ABSTRACT. Seeds of three species of lupine (Lupinus tennis, L. triticale and L. albus) were tested to determine if the seed
contains diffusable substances toxic to bradyrhizobia. L. albus seeds were less toxic to bradyrhizobia, followed by L. triticale.
Six strains of Bradyrhizobium were evaluated for their resistance to the toxic substances in lupine seeds. Zones of growth inhibition
were determined on yeast-mannitol-agar medium surrounding surface-sterilized seed. The effect of surface sterilization
of seeds by different chemical treatments on seed toxicity was assessed. Seeds soaked in water for 1 h before placing on agar
surface significantly decreased the inhibition zone. Also, the effect of soaking seeds in water for 4 h before planting and inoculation
on nodulation, nitrogen fixation and plant growth were investigated. Addition of seed diffusate to soaked seeds significantly
decreased nodulation and plant growth. Autoclaving the seed diffusate had no effect on the toxicity of the seed diffusate.
Addition of the absorbent polyvinylpolypyrrolidone (PVPP) to seed diffusates significantly decreased the inhibitory effect of
seed diffusate on nodulation and plant growth. Seed diffusate substances were water-soluble, heat-stable and partially bound to
PVPP.