the woody parts (where the primary transport pathway is the xylem). Furthermore, the concentration of glutamine in the roots was also assessed. The findings indicated a positive correlation between the nitrate concentration in the roots and the activity of glutamine synthetase in the roots on both sides. The levels of asparagine in the roots, specifically those in the basal root peeled skin and woody part on both sides, rose when nitrate was added and declined when nitrate was removed. There was no significant change in glutamine concentration within the roots of soybeans. Hence, we deduced that the local provision of nitrate to the dual-root soybeans might enhance the absorption of nitrate into the roots on both sides. Additionally, the assimilated substances were predominantly carried as asparagine through the peeled skin and woody sections of the basal root, facilitating transportation in both directions within the soybean plants (from shoot to root and from root to shoot).