Patterns of Nectar Production in &lt;i&gt;Asclepias curassavica&lt;/i&gt; (Apocynaceae))

Steven Broyles

doi:10.26786/1920-7603(2019)541

Abstract

Milkweeds are important nectar resources for insects in the New World. In addition, nectar is the germination medium for milkweed pollen. This study is the first controlled, greenhouse examination of patterns of nectar production in a milkweed species. We measured nectar volume, concentration, and mg of sugar in the pantropical, weedy milkweed Asclepias curassavica. Our results show that A. curassavica secretes nectar primarily during daylight hours and it continues at a constant daily rate for four to five days. Freshly secreted nectar is lower in sugar concentration than older nectar. This provides an opportunity for milkweed pollen to germinate throughout the day, but pollen germination could be inhibited at times when the sugar concentration increases. Nectar production in A. curassavica is adapted to attract diurnal insect pollinators over several days and to allow pollen germination to occur quickly. Significant differences in nectar production exist among plants and among inflorescences within plants. Nectar production increases in flowers when nectar is extracted using paper wicks that simulate removal by insects in nature. Removal-enhanced nectar production in milkweeds may allow plants to adjust resources to inflorescences receiving insect visitation. Significant inter-plant differences in nectar production and the unique milkweed flower provides a model system for examining the role of pollinator-mediated selection on nectar traits.

Highlights

Flower color, anthesis, and nectar production can be assembled into suites of traits recognized as pollination syndromes (Faegri & van der Pijl 1979)
Differences in nectar production, nectar concentration, and sugar content exist among plants of A. curassavica (Tab. 1B)
Our results suggest that A. curassavica nectar production exhibits considerable variation among plants

Summary

Introduction

Flower color, anthesis, and nectar production can be assembled into suites of traits recognized as pollination syndromes (Faegri & van der Pijl 1979). Selection for floral traits occurs through the interaction with pollinator functional groups whose unique behavioral and morphological attributes affect the pollination efficiency of the flowers (Fenster et al 2004). Floral nectar is frequently the reward to pollinators and may be matched to the extraction mechanics and energetics of the pollinator. Nectar production is often greatest in larger flowers, or flowers with unique attributes that pollinators learn to associate with greater nectar rewards (Cresswell & Galen, 1991; Fenster et al 2006). The variance in nectar production among flowers and inflorescences may be associated with sexual stage of the flower (Devlin et al 1987), movement of pollinators (Zhao et al 2016), pollination success (Pleasants & Chaplin 1983; Mitchell 1993), and levels of geitonomy/xenogamy

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Discussion

Conclusion