ISOFLAVONOID METABOLISM IN RESISTANT AND SUSCEPTIBLE CULTIVARS OF WHITE CLOVER INFECTED WITH THE STEM NEMATODE DITYLENCHUS-DIPSACI

Seedlings of white clover (Trifolium repens), which were either resistant or susceptible to the white clover race of Ditylenchus dipsaci, were infected with the white clover race and with the oat race, which is avirulent on clover. In all tissues of uninfected clover seedlings isoflavonoids occurred predominantly as their glycosidic conjugates, in the order roots > meristems > leaves with formononetin-7-O-glucoside-6''-O-malonate (FGM) and medicarpin-3-O-glucoside-6''-O-malonate (MGM) as the major metabolites. The conjugates accumulated with age in all tissues and no differences were observed between the isoflavonoid content of healthy susceptible and resistant plants. Inoculation of the bud tissue of the susceptible, but not the resistant, seedlings with the white clover nematode race resulted in hypertrophy accompanied by reproduction of the nematodes. In contrast the oat race initialy invaded the seedlings but was unable to reproduce. Infection with either race elicited the accumulation of medicarpin, MGM, and FGM in the meristems to a similar degree in resistant and susceptible seedlings. However, formononetin accumulated only in the infected meristems of the resistant plants. Infection did not induce the accumulation of appreciable amounts of isoflavonoids or their conjugates in the expanded leaves nor in the roots. We conclude that infection with D. dipsaci elicits the localized accumulation of antibiotic isoflavonoids and their conjugates and that the accumulation of formononetin was associated with the race specific resistance.