Effects of juglone concentration on growth in vitro ofFrankia ArI3 andRhizobium japonicum strain 71.

Nitrogen-fixing nurse crops and cocrops of plant species nodulated byFrankia andRhizobium have been used to promote the growth of black walnut trees. Although walnut is known to inhibit the growth of certain associated plants due to its allelopathic derivative juglone (5-hydroxy-1,4-napthoquinone), juglone inhibition of symbiotic, nitrogen-fixing soil microorganisms had not been investigated. This research revealed that a concentration of 10(-3) M juglone absolutely inhibited the growth in vitro of aFrankia isolate from root nodules of red alder and ofRhizobium japonicum strain 71. Lesser concentrations of juglone inhibited the growth of these bacteria relative to the controls. The more-rapidly growingRhizobium strain exhibited slight growth at 10(-4) M juglone concentration, whereasFrankia growth was completely inhibited. Considering both the susceptibility of the host plant and nitrogen-fixing endophyte to the allelochemical juglone, caution should be exercised in selecting nitrogen-fixing plants as nurse crops for black walnut.