Physiological Studies on Nodule Formation

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Abstract
A comparative study is made of nodule and lateral root formation on red clover grown in test-tubes on slopes of a N-deficient agar medium, using plant material selected for high and low nodule number. Sparsely nodulating plant types show low rates of lateral root formation and abundantly nodulating types a high rate. With either effective or ineffective bacterial strain inoculation, nodulation takes place on all types of plant material at a constantly decreasing rate (i.e., on a logarithmic time scale the rate is constant). Lateral root formation with effective strain inoculation occurs in 2 distinct stages, a seedling phase at a decreasing rate and a later phase at a much increased rate, whereas with ineffective strain inoculation a single phase with a decreasing rate is maintained. Inoculated plants have fewer lateral roots than uninoculated plants throughout the first developmental phase independently of type of inoculum. Abundantly nodulating material is found to have a root system of about the same extent as the sparse type so that the number of nodules formed is not dependent upon the size of the root system open to infection. Delay in inoculation leads to an enhanced rate of nodulation with effective strains of bacteria. It is concluded that the nodules and laterals are physiologically homologous, the infection of the root taking place at predetermined foci corresponding to the sites of initiation of lateral roots.