Partitioning of Nitrogen Derived from N2Fixation and Reserves in NodulatedMedicago sativaL. During Regrowth

Abstract

Nodul¯ted 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 15N. 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 15N 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.