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

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Abstract
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.