Effects of gibberellic acid on mentha peperita

1. The morphological changes observed were characterized primarily be stem elongation and to a lesser degree by changes in leaf shape. The latter changes were related to the position of the leaf on the stem; the lower leaves were similar in shape but larger leaves were more lancoeolate and smaller in area. 2. The fresh weight yield of peppermint herb was not significantly different from that of the control plants. 3. Following harvest of the herb of treated peppermint plants, a new crop of peppermint was allowed to grow without further treatment. The second crop of GA-treated peppermint plants was markedly smaller than the crop from the control plants. 4. The yield of oil was reduced when compared on both a fresh weight and dry weight basis. 5. A single spraying of GA produced stem and internode elongation which was followed by a reduction of the rate of stem elongation and the length of the internodes being produced. In some stems this reduction in elongation was accompanied by loss of apical dominance. 6. From observations of histological sections of the stem it was concluded that cell elongation primarily accounts for the change in intnernode length. There was no significant increase in the number of cells in the longitudinal direction. 7. In the stem cross section, a decreased number of cells in all tissues was observed. The reduction of cells resulted in a thinner stem. 8. Leaf cross sections revealed that the leaves were thinner than the leaves of the control plants. Changes in the shape of the cells were observed. 9. The powdered herb obtained from plants treated with GA was indistinguishable from the powered herb of control plants or from powdered U.S.P. peppermint. 10. Assays were run on the oils obtained from the treated peppermint plants for the physical and chemical properties specified by the U.S.P. under peppermint oil. In genera, the oils from plants treated with GA had higher specific gravities than the oils from control plants, were more levo-rotatory, had higher refractive indicies, and contained larges percentages of esters. Calculated as menthyl acetate and of alcohol calculated as menthol. Only one oil produced with within U.S.P. specification and that was from a plant sprayed with 10 ppm of GA and harvested in the summer of 1959. 11. From the observation made to date, no recommendation can be made for the use of GA on peppermint crops. Further studies should be run to evaluate the effects of other concentrations than those used and other methods of application. Because the control peppermint plants grown n Utah did not yield U.S.P. grade oil, any further work done should be in an area of commercial peppermint production. It would appear, however, the 10 and 100 ppm GA as weekly sprays is either too high of concentrations or is too frequent a spraying schedule for optimum production of peppermint herb or peppermint oil.