Studies on Physiology of Nodule Formation: VI. The influence of Bacterial Numbers in the Rhizosphere on Nodule Initiation

The direct examination of the relation between inoculum size and infection rate for the nodulation of a legume is difficult because of the rapid multiplication of the nodule bacteria in the rhizosphere. This has been overcome by (i) using mixtures of normal (virulent) and mutant avirulent strains of clover nodule bacteria, the avirulent acting as a diluent of the virulent, and (2) by employing large original inocula to reduce multiplication. By these means the numbers of virulent clover bacteria were maintained during an experiment at approximately predetermined levels. When a strain of lucerne nodule bacteria was used as diluent the multiplication of clover nodule bacteria was not suppressed.

Nodule number was found to approach the maximum asymptotically with bacterial density; very few bacteria (of the order of 10) were required to give rise to a single nodule only on a plant during an experiment, under conditions where the full nodulating capacity of a root is saturated by a total rhizosphere population of the order of 104 bacteria. This relationship supports the view that infection only takes place at discrete foci on the root.

A good fit of the experimental data was given by a compound Mitscherlich curve and by a combination of a normal and a modified Mitscherlich equation. This latter model takes into account the multiplication of sites of infection which occurs in the early seedling stage from foci not occupied at high dilution of virulent bacteria. Multiplication of this kind has also been demonstrated by delayed inoculation experiments.

In the preliminary experiments it was found that comparable rhizosphere stimulation of bacterial growth took place in sand culture, water culture and agar culture and with plants of diverse nodulating habit. RESP-3833