Plant secondary metabolites in nectar: impacts on pollinators and ecological functions

Abstract

Summary1. The ecological function of secondary metabolites in plant defence against herbivores is well established, but their role in plant-pollinator interactions is less obvious. Nectar is the major reward for pollinators, so the occurrence of defence chemicals in the nectar of many species is unexpected. However, increasing evidence supports a variety of potential benefits for both plant and pollinator from these compounds. 2. Beneficial effects may include: (i) mediating specialization in plant-pollinator interactions, (ii) protecting nectar from robbery or larceny and (iii) microbial activity including preservation of nutrients in nectar from degradation and reduction in disease levels in pollinators. 3. Secondary metabolites in nectar can be toxic or repellent to flower visitors, but equally they can go undetected or even make nectar more apparent or attractive. These biological effects are concentration dependent, so must be considered at a range of ecologically relevant doses. For example, caffeine occurs in nectar and improves honeybee memory for odours associated with food rewards, which enhances pollen transfer at naturally occurring concentrations but is repellent to honeybees at higher concentrations. 4. This review synthesizes evidence from recent literature that supports selection for secondary metabolites in floral nectar as an adaptation that drives the co-evolution between plants and their pollinators. However, their presence in nectar could still simply be a consequence of their defensive role elsewhere in the plant (pleiotropy). We highlight the need for more studies demonstrating measurable benefits to the plant, the importance of exposure levels and effects on target species beyond the current emphasis on alkaloids and bees.

New perspectives in nectar evolution and ecology: simple alimentary reward or a complex multiorganism interaction?

“…Secondary metabolites such as tannins, phenols, alkaloids, and terpenes have been detected in floral nectar in several angiosperm families since the 1970s and they are considered to be toxic deterrents against predators as well as a defense against microorganisms [30,31]. Their effect on feeding animals is dose-dependent: they can be lethal in high doses, but in low doses their effects can differ, depending on the sensitivity of the interacting organism [31,32].…”

“…Their effect on feeding animals is dose-dependent: they can be lethal in high doses, but in low doses their effects can differ, depending on the sensitivity of the interacting organism [31,32]. The concentrations of secondary compounds in nectar are lower than in other plant parts, such as leaves, stems, or flowers, where they deter herbivores [31,32].…”

“…Their effect on feeding animals is dose-dependent: they can be lethal in high doses, but in low doses their effects can differ, depending on the sensitivity of the interacting organism [31,32]. The concentrations of secondary compounds in nectar are lower than in other plant parts, such as leaves, stems, or flowers, where they deter herbivores [31,32]. It is not likely that secondary compounds in nectar coevolved with pollinators but rather that their presence is a pleiotropic trait, i.e., they occur in other plant organs (leaves, stems) and are transported by phloem/xylem during nectar production [31].…”

“…The concentrations of secondary compounds in nectar are lower than in other plant parts, such as leaves, stems, or flowers, where they deter herbivores [31,32]. It is not likely that secondary compounds in nectar coevolved with pollinators but rather that their presence is a pleiotropic trait, i.e., they occur in other plant organs (leaves, stems) and are transported by phloem/xylem during nectar production [31]. Pollinators presumably impose selection to lower their concentrations in nectar rather than to produce specific nectar secondary compounds [31].…”

“…Most studies (reviewed in Stevenson et al [31]) involved bumblebees infected with the gut parasite Crithidia bombi. Consumption of these secondary compounds by bees generally lowered their pathogen load after infection but pre-exposure of the pathogen to the same substances did not decrease its viability.…”