Reproductive biology of the Red List speciesPolemonium caeruleum(Polemoniaceae)

Abstract

The majority of flowering plants, including many rare and threatened species, are pollinated by animals, but little is known of pollination and breeding systems of many endangered species. Polemonium caeruleum (Polemoniaceae) is a red-listed species and is regarded as dichogamous, self-compatible and bee pollinated. However, some studies show that it is visited by a vast assemblage of anthophilous insects from many taxonomic orders and that breeding systems vary greatly between closely related taxa of this genus. Over a period of 3 years we investigated breeding system, dichogamy, nectar secretion and composition, insect visitations and pollen loads in flowers of P. caeruleum in north-eastern Poland to determine whether the reproductive biology of the plant explains its rarity. Contrary to published data, our study plants were self-incompatible and showed a high degree of outcrossing. Our experimental work confirmed the occurrence of protandry in this species, revealed that nectar is sucrose-dominant and proline-rich and, for the first time for Polemoniaceae, that nectar secretion and nectar sugar concentration in flowers of P. caeruleum is female-biased. Although flowers were visited by at least 39 species of insects from five taxonomic orders, overall the plant exhibited many characters associated with bee pollination, and analysis of insect performance showed that bumblebees and honeybees are the key pollinators; occasionally hoverflies and butterflies may also be involved. We conclude that, in terms of pollination system, P. caeruleum demonstrates high apparent generalization, but low realized generalization, and is a functional specialist, as most pollinators belong to a single functional group (guild). Its conservation status, at least in our study population, cannot be explained in terms of the biological properties of its breeding or pollination systems; rather, the present decline of the species is caused by habitat loss. However, if this process and bumblebee decline in Europe continue, P. caeruleum populations may diminish in numbers and density and, owing to the self-incompatibility of the species, quickly become severely pollen-limited, thereby accelerating further local extinctions.

Breeding system variability, pollination biology, and reproductive success of rare Polemonium caeruleum L. in NE Poland

“…The blue campanulate flowers are produced in a corymbose inflorescence, which develops gradually from the beginning of June to the middle of July. Flowers are protandrous and the anthers open first, then after a few days the 3-lobed stigma open, but these two stages slightly overlap [21,22]. Flowers produce nectar rich in sugar, which is secreted and stored by the ring-shaped nectary located at the base of the ovary [25,26].…”

“…For example, in self-incompatible Polemonium foliosissimum [19] self-compatible populations have been suggested [20]. Recent studies also indicate SC-SI transition in Polemonium caeruleum populations [21,22], which are differentiated in size, density, population structure, and can occupy both natural and anthropogenic habitats. Flowers of this species are protected against autonomous selfing by the temporal separation of male and female phases due to protandry [20,21].…”

“…Flowers of this species are protected against autonomous selfing by the temporal separation of male and female phases due to protandry [20,21]. Dichogamous protection is not complete, however, as a few hours of overlap between both sexual phases were detected [22]. Additionally, P. caeruleum inflorescence is composed of many simultaneously open flowers, being at the same time under the male-or female-stage [22].…”

“…Dichogamous protection is not complete, however, as a few hours of overlap between both sexual phases were detected [22]. Additionally, P. caeruleum inflorescence is composed of many simultaneously open flowers, being at the same time under the male-or female-stage [22]. This facilitates mixed-mating in SC populations, especially since key pollinators are social bees whose foraging behavior on the inflorescence promotes crossing, geitonogamy, and facilitated autogamy [20,22,23].…”

“…Additionally, P. caeruleum inflorescence is composed of many simultaneously open flowers, being at the same time under the male-or female-stage [22]. This facilitates mixed-mating in SC populations, especially since key pollinators are social bees whose foraging behavior on the inflorescence promotes crossing, geitonogamy, and facilitated autogamy [20,22,23]. Understanding the relationship between the demographic properties of P. caeruleum populations, pollinator abundance, and reproductive success in the context of SC-SI transition of these populations is currently the subject of intensive investigations.…”