Analysis of root growth characteristics based on a pipe-model theory and distribution of root nodules in soybean (Glycine max Merr.)

Based on the pipe-model theory, the root growth characteristics and root nodule distribution in soybean grown in pots were analyzed, using the variety Fukuyutaka and two mutants derived from the variety Enrei, super-nodulating (En6500) and non-nodulating. Fukuyutaka plants were grown under various nitrogen levels, soil hardness, shading levels and bacillus densities, in soil. The ratio of the amount of thin roots which does not conform to the pipe model to that of thick roots which conformed to the model was nearly the same regardless of cultivation conditions. The formation of nodules began in minute radicles of 0.2-0.3 mm diameter class, and about 95% of root nodules were distributed in the thin roots of 1 mm or less. A high correlation was observed between the amount of roots and the number of root nodules in every root-diameter class. However, the big root nodules (larger than 3 mm in diameter) tended to be distributed over all root-diameter classes equally. It was suggested that the distribution of the root nodules with high nitrogen-fixing ability is closely related to the amount of the vascular bundles in each root diameter class