PARTITIONING OF NITROGEN DERIVED FROM N-2 FIXATION AND RESERVES IN NODULATED MEDICAGO-SATIVA L DURING REGROWTH

Nodulated alfalfa plants were grown hydroponically. In order to quantify N2 fixation and remobilization of N reserves during regrowth the plants were pulse-chase-labelled with N-15. Starch and ethanol-soluble sugar contents were analysed to examine changes associated with those of N compounds. Shoot removal caused a severe decline in N2 fixation and starch reserves within 6 d after cutting. The tap root was the major storage site for metabolizable carbohydrate compounds used for regrowth; initially its starch content decreased and after 14 d started to recover reaching 50% of the initial value on day 24. Recovery of N2 fixation followed the same pattern as shoot regrowth. After an initial decline during the first 10 d following shoot removal, the N2 fixation, leaf area and shoot dry weight increased so rapidly that their levels on day 24 exceeded initial values. Distribution of N-15 within the plant clearly showed that a significant amount of endogenous nitrogen in the roots was used by regrowing shoots. The greatest use of N reserves (about 80% of N increment in the regrowing shoot) occurred during the first 10 d and then compensated for the low N2 fixation. The distribution of N derived either from fixation or from reserves of source organs (tap roots and lateral roots) clearly showed that shoots are the stronger sink for nitrogen during regrowth. In non-defoliated plants, the tap roots and stems were weak sinks for N from reserves. By contrast, relative distribution within the plant of N assimilated in nodules was unaffected by defoliation treatment.