Studies on the Physiology of Nodule Formation: With two Figures in the Text: A Reappraisal of the Effect of Preplanting

Nutman (Arm. Bot. 21, 321, 1957) found that preplanting agar slopes with Trifolium pratense L. or Medicago sativa L. advanced the time when second lots of plants of these species grown on the same slopes initiated nodules, and depressed the total number of nodules formed. He attributed these effects to root secretions which at low concentration hasten initial nodulation but at higher concentrations inhibit nodule formation. Further work has now shown that initial nodulation is stimulated because the preplant removes traces of nitrate from the medium. The amount of nitrate in the tap water used to prepare the medium (6·5 p.p.m. N) also increases the number of nodules formed on the control plants, and this effect explains to a considerable extent the depression of nodule numbers by preplanting. Initial nodulation was delayed by small amounts of nitrate and nitrite but not by other forms of combined nitrogen (ammonium, asparagine, and urea). All forms of combined nitrogen tested increased the number of nodules formed over a period of 8 weeks when supplied at an initial concentration of 20 p.p.m. N. RESP-438