Variation among commercial cultivars of Japanese mint (Mentha arvensis L.) in the morphological and metabolite characters associated with essential oil yield

Intraspecific variation in four widely cultivated cultivars of Japanese mint (Mentha arvensis L.) was studied to understand the physiological basis of regulation of essential oil accumulation. The cultivars had the same oil biosynthetic route but differed in morphological/physiological characters. Oil content showed significantly positive correlations with yield traits like leaf and stem fresh and dry weights, relative growth rate and CO2 exchange rate. Leaf area per plant was positively associated with herb yield. Observation that leaf fresh and dry weights possessed higher significant genotypic correlation coefficients than phenotypic correlation coefficients indicated that these traits express a strong genetic influence on oil yield, as the latter incorporates the former. Photosynthetic efficiency measured by uptake and fixation of 14CO2 and 14C-sucrose showed differential utilization of these precursors. Sucrose was more efficiently utilized for oil biosynthesis than photoassimilated (14CO2) precursors. The levels of metabolites fixed through 14CO2 remained lower than those fixed via 14C-sucrose. Efficient cultivars translocated a greater portion of metabolite towards essential oil and also accumulated higher concentrations of nutrients. The concentrations of the micronutrients Fe, Mn, Cu, Zn varied significantly among cultivars with leaves showing higher concentrations than stems. It is evident that the physiological capability of mints in terms of leaf growth, nutrient uptake, photosynthetic capacity, partitioning and distribution of assimilated metabolites in leaf and stem, contribute significantly to biosynthesis and accumulation.