Sisymbrium irio

Hardcore plants

Plants are incredibly resilient organisms, surviving fire, ice and even nuclear disasters, finds Ellie Pinney

Sixty-six million years ago a giant asteroid hit the Earth, killing off the dinosaurs in what’s known as the KT (Cretaceous–Tertiary) event. 80 per cent of all animal species became extinct, yet somehow plants fared much better. While the number of seed-producing plant species (gymnosperms) temporarily fell, this didn’t last long. What’s more, for flowering plants (angiosperms) there was actually an explosion of new species straight after the asteroid hit. By analysing fossils, scientists have shown that this wasn’t a one-off: plants have bounced back from extinction events faster than animals throughout our planet’s history.

Plants thrive in some of the world’s most hostile environments: from scorching deserts and high-altitude mountain ranges to the darkest depths of the ocean. They can even protect humans from destructive forces of nature. Mangrove forests, for example, act as natural buffers against tsunamis and tropical storms. Plants are incredibly resilient, and they do it all without being able to move. So how do they survive?

Burnt-out bombsites

In September 1666, the Great Fire swept through London, destroying one-third of the city and leaving behind a trail of destruction. But within days, Sisymbrium irio, a bright-yellow mustard plant, began to flower everywhere, thriving among the wreckage. It became known as London Rocket. The weed – and several of its cousins – would colonise London again during World War II, emerging from the ruins of burnt-out bombsites.

Fire is destructive to life, but some plants can use it to their advantage. In the USA, for example, forest fires devastate up to 2 million hectares of land every year. In the short term these fires wipe out much of what they touch, but over time they actually help plants thrive. By removing diseased plants and harmful insects, like beetles and moths, the flames ‘disinfect’ the forest ecosystem. They also remove dead or decaying matter, clearing the way for a new generation of plants and leaving behind nutrient-rich ash for them to grow in. By burning through thick canopies and undergrowth, wildfires also clear shade, providing light for new growth on the forest floor.

Some plants, like California’s giant redwood (Sequoiadendron giganteum), even rely on forest fires to help release their seeds. Their cones can hang on the tree for years without opening up, but when a forest fire sweeps through, the heat melts the resin that glues them shut, allowing them to open and scatter their seeds below. The trees themselves are covered with fibrous bark up to two feet thick, protecting their vital tissues from the flames.

Ice and rocks

Antarctica: the coldest, driest, and (for half the year) darkest continent on Earth. Not necessarily a place you’d expect to see much plant life, but this winter wonderland is home to a variety of ‘lower plants’ – mosses, lichens and liverworts. Even more surprisingly, certain foreign plant seeds, like chickweed (Cerastium arcticum) and annual bluegrass (Poa annua), have managed to hitch a lift to Antarctica on visitors’ clothes and boots, and establish themselves among the ice and rocks. So how do plants survive here?

Antarctica’s native plants – and its successful foreign colonisers – are specially adapted to protect themselves from the continent’s harsh conditions. For example, they tend to grow low near the ground and close together, maximising shelter and reducing damage from wind and ice particles. They can also photosynthesise in very cold weather, allowing them to store up energy to see them through the dark winter months. These plants are even adapted to deal with the dryness of the region, with small leaves to minimise water loss and shallow roots to avoid the permafrost – a layer of permanently frozen ground beneath the thin layer of soil. To cap it all, Antarctic plants are expert opportunists when it comes to reproduction – they are quick to produce seeds and can germinate in a small window of time if weather conditions allow.

While it’s unlikely that Antarctica will suddenly become covered in rogue trees and flowers, certain intruders have already made a home there. And with climate change raising temperatures, it may be even easier for them to survive in the future.

Radioactive soil

On Monday 6 August 1945 at 8.15am, a US plane dropped an atomic bomb on the Japanese city of Hiroshima. 80,000 people were killed and the city was flattened instantly. Plants were incinerated, soils were charred, and radiation was everywhere. Experts said Hiroshima would be barren of life and nothing would grow for 70 years.

But they were wrong. Among the rubble of a city virtually wiped off the map, 170 ‘survivor trees’ stood defiantly – broken, and badly burned, but alive. One, a weeping willow (Salix babylonica), was just 370 metres from the centre of the blast. Soon new buds were sprouting again, and today the trees stand under the care of the Hiroshima government. As well as the trees, bright red Canna flowers started springing up among the debris just a month after the bomb was dropped, bringing the survivors hope and courage.

Plants can withstand a great deal of radioactivity. In fact, the world’s worst nuclear disaster – the explosion at the Chernobyl nuclear power plant in Ukraine in 1986 – holds a similar tale of plant resilience, despite far higher levels of radiation than in Hiroshima. Today, animals living around the Chernobyl site are still sick and deformed, and the area remains off-limits to people. Yet plants are thriving. Trees, bushes and vines have taken over the town’s abandoned streets, despite the highly radioactive soil.

To understand how these plants have survived, scientists grew some soybean plants just 5 km away from the remains of the power station, and then analysed their beans. They turned out to be very different to beans collected from other plants further away, where radiation levels were much lower. The high-radiation beans weighed less, took up water more slowly, and contained much higher levels of a protective protein that binds to damaging heavy metals – like the radioactive caesium-173 in the ground near Chernobyl.

The plants have clearly protected themselves from Chernobyl’s radioactive soil, but we don’t yet understand how these changes help. Nor do we know why plants can seemingly shrug off deadly radiation while humans can’t. One theory is that plants ‘remember’ prehistoric conditions on Earth, when the atmosphere was full of harmful radiation. More research is needed, but if we can understand how plants respond to radiation, scientists could start to modify crops to withstand – or even remove – nuclear contamination.

Lead image:

Sisymbrium irio, also known as the London candelabra or the London rocket.

Anne Reeves/Flickr CC BY NC ND

References

About this resource

This resource was first published in ‘Plants’ in May 2016.

Topics:
Ecology and environment, History
Issue:
Plants
Education levels:
11–14, 14–16, 16–19, Continuing professional development