Abstract
The specific mechanisms by which nitrogen affects nodulation and nitrogen fixation in leguminous crops are still unclear. To study the relationship between nitrogen, nodulation and nitrogen fixation in soybeans, dual-root soybean plants with unilateral nodulation were prepared by grafting. At the third trifoliate leaf (V3) to fourth trifoliate leaf (V4) growth stages (for 5 days), nitrogen nutrient solution was added to the non-nodulated side, while nitrogen-free nutrient solution was added to the nodulated side. The experiment was designed to study the effects of exogenous nitrogen on proteins and metabolites in root nodules and provide a theoretical reference for analyzing the physiological mechanisms of the interaction between nitrogen application and nitrogen fixation in soybean root nodules. Compared with no nitrogen treatment, exogenous nitrogen regulated the metabolic pathways of starch and sucrose metabolism, organic acid metabolism, nitrogen metabolism, and amino acid metabolism, among others. Additionally, exogenous nitrogen promoted the synthesis of signaling molecules, including putrescine, nitric oxide, and asparagine in root nodules, and inhibited the transformation of sucrose to malic acid; consequently, the rhizobia lacked energy for nitrogen fixation. In addition, exogenous nitrogen reduced cell wall synthesis in the root nodules, thus inhibiting root nodule growth and nitrogen fixation.
Highlights
High soybean yields depend on the combined effects of nitrogen fixation in the root nodules and fertilization with nitrogen
With nitrogen supplied to non-nodulated side roots for 5 days, no significant difference in the root nodule number and weight was found on nodulated side roots
The results showed that nitrogen supplied for 5 days had no significant effect on nodule growth and development
Summary
High soybean yields depend on the combined effects of nitrogen fixation in the root nodules and fertilization with nitrogen. Exposing soybean roots to high concentrations of nitrogen can inhibit root nodule growth, accelerate root nodule senescence, and reduce the efficiency of nitrogen fixation [1,2]. The effects of exogenous nitrogen on nodulation and nitrogen fixation in soybeans are influenced by nitrogen concentration, application site, and treatment time, among other factors [3,4,5,6,7,8]. Exogenous nitrogen was separately supplied to the upper and lower layers of soybean roots [9,10] or supplied to the left and right sides of dual-root soybeans [11,12], and all showed a rapid and reversible effect of nitrogen on root nodule growth and nitrogenase activity. Tanaka et al [15] and
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