‘Native’ spearmint, Mentha spicata L., is one of the two widely grown spearmints in the United States and throughout the world (Bienvenu et al., 1999; Lawrence, 2006; Topalov, 1989). The other spearmint is ‘Scotch’ spearmint, which actually is a different species (Mentha ×gracilis Sole.; syn. M. cardiaca L.). ‘Native’ spearmint is a major essential oil crop grown in the Midwest and in the northwestern United States (National Agricultural Statistic Service, 2009). However, ‘Native’ spearmint has a wide environmental adaptation, and recent studies demonstrated feasibility of growing it under the humid conditions of Mississippi (Zheljazkov et al., 2010a, 2010b, 2012a). Indeed, the natural distribution of Mentha spicata L. spans from Alaska and British Columbia to Texas and Florida in North America [U.S. Department of Agriculture (USDA), 2012].
Recent studies in Wyoming indicated that ‘Native’ spearmint can withstand the first fall frost and apparently could be developed as a crop for northern Wyoming at 1170 m a.s.l. elevation (Zheljazkov et al., 2012a). Developing means for increased biomass and oil yields and increased concentrations of carvone, the main essential oil constituent of ‘Native’ spearmint, would be beneficial for the existing and future spearmint essential oil industry. There have been various studies aiming to improve essential oil yield and composition of spearmints. For example, previous studies demonstrated that some plant hormones such as MJ may increase carvone concentration of ‘Scotch’ spearmint (Zheljazkov and Astatkie, 2011a). In another study, the application of MJ had no significant effect on essential oil content of ‘Native’ spearmint (Zheljazkov et al., 2010b). In the same study, the application of distillation water from wild bergamot (bee balm) (Monarda fistulosa L.) increased the essential oil content, whereas the distillation water of absinthe wormwood (Artemisia absinthium L.), lavender (Lavandula vera DC), and wild bergamot increased oil yields of ‘Native’ spearmint (Zheljazkov et al., 2010b). In other studies, it was shown that plant hormones or plant extracts may have a significant effect on spearmint and peppermint essential oil yields and composition (Zheljazkov and Astatkie, 2011b, 2012). These reports suggest that MJ and some plant extracts may have potential for improving spearmint oil content and composition.
Rocky mountain juniper (Juniperus scopulorum Sarg.) and big sagebrush (Artemisia tridentata Nutt.) are two of the most widely spread native plants in the semiarid regions of western United States, Mexico, and Canada (USDA, 2011). These species provide cover for a number of wildlife species and food for many birds, whitetail deer, elk, bighorn sheep, pronghorn antelope, but also for cattle and sheep (Scher, 2002). Both species contain a significant amount of plant secondary metabolites. Rocky Mountain juniper contains podophyllotoxin and essential oil (Cantrell et al., 2013; Zheljazkov et al., 2013). Big sagebrush contains essential oil, MJ (Jassbi et al., 2010), and other plant chemicals. Junipers and sagebrush have been shown to exert allelopathic effects on surrounding species (Jassbi et al., 2010; Preston et al., 2002; Yager and Smeins, 1999). On the other hand, Tween20 has also been investigated as a means for improving essential oil content of some aromatic plants (Dobreva et al., 2011).
There are no studies in the literature whether foliar application of Rocky Mountain juniper or sagebrush extracts could modify essential oil content and composition of spearmints. The hypothesis of this study was that Rocky Mountain juniper and sagebrush essential oil and plant extracts may have significant effect on essential oil yields, content, and composition of ‘Native’ spearmint grown under Wyoming conditions.
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