In the soyabean [Glycine max (L.) Merr.]–B. japonicum
symbiosis, genistein has been identified as one
of the major compounds in soyabean seed and root extracts responsible for inducing the expression
of the B. japonicum nod genes. High combined nitrogen in the growth medium inhibits nodulation and
nitrogen assimilation. Two experiments were conducted to test the possibility of overcoming this
inhibition by adding genistein to the rooting medium and by incubation of B. japonicum cells with
genistein. One soyabean cultivar was used in the first experiment, and two in the second experiment.
The experiments were conducted in a glasshouse using a completely randomized design with three
rooting medium nitrate concentrations (0, 5 and 10 mM) and four genistein treatments. The genistein
treatments were 0 (control), incubation of B. japonicum cells with 5 μM genistein, and regular watering
with 5 μM or 20 μM genistein. A two way interaction existed in the first experiment, and two and three
way interactions existed in the second experiment. Root growth was inhibited by repeated watering
with 20 μM genistein. Weight per nodule was greater at 5 mM than at 0 mM nitrate. At 10 mM nitrate
watering with genistein resulted in significant increases in nodule dry weight per plant. Shoot nitrogen
contents were significantly increased at 5 mM nitrate by genistein incubation and watering with 20 μM
genistein. Watering with 5 μM genistein significantly increased nodule nitrogen concentrations at both
5 and 10 mM nitrate. The two soyabean cultivars responded differently to the genistein and nitrate
treatments in terms of nodule number, nodule weight, leaf nitrogen concentration and nodule nitrogen
content. Genistein could, at least partially, overcome the inhibition of soyabean nodulation and
nitrogen assimilation by nitrate.
