N-feedback Regulation of N2 Fixation in Medicago Truncatula under P-deficiency

The objective of the present study was to determine a possible interconnection between a P-deficiency induced decline in nitrogen fixation of Medicago truncatula Gaertn. (barrel medic) cv. Jemalong and the accumulation of nitrogen or amino acids in different organs of the plants as a possible agent of a N-feedback mechanism. Nodulated plants were grown in N-free nutrient solution at increasing levels of P-concentration in the solution (0.2-5 μM P). Increased P-supply improved plant growth and nodule functioning. When nitrogenase activity (H2evolution) was measured at the 1 and 5 μM P-level, the activity was more than doubled at the 5 μM P supply. Coinciding with the improved nitrogenase activity, nodule number and plant Pand N-content improved with higher P-supply. With increasing P-supply the N-concentration in shoots and roots declined, while the C/N ratio decreased. In nodules the total amount of free amino acids was higher at low P and this was almost exclusively a result of higher asparagine concentrations. Thus, asparagine might be a substance that exerts N-feedback effects in nodules and it might be increasingly retranslocated to nodules via the phloem when the nitrogen content in the shoot is high as in our experiments under conditions of low P supply. Overall, the results are consistent with an effect of P-deficiency on nitrogen fixation that is mediated through an N-feedback mechanism.