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?

cuttings

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

KEW Summer 2010 l 15