Coyote tobacco can alter its flowering to suit different pollinators – moths or hummingbirds
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).
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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 delayed ( Proceedings of the National Academy of Sciences, vol 107, p3,912).
Investigations showed that when water enters a seed, it dissolves the sugars inside – a process that causes cooling. This is a purely physical process, requiring no input from the seed. Once rehydrated, healthy seeds begin to break down stored starches into sugars, which dissolve, prolonging the cooling effect. Old seeds need to repair the damage caused by long-term storage before they begin to germinate, so there’s a delay before they start to mobilise starch reserves.
‘In pea seeds, you could clearly distinguish between live and dead seeds,’ says Kranner. ‘What was so exciting was that you could tell them apart within two to three hours of adding water, which means you can re-dry them without any problem.’ To prove the technique worked with other types of seeds, the researchers repeated it with wheat and oilseed rape. ‘It works well, but rape seeds are so small they pushed the technology to the limit.’
The final step was to build a library of ‘thermal fingerprints’ to predict the viability of individual seeds of unknown quality. That meant producing and analysing images of hundreds of seeds, a timeconsuming and tedious process. Every species requires its own set of thermal fingerprints, so the test is of most use to those who work with one or very few species, such as seed companies and crop seed banks. Conservation seed banks such as the Millennium Seed Bank Project (MSBP) at Wakehurst will have to wait for a more generic test,
says Kranner. ‘In theory you can use it, but in practice there are too many species – the MSBP has 28,000. If necessary, for a seed that’s vital to conservation, we could work it out.’
Infra-red thermography shows the temperature changes inside a seed as it takes up water
Will this wasp hold up its side of the bargain and pollinate the fig flowers while laying its eggs?
Crime and punishment
Fig trees and fig wasps need each other. Female fig wasps lay their eggs inside the enclosed inflorescences we think of as fig fruits, which are a safe place for developing larvae. In return, the wasps pollinate enough flowers to ensure the tree produces seed – or at least that’s how it should be. But sometimes a wasp cheats, laying eggs without pollinating the flowers. If too many wasps try that trick, the once mutually beneficial relationship breaks down: the wasp gains and the tree loses.
Yet the fig-wasp partnership is hugely successful – it goes back more than 80 million years, and there are more than 700 species of fig, each with its own wasp partner. For these relationships to work, there must be a way to prevent wasps from cheating. According to Swedish biologist Charlotte Jandér, there is: when wasps cheat, figs dump the fruits and the wasp’s offspring die (Proceedings of the Royal Society B, vol 277, p1,481).
While working at the Smithsonian Tropical Research Institute in Panama, Jandér tested the idea that figs keep their wasps in line by applying sanctions to those that cheat. In the most ancient fig-wasp pairings, wasps are covered with pollen as they crawl inside the fig. In partnerships that evolved more recently, wasps actively collect and stash pollen in special pouches. This is better for the fig because it needn’t produce so much pollen, but it requires more effort by the wasp.
Passive pollinators have no reason to cheat and can hardly avoid picking up pollen. Active pollinators have both the means and the motive – cheating saves them energy. Sure enough, Jandér only found cheats among these species of wasp. And only trees with active pollinators dumped their unpollinated figs – so ensuring wasps with cheating tendencies don’t pass on their genes. Such a strategy ensures the number of cheats remains low. ‘Relationships require give and take,’ says Jandér. ‘Sanctions seem to be a necessary force in keeping this relationship on track.’
Looking for cheating wasps, Charlotte Jandér studied many fig species and their pollinators
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