In-Vitro Selection for Stress Tolerant Spearmint*

Medicinal and aromatic plant species are important sources of secondary metabolites with therapeutic, cosmetic, and food processing applications. The quality of the metabolites, however, frequently varies due to inconsistencies in secondary metabolite composition of plant extracts, especially within the family Lamiaceae. A major contributing factor to these metabolite inconsistencies is undoubtedly due to plant heterogeneity, a characteristic of open-pollinated species. Thus, various genotypes occur within a plant population, differing in physical and metabolic traits that control plant growth, chemistry, and environmental susceptibility. Spearmint (Mentha spicata L., Lamiaceae) is an essential oil crop with a number of culinary and aromatic uses (Yanishlieva et al. 2006; Szczerbanik et al. 2007). Similar to other Lamiaceae species, however, uniformity in essential oil content and composition and consistency in growth and development are especially susceptibility to environmental stress due to plant heterogeneity. Thus, crop yields and quality in major spearmint production regions of the US and in Egypt, areas that are frequently subject to dry periods, can fluctuate. The use of irrigation over the past several years to promote crop growth has increased the salt content of the soil, frequently forcing growers to apply 10% to 20% excess water to lower salt concentrations in the root zone (Takabayashi et al. 1994; Takabayashi and Dick 1996; Arndt et al. 2001; Mohamed et al. 2002). Water stress in plants from a lack of moisture or from drought induced by salt stress is associated with increased proline synthesis and other metabolic changes, including the increased synthesis of phenolic acids and essential oil metabolites that are stimulated in response to stress (Matticci et al. 1995; Paré and Tuminson 1999; Mohamed et al. 2002). The exact nature and level of the response to stress is dependent upon the susceptibility of the plant genotype to stress (Rena and Splittstoesser 1975; Premachandra et al. 1992; Fatima et al. 2002). This variability among the assortment of genotypes within open-pollinated spearmint suggests that such variability could be exploited for development of drought tolerant types of spearmint (Rena and Splittstoesser 1975; Premachandra et al. 1992; Fatima et al. 2002) that over-express phenolic acids and other essential oil constituents (Mohamed et al. 2002).