Raindrops drive flower evolution

Matt Walker
Editor, Earth News

Soggy pollen is of little use

The humble raindrop may have played an important role in the evolution of flowers, scientists in China have discovered.

A study of 80 species has revealed that flowers evolve different shapes and structures in part to prevent their pollen getting wet.

Other flowers get round the problem by evolving waterproof pollen.

The finding may help explain why so many species in rainy areas either have droopy flowers or close their petals.

Many researchers, including Charles Darwin, have speculated that flowers may have evolved certain traits or structures to protect themselves against the damaging effects of rain, which can wash away pollen grains and dilute nectar.

But few have experimentally tested the idea.

So Yun-Yun Mao and Shuang-Quan Huang of Wuhan University in China decided to do just that by studying the response to rain and water of 80 species of flower living locally around the University's campus and in the nearby Wuhan Botanical Garden.

"Animals as well as ourselves like to be sheltered when it rains. We were wondering how flowers reduce rain effects on pollen grains, given that plants are immobile," says Huang.

A hole at the flower's base allows water to drain away

Firstly, they recorded how the flowers of each species respond to rainfall, including whether the pollen becomes wetted or washed away and whether the flower moves or closed its petals.

They also measured the effect of rain on pollen performance, by determining how long pollen produced by each species remains viable in water.

What they found confirmed a strong link between rainfall and flower design, the researchers report in the New Phytologist.

For example, of the 80 species studied 20 produce flowers that completely protect their pollen. As a result, none of these 'waterproof' flowers produce pollen that is resistant to water.

That supports the idea that the flowers have evolved certain structures that can keep their pollen dry and viable and prevent it being washed away before it can be picked up by pollinators.

"Some plants shelter their pollen grains through a change in floral orientation or closing their corolla on rainy days," explains Huang. "For example, tulip flowers close their petals rapidly when rains come."

Others have flowers that droop downward, while Araceae species have outlets in the base of the flower that let water quickly drain away.

Drooping flowers make good umbrellas

But 44 species expose their pollen completely, giving it no protection. Of these species, 13 produce pollen that is highly resistant to water, suggesting they have evolved an alternative way to deal with the rain.

"We were surprised that some plants without pollen protection structures develop water resistant pollen," says Huang.

"The finding of a high proportion of resistant pollen in no-protection species suggests that selection by rain contact favours pollen resistance to water."

Previous research has also suggested that places which have high levels of rain are home to a higher than normal proportion of flowers that nod downward, droop or are able to close their petals.

Mao and Huang hope to investigate this further, and also whether flowers also protect their nectar from the rain.