Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth |
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Authors: | KIM, T. H. OURRY, A. BOUCAUD, J. LEMAIRE, G. |
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Abstract: | Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N2 fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with 15N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N2 fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N2fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N2 fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of 15N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N2 fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N2 fixation, N remobilization, N2 partitioning, regrowth |
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