Nodule Position Research Articles

Evaluation of Nitrogen Fixation in Legumes by the Regression of Total Plant Nitrogen with Nodule Weight

THE relation of nitrogen fixation to nodulation is not yet well understood. Nodule position, nodule size, haemogoblin content and longevity of nodule tissues have been taken into consideration1,2. Chen and Thornton3 showed, from considerations of the structure of effective and ineffective nodules of clover, pea and soybean, that differences in efficiency of nitrogen fixation were due to the volume of active bacterial tissue. Bowen4 found a highly significant regression of dry weight of plants with nodule weight in Centrosema pubescens, inoculated with seventeen different Rhizobium strains. He suggested that Rhizobium strains have the same nitrogen fixing ability weight for weight of nodule tissue, and that differences in effectiveness resulted from the different amounts of nodule tissue formed.

The relation between root hair infection by Rhizobium and nodulation in Trifolium and Vicia

The infection of the root hairs of young seedlings of twelve species of Trifolium and of Vicia hirsuta was examined. The amount of infection (numbers of hairs containing infection threads) at 2 weeks varied much between species of host and was less affected by bacterial strain ; host and strain differences were independent. In most hosts a high proportion of infections did not result in nodule formation. The relative rate of increase in numbers of infected hairs was constant before nodulation began. The duration of this pre-nodulation phase of exponential increase in infection, but not its rate, differed between species. Nodulation (and lateral root formation) caused an abrupt lowering of the initial rate of infection. Post-nodulation infection also increased exponentially. Low concentrations of nitrate nitrogen delayed nodulation and increased the number of hairs infected. Infected hairs were not randomly distributed along the root, infection beginning at a few well-separated points. Later infections occurred near these primary foci to give zones of infection which then spread up and down the root. The positions of nodules or lateral roots were not related to the primary foci of hair infection.

Nodulation Patterns on Legumes

Fred, Baldwin and McCoy1 made the generalization that effective strains of Rhizobium produce few but large nodules, usually located on the upper root system of the host, whereas ineffective strains form numerous small nodules widely distributed over the root system. Subsequent detailed studies on the physiology of nodulation have been summarized by Nutman2, with a hypothesis relating nodule density on the root to the number of preformed foci, themselves determined by genetic factors affecting root morphogenesis. Dart and Pate3 recently reported studies concerning the effects of delayed inoculation on nodule production and position.