Evaluation of Peppermint Field Performance from Plants Regenerated from Meristem Tip Culture, and Investigations of Virus Infection1

In 1994, its second full production year, field-grown meristemmed 'Black Mitchum' peppermint performed differently than in 1993, when compared with a non-meristemmed treatment in a field trial at the OSU-COARC. The difference between years may be partly attributed to better meeting the fertility and soil moisture needs of meristemmed plants in 1994. Plants were not moisture-stressed near harvest, and plots were fertilized according to stem and soil nitrate levels. Both visual ratings and detailed measurement of plant growth indicated little difference in the growth and growth habit between meristemmed and non-meristemmed plants in 1994. Nevertheless, in 1994, plants in meristemmed plots required supplementary nitrogen, whereas in 1993 they became nitrogen deficient, which may have contributed to reduced oil production, enhanced stem mass over leaf mass, reduced branching, etc., in 1993. In 1994, when yield performance was compared without the influence of rolling, oil yield was 9 percent greater in the meristemmed treatment an early harvest in late July than in non-meristemmed treatment, but two weeks later in early August, oil yield was 20 percent greater in non-meristemmed mint than in meristemmed mint. Harvested biomass of meristemmed mint was 20 percent and 7 percent greater than the biomass of non-meristemmed mint at each harvest date. Nevertheless, due to variability in data, none of these difference were statistically significant (P<0.05). Based on 1993 observations, half of all plots were rolled in late June, theoretically to induce branching and enhance more foliage production on meristemmed plants. Whereas rolling resulted in many measurable and statistically significant growth changes in plants (P<0.05), these were not necessarily greater branching, increased leaf number or size, or enhanced oil yield. For the meristemmed treatment, rolling significantly reduced both oil yield and biomass (P<0.05) for both harvest dates compared to the non-rolled meristemmed treatment. For the non-meristemmed treatment, rolling significantly enhanced yield at the early harvest date (P<0.05), but significantly reduced oil yield at the later harvest date (P<0.05), but the effect on biomass was negligible for both dates. It is recommended that future management of meristemmed peppermint focus on the potential advantages of early harvest, rather than rolling. Molecular biological analyses indicated that non-meristemmed 'Black Mitchum' peppermint taken from the COARC field trial contained a double-stranded RNA (dsRNA) entity which was not found in meristemmed 'Black Mitchum' peppermint taken from the same trial. Such dsRNA is 1This research was supported by a grant from the Oregon Mint Commission and the Mint Industry Research Council.