A Study on the Effect of Indirect Nitrate Supply on the Nitrogen Fixation Capacity of Soybean Nodules

In this study, dual-root soybean (<i>Glycine max</i> L. Merr.) plants, with one side nodulated and the other nonnodulated, were used as experimental materials. The nonnodulated lateral roots were treated with excessive nitrate (200 mg·L^-1 nitrogen) for three days, followed by a three-day nitrate withdrawal, and then subjected to excessive nitrate again for another three days. Meanwhile, the nodulated side was continuously supplied with nitrogen-free nutrient solution. We measured the nitrogenase activity, nodule quantity, and concentrations of sucrose, starch, and soluble sugars, along with the microstructure of the nodules. By analyzing these data, we aim to provide theoretical insights into how indirect nitrate supply affects the nitrogen fixation capacity of nodules. The results demonstrated that indirect supply of nitrate to the soybean nodules reduced the nodule nitrogen fixation ability, which was manifested in the decrease in nodule dry weight, nodule number, and nitrogenase activity. The reason was found to be related to the decrease in carbon sources (sucrose, starch, and soluble sugar) allocated to the nodules. Further observation of the internal structure of the nodules found that the number of infected cells in the nodules decreased with the addition of nitrate, and increased with its withdrawal. However, the addition and withdrawal of nitrate did not change the effect of nitrate on the structure of infected cells around the nodules after the first addition of nitrate. These may be one of the important reasons why nitrate indirectly affects the activity of nitrogenase in nodules.