The Effect of Potassium on the Fixation of Molecular Nitrogen by Root Nodules of Vicia faba

The effect of potassium supply of Vicia faba on the fixation of molecular nitrogen by root nodules was studied by using (15)N-labeled molecular nitrogen. Plants well supplied with potassium showed higher contents of (15)N in the soluble amino fraction and in the protein fraction of various plant organs as compared with plants of a lower potassium status. This effect was evident particularly in the root nodules. Assimilation experiments, carried out with (14)CO(2), revealed that the content of radioactivity in the sugars and amino acids of the root nodules was increased by the potassium supply of the host plants. In particular, the content of (14)C amino acids in the root nodules was influenced beneficially by potassium, which means that potassium favored the provision of reduced nitrogen (NH(3)). It is postulated that the better carbohydrate supply of nodules, by plants well supplied with potassium, results in a higher carbohydrate turnover in the nodules and thus the provision of ATP and reducing electrons required by the nitrogenase is enhanced.