Doubling the Chromosome Number of Trifolium Species Using Nitrous Oxide¹

To determine the effect of nitrous oxide on chromosome doubling in diploid red dover ( Trifolium pratense L.), excised heads were treated at 6 bars atmospheric pressure. Treatment commenced2 4 hours after crossing and continued for the following 24 hours. Of the plants which flowered, 71% were classified as tetraploid based on pollen size. Subsequent root‐tip and PMC chromosome counts of 136 of these plants revealed that 119 were tetraploid, 3 were diploid, and 12 were aneuploid. Two plants died before chromosome counts were made. The technique produced 49 and 79% tetraploids in T. alpestre L. and T. rubens L. respectively. The chromosome numbers of T. noricum Wulf. and T. pallidum Wald. and Kit. were successfully doubled also. Ease and frequency of chromosome doubling was considerably higher and production of mixaploids was lower than in previously‐reported colchicine treatments. The treatment was toxic to T. hirtum All. and T. heldreichianum Hausn.