Sexual dimorphism and pollination mechanism of Zanthoxylum armatum, a vulnerable medicinally important dioecious tree

Abstract

Among dioecious species, the differential strategy of resource allocation between male and female plants is depicted in their life-history traits and reproductive features. This is classified as sexual dimorphism. The strategies of resource allocation along with the habitat of these species pose an impact on their pollination mechanism and reproductive success. Zanthoxylum armatum DC (Rutaceae) is a small tree with wide economic and medicinal value. The species is naturally distributed in tropical parts of India and other countries. Due to overexploitation of natural populations in India, the species is rapidly declining. To restore the population of the species in the wild and establish commercial plantations, detailed knowledge of its reproductive biology is essential. Thus, the present study was conducted in natural population of Z. armatum growing in the union territory of Jammu and Kashmir, India. The study revealed sex-biased allocation and sexual dimorphism both in vegetative and reproductive traits. Such traits include more height and higher number of shoots, inflorescences, and flowers in male plants than in female plants. The natural populations of Z. armatum exhibit a male-biased sex ratio, and such biasness appears to be the outcome of resource allocation strategies and other demographic characteristics. Species possibly exhibit presence of ambophilous suite, i.e., a combination of wind and insect pollination in floral features. However, pollen dispersal by wind is up to limited distance, and thus wind may or may not be effective in fruit-set.