Wednesday, May 20, 2015

Plant waits for full moon

I was alerted to this story in the latest hardcopy of New Scientist (no. 3016, p. 16): "Werewolf plant waits for full moon" and tracked down this on-line story by Ed Yong at National Geographic:

Shrub Attracts Pollinators By Glittering Under the Full Moon

On the cliffs of the Mediterranean, there grows an untidy, scrambling shrub called Ephedra foeminea. It isn’t the prettiest of plants, but once a year, in the middle of July, it becomes far more appealing. On the night of the full moon, the shrub exudes small, sweet droplets from its red cones. Without any clouds or trees in the way, these drops catch the full intensity of the moonlight, reflecting them into the eyes of passers-by. The shrub sparkles, as if covered in diamonds.

“We find it ever so beautiful,” says Catarina Rydin, from the University of Stockholm.

She thinks that the precisely timed lightshow attracts night-flying pollinators like moths and flies. After all, E.foeminea has no other obvious way of enticing these insects. It doesn’t produce scent and it doesn’t have bright white flowers that might stand out in the darkness. Instead, it relies on the moon, cloaking itself in reflective ‘pollination drops’ that shine like beacons.

This discovery was borne of frustration. Ephedra is part of an old lineage of plants that had their heyday during the reign of the dinosaurs, almost went extinct, and then stubbornly clung to existence. Rydin wanted to know why. She and her student Kristina Bolinder started studying two European species to work out how they reproduce. Bolinder became so familiar with the wind-pollinated Ephedra distachya that she could predict its pollination schedule and plan trips to Greece accordingly. But E.foeminea proved to be much harder to predict. “We made mistakes basically every year,” says Rydin. They’d go, only to find no signs of pollination drops. They always had to wait.

In 2014, they made a trip in early July and “as usual, we came at the wrong time,” says Rydin. “No pollination drops, no pollinators. We could do nothing but wait.” Wait, and read. Rydin read up on insect pollinators. She read about how some insects can navigate by moonlight. And when she reviewed records and photos from their 2012 trip, the only one where they accidentally got the timing right, she noticed that there was a bright full moon in the sky. “We talked about it. We even said that we had bad luck with the moon this year,” she recalls. “But we still didn’t get it.”
A week passed and the duo became incredibly frustrated. They tried to take their minds off with a nice Greek dinner, “but all we could talk about was why the ******* plants did not go into pollination phase,” says Rydin. “And, I am not sure why, but all of a sudden we experienced a eureka moment! The moon in the photos from another year, the darkness at the field site this year, the articles about nocturnal insect navigation… Wait a minute now… What if…?”

The duo looked at old records, from their own trips and from the scientific literature as far back as 1910, and found that E.foeminea always releases pollination drops on the night of the full moon. “Not much data is available, but it all speaks the same language,” says Rydin. The full moon of July 2014 was due on the 12th, a few days away. So, they waited “to see if that was what the plants were waiting for. And so it was!” The moon appeared, and the shrubs started sparkling.

E.foeminea’s timing is unerringly precise. Even if its cones are mature earlier in the month, it waits till the full moon to produce its pollination drops. And if some cones are immature, the plant still forces them to exude droplets on that particular day. By contrast, the wind-pollinated E.distachya is unconnected to lunar cycles. It produces pollination drops at roughly the same time, regardless of what the moon is up to.

How does E.foeminea detect moonlight? “Short answer: we don’t know,” says Rydin. It might be able to detect small changes in light intensity. It’s also unclear why the full moon matters; surely the light of a half-moon would be sufficient to guide in pollinating insects? The difference, Rydin thinks, is that a full moon is not just brighter, but brighter for longer. “We think it is all about maximizing efficiency. Only at full moon do the insects have a moon to navigate by during the entire night.”

This strategy has clearly served E.foeminea well for a long time, but also makes it vulnerable to rain, clouds, and perhaps man-made lighting. Perhaps this is why E.foeminea grows much further away from local villages than E.distachya does. And perhaps this is why it seems to be the only Ephedraspecies to retain its ancient insect-pollinated strategy, while other members of the group have trusted their fates to wind instead.

More on moonlit liaisons: Clock gene and moonlight help corals to co-ordinate a mass annual orgy

Reference: Rydin & Bolinder. 2015. Moonlight pollination in the gymnosperm Ephedra (Gnetales). Biology Letters.