Abstract
A method to bring about legume root nodulation in vitro by growing berseem plants (Trifolium alexandrinum L.) aseptically and inoculating the roots with the specific Rhizobium has been attempted. The method was used to verify the reported field observations of inhibitory effects of excess of combined nitrogen in soil on nodulation. To find out how far the presence or absence of nitrate had interfered with the amino acid composition of the plant and its root exudate, the root media and extracts of plant tissue of both nitrate-added and nitrate-free uninoculated plants at different ages during seedling growth were analyzed for free amino acids by paper chromatography. As expected, variations in the presence or absence of different amino acids occurred. However, it was observed that although asparagine was present in plant tissue in relatively higher concentration in both the nitrate-added and nitrate-free series, it was exuded by roots of nitrate-added series alone. Attention has been drawn to the presence of tryptophane in root exudates and its implication in the infection process of legumes by Rhizobium, since the amino acid is a possible precursor of indoleacetic acid, which, in turn, has been shown to effect curling and deformation of root hairs, a phenomenon observed in pre-penetration stages of legume–Rhizobium symbiosis.
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