Abstract
Strategies to increase the leaf and flower essential oil yield of Mentha species would be improved by an understanding of how phenotypic and phenologic characters interact with one another in affecting the essential oil yield. In this study, path analysis was applied within some morphological and phenological characters recorded from field studies conducted in 2000 on four Mentha spp., M. longifolia M. spicata M. piperita and M. aquatica. Several morphological, phenological, and chemical characteristics were recorded on the experimental units, based on a three times replicated completely randomized design. The characteristics recorded on five single plants in each experimental unit were: main stem length, number of lateral branches on main stem, leaf length, leaf width, stem diameter, flowering date, percentage of flower essential oil, and percentage of leaf essential oil. After a confident result of differences between the species for the characters, path analysis was performed on the data. The analysis on flower essential oilโrelated characters revealed that the percentage of leaf essential oil and leaf length had a high direct effect on the dependent variable, flower essential oil yield. Flowering time showed a positive direct effect but a high negative total indirect effect through other independent variables on the dependent variable, resulting in a negative total effect on essential oil yield. The analysis on leaf essential oilโrelated characters showed that both stem diameter and flower essential oil yield had almost equal positive direct effects on leaf essential oil yield. Flowering date showed negative direct and total indirect effects on leaf essential oil yield. Stem length had a high positive indirect effect on the leaf essential oil yield through independent variables, counterbalancing negative direct effects to a positive total effect of 0.487. Among the species in the current study, genetic improvement in the stem diameter and length, leaf length and width would have beneficial effects on essential oil yield. Therefore, indirect selection may be done at the field for high essential oil yield based on morphological characters. Copyright ยฉ 2001 John Wiley & Sons, Ltd.