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A hotspot on your doorstep

For a continent whose flora has been studied for longer and by more botanists than any other, Europe can still spring surprises. As researchers from the Royal Botanic Gardens of Madrid and Kew have discovered, one familiar group of plants – the carnations and pinks – diversified into today’s huge array of species at record-breaking speed. The rate of speciation in the genus Dianthus outstrips anything found in regions regarded as hotspots of rapid evolution.

As part of a broader study of biodiversity in the Mediterranean region, Spanish botanists Luis Valente and Pablo Vargas decided to investigate one very diverse genus and picked Dianthus. There are some 300 Dianthus species worldwide, but the largest group, with more than a hundred species, is found around the Mediterranean. To learn more about the origins of this group, Valente and Vargas teamed up with Vincent Savolainen in Kew’s Jodrell Laboratory, where they could work out the relationships between species using the latest techniques of molecular phylogeny. These studies revealed little genetic variation between species, suggesting that Europe’s carnations had experienced a sudden and rapid burst of speciation.

Such an explosion in the number of species is generally associated with more exotic locations, such as equatorial forests, oceanic islands, the Andes and the Cape region of South Africa. Intrigued by the possibility that Europe might also be an evolutionary hotspot, the team searched for an explanation.

Using well-established molecular techniques to calibrate the Dianthus family tree, they found the ancestral Dianthus gave rise to new species at an unprecedented rate of almost eight species per million years – faster than any other group of plants. This rate was not constant however. To begin with, new species appeared slowly, with at most 0.3 species per million years. Then, at some point between 2 and 1.3 million years ago, the rate of diversification rocketed to seven or eight times as fast (Proceedings of the Royal Society B, vol 277, p1,489). ‘We thought that carnations were a young group, but never suspected they had diversified so rapidly,’ says Valente.

Above and below: Dianthus evolved into many different forms to attract pollinators

What triggered this blooming of carnation species? The likeliest answer is the sudden drying out of the Mediterranean Basin 2 million years ago, when the hot, humid climate gave way to a seasonal one, with mild, wet winters and hot, parched summers. Today’s flora is dominated by plants that bloom in spring, when insects are plentiful, but most Dianthus bloom in summer, when insects are scarcer. Valente suspects that pressure to attract rare pollinators drove the evolution of different floral features and then geography did the rest. The Mediterranean region has a remarkably varied topography, with hundreds of islands, peninsulas and many mountains – a landscape that helps to isolate plant populations and encourages divergence into new species. ‘In theory, this is the ideal setting for fast speciation, and for some reason that we haven’t got to the bottom of yet, carnations seem to have been particularly prone to it.’

Carnations and pinks diversified into today’s huge array of species at record-breaking speed

Other European genera with large numbers of species, such as Silene, Astragalus and Centaurea, are likely to have behaved in the same way as Dianthus, suggests Valente. ‘Hopefully this study will persuade young botanists to take more interest in Europe. To think that the field next to where you live may be a hotspot for the origin of new species is an exciting thought.’

12 l KEW Summer 2010 Central heating for hellebores

Some flowers warm themselves by absorbing heat from the sun, some even track its movement across the sky. Others, such as the titan arum and some of its smellier relatives, generate heat metabolically to encourage their evilsmelling odours to waft further afield. Spanish researchers have now found a third means by which flowers can raise their temperature – by harnessing heat generated by yeasts living in their nectaries.

Carlos Herrera, an ecologist at the Doñana Biological Station in Seville, knew from earlier studies that many flowers harbour sugar-loving yeasts. These specialist yeasts are spread from plant to plant by bumblebees, and once inside a nectary they proliferate rapidly. On the face of it, the yeast benefits at the plant’s expense, consuming nectar meant to attract pollinators. Yet these yeasts are so common that Herrera suspected there might be something in it for the plants too.

When yeasts break down sugars, they generate significant amounts of heat. Were these plants trading sugar for warmth,

Warmth created by yeast growing in the nectaries of stinking hellebores offers several benefits

Herrera wondered. He decided to test his hunch in a plant that could benefit from a little extra heat, the winter-flowering stinking hellebore (Helleborus foetidus).

Herrera and his colleague María Pozo studied hellebores growing in the mountains of south-eastern Spain. They found that the flowers almost invariably contain yeasts, often at very high densities. The question was whether the yeast made cuttings any noticeable difference to temperature. To find out, the biologists compared yeastfree flowers, kept ‘clean’ by the exclusion of bees, with flowers they had inoculated with a culture of yeast collected from a bumblebee’s tongue.

The effect was dramatic. Flowers with yeast in their nectaries had markedly warmer interiors than flowers without, and the more yeast, the bigger the difference (published online in Proceedings of the Royal Society B). On average, nectaries were 2°C warmer, but 7°C ‘was not exceptional’. Warmer nectaries kept the air within the flower significantly warmer than outside.

For winter-blooming plants like the hellebore, a little extra warmth can pay dividends. Pollen germinates and pollen tubes grow faster when it’s warmer, improving the prospects of successful fertilisation. Extra heat also encourages fruits to develop and seeds to grow larger. But first the flower has to be pollinated, and few insects are active when hellebores bloom. Those that are, suggests Herrera, might find a warm flower that bit more attractive. ‘In the early part of the flowering season, when there’s regular snowfall and daily frosts, floral warming will benefit pollinators directly by providing some sort of small thermal oasis.’

CALVERT/ALAMY

,ROSEMARY

ILFORD

W,RICHARD

VARGAS

:PABLO

PHOTOGRAPHS

Phew, not a scorcher!

Don’t water your garden in the noonday sun or your plants may suffer from sunburn – or so it’s said. The belief that drops of water focus the sun’s rays like tiny magnifying glasses, burning and scorching leaves, is deeply ingrained in gardening lore. Some even believe forest fires can start this way. Yet no one had gone to the trouble of checking if it’s possible. ‘It’s not a trivial question,’ says Gábor Horváth, of the environmental optics laboratory at Eötvös University in Budapest. He and his colleagues decided to investigate.

An experiment with small glass balls laid on the surface of maple leaves proved that transparent spheres can indeed focus the sun’s rays to an intensity that causes burns. But no drop of water can focus sunlight quite so effectively, partly because water refracts light less powerfully than glass, and partly because water droplets are rarely spherical.

The shape of a drop depends on how smooth and wettable the surface of a leaf is. On a maple leaf, for instance,

drops spread and flatten, while on water-repellent ginkgo leaves, drops hold their shape better and stand taller. In tests, however, even the most spheroid-shaped drops failed to cause burns. In optical terms, the drops were unable to focus sunlight to a point on the leaf, so the heat was never intense enough to burn (New Phytologist, vol 185, p979).

Horváth calculated that the only conditions where drops might cause burns would be if they were held off the leaf surface – removing any cooling effect of water – and high enough to bend the light to a point on the leaf itself. This might be possible with hairy leaves, where drops are trapped among the hairs – and so it proved. Tests with the waxy-haired leaves of the floating fern Salvinia natans resulted in scorch marks after two hours in bright sun. As Horváth points out, burning is unlikely in nature because the drops would almost certainly roll off long before any harm is

Droplets of water are said to scorch leaves by focusing the sun’s rays – but do they?

done. As for starting forest fires, he advises taking that with a pinch of salt.

Even so, it’s still best not to water at midday because it’s wasteful, as much of it will evaporate. Stick to early evening and stick to the parts that need it – the roots.

KEW Summer 2010 l 13