Index >> Rhizobium and Legume Root Nodulation >>Growth Substances

Lower levels of IAA are conducive to early nodulation whereas higher doses result in stunting and other morphogenetic effects on roots. Naphthalene acetic acid inhibits nodulation in subterranean clover whereas p­chlorophenoxy-isobutyric acid stimulates nodulation. Growth retarders such as B-nine (N-dimethylamino succinamic acid) and benzimidazole alone or in combination with IAA reduce the size of roots and the number of nodules in cowpea. Other growth substances such as kinetin and gibberellic acid do not induce any change in nodulation.

Among B-vitamins, thiamine, pyridoxine, biotin and riboflavin have no stimulatory effect on nodulation whereas meso-inositol increases the number of nodules and the percentage of nodulating roots on the root system. On the other hand, vitamin B₁₂ depresses nodulation. Certain other unknown factors present in coconut water and extracts of alfalfa seeds also promote nodulation.

Since these studies have been done at one fixed concentration of such compounds, further studies with a range of concentrations of individual metabolites are necessary before any generalizations are made with regard to their influence on nodulation.

The effects pf antimetabolites on nodulation and growth of leguminous plants have also been investigated. The number of nodules in birds foot trefoil (Lotus corniculatus) is increased by indole 2-phenyl-n-butyric acid, D- and L-Ieucine, barbituric acid, oxythiamine and quercetin, although L­picolinic acid, a niacin antagonist, prevents nodule formation in spite of the compound possessing no detrimental influence on the growth of Rhizobium. Interestingly enough, another niacin antimetabolite, pyridine­3-sulfonate, does not exert an inhibitory effect in the same legume species.