Evaluating honey bees as pollinators of virgin flowers of Echium plantagineum L. (Boraginaceae) by pollen tube fluorescence

SUMMARY

A novel technique was developed which simply, directly and quantitatively evaluated flower visitors as pollinators of Echium plantagineum in Canberra, Australia, based on microscopic detection of the fluorescence of the callose content of stylar pollen tubes stained with aniline blue. E. plantagineum is self-compatible, so that germination and pollen tube growth of both cross- and self-pollen to the ovules at the style base occur. The technique utilized ‘virgin’ flowers, bagged as buds and which subsequently opened within the bags. After careful unbagging, the majority (93.5%) of initial visitors to these previously-unvisited flowers were honey bees (Apis mellifera). The technique demonstrated that although autogamy was possible, it was not predominant in E. plantagineum because permanently-bagged flowers had very low numbers of pollen tubes at their style bases (usually none), and emasculated and intact flowers exposed to single honey bee visits usually had the same numbers of tubes. Within treatments (intact, emasculated), honey bee visits for pollen, nectar or both, did not result In differences in pollen tube number, nor was there any connection between degree of pollination and time spent by a single honey bee per virgin flower. Although most honey bees attending virgin flowers carried pellets of E. plantagineum pollen on their hind legs, as a result of grooming actions to gather grains from their bodies, they were still as effective as pollinating agents as bees lacking corbicular pollen. Because one-third of single honey bee visits to emasculated virgin flowers (bearing receptive stigmas) introduced sufficient pollen to achieve fertilization of all four ovules, and almost two-thirds of such visits resulted in pollination, it appears that honey bees were effective cross-pollinators of E. plantagineum. Various advantages of the technique, including applications to agriculture, are discussed.

Keywords:

• honey bees
• Apis mellifera
• pollinators
• pollination
• foraging
• Echium plantagineum
• pollen tubes
• callose
• staining
• aniline blue
• fluorescence microscopy