Accumulation of N reserves and vegetative storage protein (VSP) in taproots of non-nodulated alfalfa ( Medicago sativa L.) are affected by mineral N availability

Abstract

Our objective was to understand how mineral N availability alters accumulation of N reserves (nitrate, amino acids, soluble proteins and vegetative storage proteins known as VSP) in alfalfa ( Medicago sativa L. cv Lodi) taproots. The effects of variation in NH 4NO 3 availability were followed by studying non-nodulated plants grown under hydroponic conditions during 21 days with (i) different N supplies which corresponded to N-replete plants (N100%=optimal N) and N-limited plants receiving only 50% (N50%) or 25% (N25%) of optimal N (Experiment I), or (ii) decreasing concentrations of NH 4NO 3 (1000, 250, 100 or 50 μM, Experiment II). Regardless of the N-limitation mode (Experiments I or II), and compared with higher N treatments (N100 or 1000 μM), there was a significant reduction of total shoot dry matter per plant for lowest N treatments (N25 or 50 μM). This was accentuated by the degree of N deficiency in Experiment I only. In Experiment II, taproot biomass significantly increased for low N treatments. In both experiments, total N, nitrate and amino acid concentrations in taproots increased for high N treatments, while the concentration of soluble proteins, and particularly VSP, increased for low N treatments. These results indicated that non-nodulated alfalfa was able to accumulate N reserves (mainly as VSPs), even under N-limited conditions, while under high mineral N availability, taproot amino acid concentrations (mainly asparagine) increased without a corresponding increase in soluble protein concentration. These results show that alfalfa was capable of optimizing N cycling and storage as a function of mineral N availability. These adaptive responses to low soil N environments also allow alfalfa to go dormant and perenniate, while awaiting more favorable conditions for shoot growth.