The interaction of nitrate and carbohydrates in rhizobial root nodule formation.

Abstract

The interaction of nitrate and carbohydrates on the nodulation of Phaseolus vulgaris has been examined using an isolated root culture system which allows feeding of nutrients via the base of the root or external feeding via the medium in which it grows. The evidence presented is not compatible with the hypothesis that a carbohydrate-nitrogen balance in the host regulates rhizobial nodule formation. It supports, instead the concept that there are 2 apparently separate effects of nitrate which depend on the place of application to the root. Externally supplied nitrate inhibited the nodulation of roots fed 2% sucrose via their bases. The inhibition of 4.4 x 10/sup 5/ M nitrate could be prevented by raising the level of sucrose fed via the bases to 5 or 10% or by externally adding 0.2% sucrose, mannitol or L-arabinos. L-arabinose, which was inhibitory to nodulation and root growth by itself, was also tested at external concentrations as low as 0.005%, but no prevention of inhibition by 4.4 x 10/sup 3/ M nitrate was obtained below 0.05% arabinose. Nitrate supplied via the base of the root had no effect on nodulation with roots fed 2% sucrose and promoted the nodulation of roots fed 5 ormore » 10% sucrose. The inhibition of nodulation of 2% sucrose roots by nitrate added externally was unaffected when nitr« less