CHANGES IN THE ACCUMULATION OF FLAVONOID AND ISOFLAVONOID CONJUGATES ASSOCIATED WITH PLANT-AGE AND NODULATION IN ALFALFA (MEDICAGO-SATIVA)

Medicarpin 3-O-glucoside-6''-O-malonate (MGM), formononetin 7-O-glucos ide-6''-O-malonate (FGM) and two glycosides of coumestrol (CG), one of which was characterised as a malonylated glucoside, have been identif ied as major isoflavonoid metabolites in the roots of healthy alfalfa (Medicago sativa) plants. Foliage contained a conjugate of the flavono id apigenin, very low levels of medicarpin and MGM, and depending on t he cultivar studied, formononetin and FGM. In the foliage the isoflavo noids were restricted to the stems. Seeds contained conjugates of quer cetin, luteolin and 7,4'-dihydroxyflavone but no isoflavonoid conjugat es. When alfalfa plants were grown under controlled conditions, isofla vonoid conjugates were first observed in the roots 24 h after emerging , and continued to accumulate as the plants matured over a 57 day peri od. Inoculating the plants with a commercial formulation of Rhizobium meliloti (Nodulaid) reduced FGM accumulation in the roots but did not affect MGM or CG content. Under field conditions plants accumulated hi gher concentrations of the isoflavonoid conjugates in both leaves and roots than plants grown under controlled conditions. Field-grown plant s which were not inoculated with Nodulaid contained higher levels of b oth MGM and FGM in the roots than nodulated plants. The conjugation of isoflavonoids in alfalfa cell cultures resembled that of the roots in that they accumulated FGM and MGM as major metabolites. However, cult ures accumulated a higher proportion of their conjugated isoflavonoids as MGM and formononetin glucoside than did the roots and did not synt hesise coumestrol. Comparisons with clover species revealed that FGM a nd MGM were common metabolites in forage legumes bur alfalfa was unusu al in having negligible levels of isoflavonoid conjugates in the folia ge. Our results clarify the nature and distribution of isoflavonoid co njugates in alfalfa and demonstrate that their accumulation is regulat ed in response to both plant age and nodulation.