Abstract
Fifty‐one genotypes from each of 20 cultivars of white clover (Trifolium repens) were raised in sand culture free of combined nitrogen. After treatment with a pulse of ammonium nitrate in solution, specific nitrogenase activity was determined by acetylene reduction for each plant. Selections were made of the 30 plants with the lowest residual nitrogenase activity, and 30 plants with the highest residual nitrogenase. An open polycross was done for each selection, and the progeny were tested as before after treatment with mineral nitrogen. The progenies showed a small but highly statistically significant difference in residual nitrogenase activity following N addition that confirmed the first selection process. A second selection for types showing extremes of specific nitrogenase activity was done on the two progeny populations, and a second polycross completed. The progeny from these crossings were tested for response to mineral nitrogen using a 15N dinitrogen gas incubation method. There were no differences in nitrogenase activity between the selections.
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