AMINO-ACID CONTENT IN XYLEM SAP OF REGROWING ALFALFA (MEDICAGO-SATIVA L) - RELATIONS WITH N-UPTAKE, N2 FIXATION AND N-REMOBILIZATION

During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N2 and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N2 fixation and endogenous N remobilization were estimated by N-15 labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N2 or from NH4NO3 during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N2 fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH4NO3 grown and in N2 fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine.