Coyote tobacco can alter its flowering to suit different pollinators – moths or hummingbirds

Changing allegiances

GUERRA

,MARCOS

ZIEGLER

,CHRISTIAN

KRANNER

ILSE

AND

KASTBERGER

,GERALD

KESSLER

:DANNY

PHOTOGRAPHS

When pollinators turn bad, what’s a plant to do? For coyote tobacco (Nicotiana attenuata), the answer is to hide from its friend-turned-foe and seek a pollinator with less destructive tendencies.

Coyote tobacco grows wild in the western United States, springing up after wildfires and often carpeting large areas. Usually, the flowers are pollinated by nightflying hawkmoths, attracting them from far and wide with an alluring scent and the promise of a meal of rich, sugary nectar. Unfortunately, female hawkmoths also have a habit of laying eggs on the plants they visit – eggs that hatch into voracious hornworms. A few hornworms are fine, but as researchers from Germany discovered, when a few becomes an infestation, the plants fight back.

In 2007, Ian Baldwin and some of his postgraduate students at the Max Planck Institute for Chemical Ecology in Jena were doing field work in the Great Basin Desert of Utah when there was a huge outbreak of hornworms. To their surprise, coyote tobacco plants began to produce flowers that opened at dawn rather than dusk. Not only that, the corollas of the morning flowers opened only a third of the normal extent. Tests showed too that these flowers stopped producing benzyl acetone, the chemical that made their perfume so irresistible to hawkmoths. In effect, they had made their flowers impossible for moths to find (Current Biology, vol 20, p237).

14 l KEW Summer 2010

Experiments over the next two years revealed that when hornworms start chewing, the chemicals they inject into leaves trigger a change in the flowers’ development. ‘You usually start seeing morning flowers after two days of feeding,’ says Baldwin. ‘By the third instar, about 12 days after hatching, the proportion of morning flowers has increased to 40 to 50 per cent of the flowers.’

Hawkmoths stopped visiting. In their place came very different pollinators – nectar-sipping hummingbirds that are active by day and forage by sight. Observations showed that hummingbirds almost invariably ignored night-opening blossoms and fed only from those that opened in the morning, probably recognising them by their distinctive shape. In the absence of moths, only morning flowers set seed – proof that the switch to day-flowering produced results without the downside of playing host to hornworms.

If hummingbirds can do the job without harming the plants, why bother with hawkmoths at all? Baldwin thinks that for plants that grow in huge patches, it makes sense to emit a powerful scent that can draw insects long distances. Relying on birds that forage by sight restricts the pool of potential pollinators to those that live nearby or happen to be passing.

The plant seems to be hedging its bets. Hawkmoths are more effective pollinators, says Baldwin, but if the price of pollination is a plague of hornworms, then it’s better to play safe with the hummingbirds.

It’s cool to be alive How do you tell if a seed is any good? Until now, the answer was to germinate it, which took days, sometimes even months, and meant the loss of the seed. With an infrared camera and endless patience, Ilse Kranner of Kew’s seed conservation department at Wakehurst Place, and Gerald Kastberger of the University of Graz in Austria, have developed a test that diagnoses the health of a seed in less than two hours, without destroying it.

The key to the test is the infra-red camera’s ability to record subtle changes in the temperature of individual seeds as they gear up to germinate – or not. Starting with a good-sized seed, the pea, Kranner and Kastberger added water, placed the seeds beneath an infra-red camera and set it snapping – one image every 20 seconds for five days. The results astonished them. ‘In the first hour, there was a huge drop in temperature. A single seed could cool by 2°C,’ says Kranner. The drop was so dramatic that initially they thought it was caused by some glitch in their technique. But it turned out to be real.

After the initial drop, the ‘thermal profiles’ recorded by the camera followed different and distinctive paths according to the quality of the seed. Dead seeds soon warmed to room temperature; living seeds remained cool, while in old seeds the whole thermal profile was