Nectar addition changes pollinator behavior but not plant reproduction in pollen‐rewarding Lupinus argenteus

Abstract

In addition to its role as the male gamete, pollen is often used as a food reward for pollinators. Roughly 20,000 species of angiosperms are strictly pollen-rewarding, providing no other rewards to their pollinators. However, the influence of this strategy on pollinator behavior and plant reproduction is poorly understood, especially relative to the nectar-reward strategy. We performed a field experiment using the strictly pollenrewarding Lupinus argenteus to explore how the absence of nectar influences pollinator behavior and plant reproduction. METHODS: We added artificial nectar to Lupinus argenteus individuals to simulate a phenotype that would reward pollinators with both nectar and pollen. We compared bee pollinator behavior, via direct observation, and female reproduction between nectaradded and nectarless control plants. RESULTS: Bees exhibited behavioral responses to the novel reward, collecting nectar as well as pollen and spending 27% longer per flower. Pollen transfer increased with flower visit duration. However, plants in the study population were not pollen-limited; consequently, the observed changes in pollinator behavior did not result in changes in female components of plant reproduction. CONCLUSIONS: The addition of nectar to pollen-rewarding plants resulted in modest increases in per-flower pollinator visit duration and pollen transfer, but had no effect on reproduction because, at the place and time the experiment was conducted, plants were not pollen-limited. These results suggest that a pollen-only reward strategy may allow plants that are visited by pollen foragers to minimize some costs of reproduction by eliminating investment in other rewards, such as nectar, without compromising female plant fitness.

Consequences of pollen defense compounds for pollinators and antagonists in a pollen‐rewarding plant

“…We also (4) tested the effect of pollen secondary metabolites on the behavior of legitimate pollinators of L. argenteus . Lupinus flowers are nectarless and produce pollen as the only reward for pollinators (Heiling et al, 2021), which allowed us to avoid potentially confounding effects of nectar chemistry on pollinators and antagonists. Pollen from this genus contains quinolizidine alkaloids (Heiling et al, 2019), which are known to be toxic to insect herbivores and mammals when present in vegetative tissues (Wink, 1992); however, the effects of these compounds on pollen‐feeding insects and microbes have not previously been examined.…”

Sexually concordant selection on floral traits despite greater opportunity for selection through male fitness

“…Similarly, the distribution of nectar volume along subsequent flowers on the same inflorescence can affect pollinator foraging (Pyke, 2016; Nepi et al ., 2018). For example, decreasing nectar volume from bottom to top along the inflorescence could reduce geitonogamy through encouraging the upward movement of pollinators in systems where flower and inflorescence ontogeny leads to male‐phase flowers being located toward the top of the inflorescence (Fisogni et al ., 2011; Zhao et al ., 2016; Heiling et al ., 2021). These functional considerations imply that the optimum phenotype may differ in terms of selfed and outcrossed reproductive fitness components, leading to functional trade‐offs (Caruso et al ., 2012; Teixido & Valladares, 2014; Bawa et al ., 2019).…”