Studies on the Physiology of Nodle Formation: V. Further Experiments on the Stimulating and Inhibitory Effects of Roots Secretions

The influence of root secretions on nodulation of clover and lucerne in agar culture is examined using a technique of preplanting. Root secretions are shown to have two effects: nodulation of the seedling is induced at an earlier stage (stimulation), but takes place at a reduced rate (inhibition). These effects do not depend upon the presence of the bacteria in the preplanting phase of an experiment. The species used differed both in production of and reaction to secretions giving these effects. As a donating plant lucerne is more active than clover, and selected early nodulating lines of clover are more active than late lines. Stimulation of lucerne is promoted by lower levels of secretion than clover and early nodulating lines are less affected than late lines. The influence of secretions on nodulation rate is greater for lucerne than for clover at lower levels and does not differ between early and late nodulating lines. In contrast to lucerne, inhibition of nodulation on clover is proportional to duration of the preplanting suggesting that secretions accumulate during growth.

The hypothesis is put forward that the stimulating reaction represents an increase in the availability of the infective foci already present on the root and that the inhibiting reaction represents a reduction in the number of these foci. RESP-3803