The Infection of Trifolium subterraneum Root Hairs by Rhizobium trifolii

Both host cultivar and Rhizobium strain influence the number of infected root hairs of Trifolium subl-errctneum, seedlings; Yarloop had fewer infections than Cranmore, Mount Barker, or Tallarook and Rhizobium trifolii strain 5 infected fewer hairs than strain TA1. Hybrid lines bred for sparse or abundant nodulation had similar numbers of infected hairs, but. as in the cultivars, these always greatly exceeded the number of nodules formed. More infection threads aborted early during growth in the root hairs of Cranmore than in other hosts and early abortion was more common with strain 5 than strain TA1

In all hosts and with both Rhizobium strains, infection began on day 3 and was initially restricted to one or two zones along the root with later infections extending these zones or initiating new ones. The exponential rate of infection (least for Yarloop) slows sharply when nodules appear.

Early nodules and lateral roots formed at different places in different hosts, and in most cultivars and hybrid lines nodules and laterals occurred in mutually exclusive zones. Primordia arising above the first nodule failed to develop. RESP-6153