Reproductive ecology of medicinally important Kashmir Himalayan species of Digitalis L.

Abstract

Reproductive biology, encompassing phenology, floral biology, pollination and breeding systems, was investigated in three pharmaceutically important and anthropogenically exploited species of Digitalis in the montane habitats of the Kashmir Himalaya. Flowers in all three species (Digitalis purpurea, Digitalis grandiflora and Digitalis lanata) are aggregated in racemes. Corollas are characteristically bell shaped enclosing four, didynamously arranged stamens. The stigma is deeply bifid only in D. purpurea. The three species over-winter as underground tubers and sprouting during the following spring occurs first in D. lanata, followed by D. purpurea and D. grandiflora. Floral bud opening in an inflorescence in all three species occurs sequentially from the bottom upwards in an acropetal succession and male and female phases are temporally separated with basal flowers entering the male phase first followed by the upper flowers in a regular sequence. Only after shedding the pollen grains completely do the flowers shift to the female phase. Resource allocation in all three species is more towards maleness than femaleness during anthesis. The species are principally insect pollinated and selfing is denied because of protandry and spatial separation of the stigma and anthers. Copious amounts of viable pollen grains produced by these species aid in successful pollination, fertilization and seed production. Studies revealed that none of these species operates autogamy because hand-pollinated flowers with self-pollen or bagged flowers (foreign pollen excluded) failed to set seed. High seed output in open-pollinated flowers and high pollen-ovule ratios also indicate the outbreeding nature of these species. Thus, any reduction in the number of individuals constituting a population would significantly reduce the floral display and consequent rewards to the pollinators, with a subsequent influence on seed production. Hence, data on the reproductive biology of overexploited species, such as Digitalis spp., is of pivotal importance in formulating effective conservation strategies.

Relative abundance of an invasive alien plant affects native pollination processes

“…In self-compatible plants, interspecific effects of abundance on reproductive success may be absent or weaker (Feinsinger et al 1991;Caruso 1999) possibly because of the reproductive assurance these species experience through delayed spontaneous autogamy (Kalisz and Vogler 2003) or facilitated selfing. In our case, although D. purpurea is incapable of spontaneous autogamy, it is self-compatible (Nazir et al 2008) and could have achieved within-flower or within-plant selfing (geitonogamy) by pollinators conducting longer visits per flower and/or visiting more flowers per inflorescence in less frequently visited plants. D. purpurea is mainly pollinated by bumblebees, i.e.…”

“…Like R. ponticum, it produces pink-purple, zygomorphic flowers which secrete copious nectar, but in D. purpurea these are arranged in one or a few inflorescences per individual plant (Percival and Morgan 1965). Although D. purpurea is a self-compatible plant species, it is incapable of spontaneous autogamy and relies on biotic vectors for pollen transfer (Nazir et al 2008). In Ireland, D. purpurea frequently co-occurs with R. ponticum, and the two species overlap in flowering time (Percival and Morgan 1965;Cross 1975) and are pollinated by the same functional group of pollinators (bumblebees; Percival and Morgan 1965;Stout 2007).…”

Rapid evolution of a floral trait following acquisition of novel pollinators

“…The purple flowers are bell-shaped and protandrous, with anthers dehiscing shortly after anthesis, while the stigma becomes receptive (by unfolding its two lobes) up to 5 days later (Darwin, 1876). The plant is self-compatible, but insect visitation is required for full seed set (Nazir et al, 2008; see also Section 3). Bumblebees typically fly upwards when foraging on an inflorescence, so on D. purpurea they travel from older female phase flowers lower in the inflorescence to male phase flowers higher up in the inflorescence, potentially reducing the incidence of self-pollination (Best & Bierzychudek, 1982).…”

The Foxgloves (Digitalis) Revisited

“…It should be noted that D. purpurea was introduced widely as an ornamental plant and can readily be found along the West Coast of the USA and Canada as well as in New Zealand. Three Digitalis species, namely D. purpurea, D. grandiflora, and D. lanata, were reported to grow in elevated montane habitats in Kashmir [34].…”