Pollen grains find safety in numbers

Behind the Cover: New Phytologist 221:2, January 2019

You might think that the image on the latest cover of New Phytologist looks a bit like falling snow, appropriate for this time of year in the northern hemisphere. But what you’re actually looking at are pollen grains.

The photo shows fragments of pollen tetrads connected by clear, sticky threads of viscin and called ‘pollen-thread-tangles’ (PTT) by Shuang-Quan Huang and colleagues, in their recent New Phytologist paper. This PTT was collected from Rhododendron fortunei, growing on campus at Central China Normal University, in Wuhan, China.

Rhododendron fortunei, growing on campus at Central China Normal University, in Wuhan, China.

To show the pollen tetrads connected by viscin, PhD student Hui-Hui Feng prepared a microscope slide, with the PTTs pasted on. Attaching a piece of double-sided sticky tape to the slide, Hui-Hui then touched the sticky tape to the tip of an anther, before gently pulling the PTT from an anther pore by moving the slide away. By turning the slide over, Hui-Hui allowed the PTT to lie flat against the slide, so that they could be photographed.

But why are the pollen grains of Rhododendron species connected and transfered in groups like this? Pollen aggregation is a characteristic of the flowers of Rhododendron species, orchids, Asclepias, and many others such as evening primrose – all these plants transfer pollen grains in groups, rather than singly.

This photo was recently taken under a Keyence Digital Microscope, showing pollen tetrads connected by viscin threads from Rhododendron fortunei.

Shuang-Quan and colleagues think that plants aggregate their pollen in response to a scarcity of pollinators. Being stuck together means that, when a pollinator does visit, it will take more pollen with it – a sticky thread of grains – than it would have done otherwise. This is only beneficial for plants if pollinators don’t collect pollen as a reward, otherwise the loss of pollen would be too great.

By comparing 13 Rhododendron species in the Cangshan mountains of southwestern China, the researchers showed that variation in pollen aggregation was indeed related to differences in the pollen collecting intensity and visitation frequency between groups of pollinators. The seven species pollinated by infrequent and weak grooming birds or insects produced significantly larger PTTs than the other six species, which were pollinated more frequently by pollen collecting bees.

When pollinators are in short supply, it seems that pollen grains find safety in numbers.

Read the paper: Song, Y., Huang, Z. and Huang, S. (2019) Pollen aggregation by viscin threads in Rhododendron varies with pollinatorNew Phytologist 221: 1150-1159. doi: 10.1111/nph.15391

Dr Mike Whitfield
Development Coordinator
New Phytologist Trust