“Dispensing pollen via catapult: Explosive pollen release in mountain laurel (Kalmia latifolia)”

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Callin M. Switzer, Stacey A. Combes, and Robin Hopkins (June 2018)

A mountain laurel flower. Each of the anthers is tucked into pockets in the petals. The filaments are under tension, ready to launch pollen into the air.
(Credit: William (Ned) Friedman, Arnold Arboretum of Harvard University)

The DOI will be https://dx.doi.org/10.1086/697220

Mountain laurels launch pollen at 400 times the acceleration due to gravity

Pollen dispensing with a catapult

Flowers are not known for their fast movements, but the mountain laurel flower propels pollen into the air at incredible speeds. These flowers are equipped with ten tiny pollen catapults. But where does this pollen go and why is it launched?

Callin Switzer, Stacey Combes, and Robin Hopkins, strived to answer these questions through an in-depth study of how this unique flower functions – they investigated the importance of the explosive pollination mechanism in reproduction and characterized the biomechanics of this quick-moving flower.

Using high-speed videos, the authors were able to track where the pollen goes after being launched from the flower. With these data, they created a three-dimensional map of where a pollinator needs to be in order to be hit by pollen. They found that the catapults are likely to launch pollen towards the center of the flower, which would hit insects like bumblebees as they collect nectar. Through careful tracking of pollen movement, this study documents one of the fastest plants ever described – mountain laurel flowers accelerate pollen at over 400 g, and the pollen reaches a top speed of around 3.5 m/s (about 8 mph).

Complementing the mechanical description of the mountain laurel flower is a pollination study that determines that insects are important for activating the catapult and pollinating flowers. When pollinators were excluded from visiting the flowers, mountain laurels produced little or no seeds.

This work represents a high-speed glimpse into the function of flower form.

Abstract

The astonishing amount of floral diversity has inspired countless assumptions about the function of diverse forms and their adaptive significance; yet many of these hypothesized functions are untested. We investigated an often-repeated adaptive hypothesis about how an extreme floral form functions. In this study, we conducted four investigations to understand the adaptive function of explosive pollination in Kalmia latifolia, the mountain laurel. We first performed a kinematic analysis of anther movement. Second, we constructed a heat map of pollen trajectories in three-dimensional space. Third, we conducted field observations of pollinators and their behaviors while visiting K. latifolia. Finally, we conducted a pollination experiment to investigate the importance of pollinators for fertilization success. Our results suggest that insect visitation dramatically improves fertilization success and that bees are the primary pollinators that trigger explosive pollen release.