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

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 NH4NO3 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 NH4NO3 (1000, 250, 100 or 50 mM, Experiment II). Regardless of the N-limitation mode (Experiments I or II), and compared with higher N treatments (N100 or 1000 mM), there was a significant reduction of total shoot dry matter per plant for lowest N treatments (N25 or 50 mM). 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. # 2003 Elsevier Ireland Ltd. All rights reserved.