Buzz pollination in eight bumblebee-pollinated Pedicularis species: does it involve vibration-induced triboelectric charging of pollen grains?

BACKGROUND AND AIMS: Buzz pollination involves explosive pollen release in response to vibration, usually by bees. The mechanism of pollen release is poorly understood, and it is not clear which component of vibration (acceleration, frequency, displacement or velocity) is critical; the role of buzz frequency has been particularly controversial. This study proposes a novel hypothesis that explosive pollen release results from vibration-induced triboelectric charging. If it does, pollen release is expected to depend on achievement of a critical threshold velocity. METHODS: Eight sympatric buzz-pollinated species of Pedicularis that share bumblebee pollinator species were studied, giving a rare opportunity to compare sonication behaviour of a shared pollinator on different plant species. KEY RESULTS: Reconsidering previous experimental studies, it is argued that they establish the critical role of the velocity component of vibration in pollen release, and that when displacement is constrained by body size bees can achieve the critical velocity by adjusting frequency. It was shown that workers of Bombus friseanus assorted themselves among Pedicularis species by body size, and that bees adjusted their buzz/wingbeat frequency ratio, which is taken as an index of the velocity component, to a value that corresponds with the galea length and pollen grain volume of each species of Pedicularis. CONCLUSIONS: Sonication behaviour of B. friseanus differs among Pedicularis species, not only because worker bees assort themselves among plant species by body size, but also because bees of a given size adjust the buzz frequency to achieve a vibration velocity corresponding to the floral traits of each plant species. These findings, and the floral traits that characterize these and other buzz-pollinated species, are compatible with the hypothesis of vibration-induced triboelectric charging of pollen grains.

In the right place at the right time: Parnassia resolves the herkogamy dilemma by accurate repositioning of stamens and stigmas.

BACKGROUND AND AIMS: Spatial (herkogamy) and temporal (dichogamy) separation of pollen presentation and stigma receptivity have been interpreted as reducing interference between male and female functions in hermaphroditic flowers. However, spatial separation leads to a potential conflict: reduced pollination accuracy, where pollen may be placed in a location on the pollinator different from the point of stigma contact. METHODS: To understand better how herkogamous flowers resolve this conflict, a study was made of a subalpine herb, Parnassia epunctulata, the nectariferous flowers of which exhibit sequential anther dehiscence (staggered pollen presentation) and stamen movements; usually one newly dehisced anther is positioned each day over the central gynoecium, while the older stamens bend away from the central position. KEY RESULTS: The open flowers were visited by a variety of pollinators, most of which were flies. Seed set was pollinator-dependent (bagged flowers set almost no seeds) and pollen-limited (manual pollination increased seed set over open pollination). Analyses of adaptive accuracy showed that coordinated stamen movements and style elongation (movement herkogamy) dramatically increased pollination accuracy. Specifically, dehiscing anthers and receptive stigmas were positioned accurately in the vertical and horizontal planes in relation to the opposite sexual structure and pollinator position. By contrast, the spatial correspondence between anthers and stigma was dramatically lower before the anthers dehisced and after stamens bent outwards, as well as before and after the period of stigmatic receptivity. CONCLUSIONS: It is shown for the first time that a combination of movement herkogamy and dichogamy can maintain high pollination accuracy in flowers with generalized pollination. Staggered pollen and stigma presentation with spatial correspondence can both reduce sexual interference and improve pollination accuracy.

Native or Exotic? Double or Single? Evaluating Plants for Pollinator-friendly Gardens.

In a series of dawn-to-dusk studies, we examined the nature and accessibility of nectar rewards for pollinating insects by monitoring insect visits and the secretion rate and standing crop of nectar in the British native plant species Salvia pratensis , Stachys palustris , S. officinalis , Lythrum salicaria , Linaria vulgaris , the non-native Calendula officinalis , Petunia × hybrida , Salvia splendens , and the possibly introduced Saponaria officinalis . We also compared single with double variants of Lotus corniculatus , Saponaria officinalis , Petunia × hybrida and Calendula officinalis . All the British species studied are nectar-rich and are recommended for pollinator-friendly gardens. They showed maximal secretion rates of about 10-90 µg sugar per flower h -1 , and most had mean standing crops of about 5-60 µg sugar per flower. In all British species studied, the corolla was deep enough for the relatively long-tongued bumblebee Bombus pascuorum , but the shallower flowers of Lythrum salicaria were also much visited by shorter-tongued bees and hoverflies, as well as by butterflies. The exotic Salvia splendens , presumably coevolved with hummingbirds in the Neotropics, has such deep flowers that British bees cannot reach the nectar except by crawling down the corolla. With a secretion rate approaching 300 µg sugar per flower h -1 and little depletion by insects, S. splendens accumulated high standing crops of nectar. S. splendens , and single and double flowers of the two probably moth-pollinated species Petunia × hybrida and Saponaria officinalis , received few daytime visits despite abundant nectar but Calendula was well visited by hoverflies and bees. We compared single and double variants of Lotus corniculatus , Petunia × hybrida and Calendula officinalis , and also Saponaria officinalis , the last being probably introduced in Britain (Stace, 1997 New flora of the British Isles. 2nd edn. Cambridge: Cambridge University Press). In Petunia , Saponaria and Lotus , double flowers secreted little or no nectar. In Calendula , where doubling involved a change in the proportion of disc and ray florets rather than modification of individual flower structure, double and single capitula had similar standing crops of nectar. Except in Calendula , exotic or double flowers were little exploited by insect visitors. In the exotics, this was probably due to the absence or scarcity of coevolved pollinators, coupled, in double flowers, with the absence of nectar.

Fruit and seed production in relation to pollination and resources in bluebell, Hyacinthoides non-scripta.

This paper explores spatial and temporal patterns in the interaction of pollination and resources as constraints on the initiation and maturation of fruits and seeds in bluebell (Hyacinthoides non-scripta, Liliaceae). Field and laboratory experiments showed a degree of self-incompatibility (cross-pollination gave greater seed and fruit numbers than self-pollination), and pollinator limitation (hand cross-pollination gave greater seed and fruit numbers than open-pollination) as well as␣resource limitation. Bud removal, designed to relax interfruit competition for resources in the remaining flowers, increased percent fruit maturation in unpollinated flowers and increased the percent of initiated seeds that matured in cross-pollinated flowers. Clear position effects were also found. In an ovary, ovules near the stylar end showed a higher percent seed maturation than basal ovules. On a raceme, terminal flowers showed lower percent fruit initiation and maturation and lower seed numbers than basal flowers. The balance between resource limitation and pollinator limitation changed with position on the raceme. Temporal patterns involved raceme death and abortion of flowers, fruits or seeds. A lattice diagram schematises the hypothetical modulation by pollination of resource-dependent serial adjustment of maternal investment.

The nectar of Justicia and Columnea: Composition and concentration in a humid tropical climate.

Studies in Costa Rica on two ornithophilous flowers, Justicia aurea (Acanthaceae) and Columnea glabra (Gesneriaceae) showed a constancy of nectar solute concentrations that was attributed to microclimatic protection by the tubular corolla and to copious nectar secretion, helped by waterproofing by a lipid film on the nectar surface in Justicia and by preferential compass orientation of the flowers of Columnea.Most of the corollas in the patch of Justicia had been pierced by nectar-robbers. A consequence of this damage, together with local microclimate effects, was flower-to-flower variation in the amount and accessibility of nectar and in the nature and concentration of its minor components, notably amino acids.McDade and Kinsman's (1980) finding that nectar secretion could be suppressed by repeated sampling or by nectar-robbing was confirmed.

Why do nectar-foraging bees and wasps work upwards on inflorescences?

Wasps (Dolichovespula and Vespula spp.) worked predominantly upwards when foraging for nectar on inflorescences of the protogynous Scrophularia aquatica, in which the standing crop of nectar sugar per flower showed no clear pattern of vertical distribution up an inflorescence. Bumblebees taking nectar (Bombus hortorum visiting legally, and certain individuals of B. terrestris which positioned themselves head-upwards while taking nectar through holes bitten in the corolla) worked predominantly upwards on the racemose inflorescences of Linaria vulgaris, although the standing crop of nectar sugar per open flower increased up the raceme. Individuals of B. terrestris which robbed Linaria flowers in a head-down position worked predominantly downwards on inflorescences. The upward or downward directionality of intra-inflorescence movements by foraging insects may depend in part on the position these adopt during their flower visits.