Effects of Plant Maturity on Oil Composition of Several Spearmint Species Grown in Indiana and Michigan

In 1970, herbage samples of 2nd-year meadow mint of the native spearmint cultivar (Mentha spicata), the Scotch spearmint cultivar (M. cardiaca), and the wild mint M. crispa were harvested on May 16, June 3, June 17, July 1, and thereafter insofar as possible every 10 days until Nov. 3. Assay data for 10 principal constituents accounting for at least 85% of the oil showed that overmature and senescent herbage had more dihydrocarvone than did mature herbage, and that M. crispa had more than both cultivars at all times. Replicated tests in 1971 showed that flower oil of both cultivars had more carvone and less dihydrocarvone than either the whole herbage or the top half of the herbage. The thesis is advanced that all inter-regional, intra-regional, and ontogenetic differences in oil composition must result from either a direct effect of environment on the amount or efficiency of a specific gene-controlled enzyme for a specific conversion or an indirect effect resulting from environmentally induced changes in plant growth influencing the proportion of young leaves to mature leaves, time of maturity, and extent of flower production. No direct effects are known. Ontogenetic differences in oil composition between juvenile, mature, and overmature plants are believed to depend upon the relative proportions of leaves of various ages and of flowers in the herbage. Ontogenetic trends will not be found unless specific inherited chemical syntheses occur at a slow rate, giving a product increase that is proportional to time and perhaps temperature, or unless there are inherited differences between leaves and flowers. All differences in stand, amount of fertilizer, cultural conditions, frequency of irrigation, weather, or climatic conditions that increase the proportion of young leaves will improve oil quality, whereas the suggestion is made that the poorer oil quality obtained from harvesting overmature herbage or herbage prematurely aged by crop mistreatment is due not only to the increased dihydrocarvone, but also to a higher alcohol/ester content.