Flowers: Umbrellas for pollen?
Pollen is key to letting plants live on land. It packages up plants’ sperm so that wind or animals can transport them to the female part of a plant, without requiring water. Ferns, which don’t have pollen, can only grow where there’s enough moisture for sperm to swim to meet eggs.
Before pollen is released, it’s allowed to dry out, making it lighter. A pollen grain which makes it to its target will take in water again, before growing a pollen tube towards the egg it will fertilise. Rain can interfere, though: either by washing pollen grains from the flower, or by soaking them, which kills them after a few hours.
The shape of flowers may protect their pollen from rain: it’s no great leap of the imagination that pollen produced by cyclamens will be better shielded in their downward-facing flowers than will pollen in the open cup of a tulip flower, for example. But is that a just-so story, or does a flower’s structure really affect its pollen?
That’s what two scientists in Wuhan, China set out to show. Their approach was to measure how resistant the pollen itself was to water, and see if it matched up to flower shape.
They found 80 species around Wuhan, both native ones and some from the Botanic Garden, and watched them during showers to put them in groups based on how well shielded the pollen was against rain: 1. “No protection”, 2. “Partial protection” and 3. “Complete protection”.
In the lab, they measured how quickly pollen died when soaked in water. Luckily for them, there’s a chemical shortcut to tell if pollen grains are alive, by soaking them in a particular chemical. The living ones take it up within ten minutes, and are stained purple, making them easy to count under a microscope.
Putting the two sets of results together gave the answer they’d hoped to see: pollen from flowers that offered little protection from the rain was able to survive longer soaking in water than pollen that was likely to stay dry. The explanation could go either way round: pollen that couldn’t stand rain had to evolve flowers that worked like umbrellas, or open, unshielded flowers may have had to evolve water-resistant pollen.
They also looked at how quickly the plant released its pollen: on sunny days, the best strategy is to get it all out quickly, but when it’s rainy, the plant is better off holding on to some of it for later. The flowers which offered least protection from the rain were able to stretch their pollen release out more, taking nearly seven times longer in a rainy spell than a sunny spell, as compared to only 3.5 times longer for the “complete protection” flowers.
It’s an interesting result: we’re used to thinking of flowers as evolving just to attract pollinators, but this research suggests another role, in protecting pollen.
I think there’s a bit of a hole in their method, however: from what I can work out, they treat each species as an independent data point, ignoring how closely related they might be. They even mention in the paper that two Rhododendron species had very water resistant pollen; counting both of these close cousins will skew the statistics. It’s not too serious a problem: they chose a variety of species from 46 different families, but that does include ten groups of two or more closely related species. An ideal way to do it would be to compare pairs of related species which had different flower shapes, but finding enough such pairs of species would be no small task. Alternatively, there are complex mathematical techniques which can take evolutionary relationships into account.
For future study, it would be nice to see this aspect of flowers linked up with their better known functions in attracting pollinators. Do pollen-shielding ‘umbrella’ flowers attract pollinators less effectively? Different sorts of pollinators? What makes some flowers go for water-resistant pollen, while others shield water-sensitive pollen? An accident of evolution, a difference in pollinators, or something else? What about wind-pollinated species, which can’t very well shield pollen?
Reference: Mao, Y., & Huang, S. (2009). Pollen resistance to water in 80 angiosperm species: flower structures protect rain-susceptible pollen New Phytologist, 183 (3), 892-899 DOI: 10.1111/j.1469-8137.2009.02925.x