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

During vegetative regrowth of Medicago sativa L., soil N, symbioticall y fixed N2 and N reserves meet the nitrogen requirements for shoot reg rowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the differ ent N sources and in xylem transport of amino acids during regrowth. E xogenous N uptake, N2 fixation and endogenous N remobilization were es timated by N-15 labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows deri ved either from N2 or from NH4NO3 during the first week of regrowth, t hereafter recovery increased linearly. Mineral N uptake as well as N2 fixation occurred mainly between the 10th and 18th day after removal o f shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants incr eased by at least three to five-fold, especially during the first 10 d ays following shoot removal. Compared to control plants, contents of a mino acids in xylem sap, during the first 10 days of regrowth, were re duced by about 72% and 82% in NH4NO3 grown and in N2 fixing plants, re spectively. Asparagine was the main amino acid transported in xylem sa p of both treated plants. Its relative contents during this period sig nificantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accomp anied by compensatory increase in the relative contents of aspartate a nd glutamine.