Root Nodule Symbiosis II

Nodule‐roots of Myrica cerifera (Southern Wax Myrtle) and Casuarina cunning hamiana (Australian Pine) have a negative geotropic curvature. Studies of their endogenotts auxin content revealed a pattern of correlation: the absence of detectable auxin when the geotropisni was negative. Non‐nodulated roots of Myrica exhibited a normal positive geotropic curvature and possessed an auxin content within an anticipated range (10 mg/kg). Root nodules of Alnus species, whose roots exhibit a positive geotropic curvature, contained measurable endogenous auxin (20 mg IAA/kg). The presence of an indoleaectic acid oxidase system in Myrica and Casuarina root nodules has heen described and correlations are drawn between non‐detectable endogenous auxin concentrations and high enzymatic activities. It is suggested that the negative geotropic curvature of the nodule‐roots of Myrica and Casuarina is due to the presence of a sub‐optimal concentration of auxin which in turn results from the activity of an indoleacetic aeid destroying system.