ABSTRACT
Nitrate is usually the principle form of mineral nitrogen available to plants in the soil. Many legume species also have direct access to biologically fixed nitrogen, but preferentially utilize nitrate rather than develop a root nodule symbiosis with Rhizobium or Bradyrhizobium. Further support for a localized effect of nitrate on nodule formation was reported by Alan Gibson and J. E. Harper. The capacity of the microsymbiont to assimilate nitrate has no bearing on its ability to nodulate its host in the presence of nitrate. Although nitrate inhibition of nodulation is a common phenomenon among legume species, there is considerable variation in the degree of regulation between different plant-bacteria combinations. Nitrite, the product of nitrate reductase activity and the first intermediate in the nitrate assimilation pathway, has been postulated to destroy hormones that may be involved in nodulation. Natural variants with increased nodulation have been identified in several legume species, but many of these were not selected specifically for nitrate-tolerant nodulation.
