RESUMO
St. John's wort [Hypericum perforatum L.] is an important medicinal plant that has been widely used for its antidepressant properties. In this study morphological characteristics variation of populations of H. perforatum were investigated. In this study, different morphological characteristics of 25 H. perforatum populations in Iran's natural habitats were evaluated in 2010. The results indicated that the highest coefficients of variations were found in flowers number, the fifth internodes' length and capsule length [respectively, 93.46, 85.28 and 84.7%]; however the lowest coefficients of variations were seen in flower width [11.72%] and sepal length [11.91%]. The largest dimensions of flowers, sepals and petals related to the population which was gathered from Zanjan/Tarum city. The highest leaves dimensions and dark glands density on the bottom leaves were observed in Alamut and Kalaleh populations. The highest positive significant correlations were seen between flower length with flower width; dark glands density on the middle leaves surface area with dark glands density on the bottom leaves surface area; petal length with flower length and width; capsule length with capsule width; non-flowering branches number with branches number; light glands density on the middle leaves surface area with light glands density on the bottom leaves surface area. The lowest significant correlations were observed between capsule length with flower length and width. The principal components analysis showed that four components explained 68.9% of total variance. The cluster analysis divided these populations into four clusters with no consistency in their geographical distributions. In conclusion, this study indicated that there were high variations among the Iranian St. John's wort populations which can be utilized in the breeding programs