Nectar and pollination drops: how different are they?

Abstract

Background: Pollination drops and nectars (floral nectars) are secretions related to plant reproduction. The pollination drop is the landing site for the majority of gymnosperm pollen, whereas nectar of angiosperm flowers represents a common nutritional resource for a large variety of pollinators. Extrafloral nectars also are known from all vascular plants, although among the gymnosperms they are restricted to the Gnetales. Extrafloral nectars are not generally involved in reproduction but serve as 'reward' for ants defending plants against herbivores (indirect defence).

Scope: Although very different in their task, nectars and pollination drops share some features, e.g. basic chemical composition and eventual consumption by animals. This has led some authors to call these secretions collectively nectar. Modern techniques that permit chemical analysis and protein characterization have very recently added important information about these sugary secretions that appear to be much more than a 'reward' for pollinating (floral nectar) and defending animals (extrafloral nectar) or a landing site for pollen (pollination drop).

Conclusions: Nectar and pollination drops contain sugars as the main components, but the total concentration and the relative proportions are different. They also contain amino acids, of which proline is frequently the most abundant. Proteomic studies have revealed the presence of common functional classes of proteins such as invertases and defence-related proteins in nectar (floral and extrafloral) and pollination drops. Invertases allow for dynamic rearrangement of sugar composition following secretion. Defence-related proteins provide protection from invasion by fungi and bacteria. Currently, only few species have been studied in any depth. The chemical composition of the pollination drop must be investigated in a larger number of species if eventual phylogenetic relationships are to be revealed. Much more information can be provided from further proteomic studies of both nectar and pollination drop that will contribute to the study of plant reproduction and evolution.

Fig. 1.

Female reproductive structures and pollination drops in gymnosperms: (A) the modified scale of Ginkgo biloba (Ginkgoaceae) contains only one ovule that produces a small pollination drop; (B) female cone of Taxus baccata (Taxaceae) containing one ovule that secrets a large pollination drop; (C) female cone of Juniperus communis (Cupressaceae) bearing three ovules, each one with its own pollination drops; (D) female cone of Cupressus sempervirens (Cupressaceae) with numerous small pollination drops each one secreted by an ovule. Scale bars = 1 mm.

Fig. 2.

Floral and extrafloral nectaries in angiosperm species: (A) extrafloral nectaries at the base of foliar rachis of Acacia collinsii (Mimosaceae; picture kindly provided by Martin Heil); (B) flowers of Fatsia japonica (Araliaceae) in which the nectary is placed above the ovary of a reduced flower and small drops of nectar are completely exposed to the environment; (C) flowers of Aloe castanea (Asphodelaceae) in which the nectary is located inside the ovary (septal nectar) and a large quantity of dark-red nectar accumulates inside the corolla tube; (D) flower of Gymnadenia conopsea (Orchidaceae) in which the nectar is produced and stored in a spur – the arrow indicates the level of nectar inside the thin spur (picture kindly provided by Malgorzata Stpiczyńska). Scale bars = 5 mm.