Meter did not return any assign values

Tuesday 26 July 2016


Plants on red alert in the battle for light

Research reveals how plants can 'see' their competitors when fighting for light.

Plant in dappled sunlight
Light brigade: plants use highly refined strategies to put them ahead of their neighbours in the battle for the sun  Photo: Alamy

Take a look out of the window at the plants in your garden. Look peaceful, don't they? It's difficult to appreciate how nervous they really are, in fact deeply worried. And what are they worried about? Each other. Plants need light, and being deprived of light means fairly rapid death.

Almost the worst thing that can happen to a plant is to be overtopped by a neighbouring plant, and thus deprived of light, so plants that are used to plenty of light make strenuous efforts to make sure this doesn't happen. In fact, a sure sign that plants such as aspidistras are genuinely shade-tolerant is that in response to shade, they do nothing.

In contrast, plants that need lots of light respond very quickly to a shortage, growing as fast as they can towards the nearest available light, often becoming pale and spindly ("etiolated") as they do so.

But how do plants know if shade is about to become a problem? If they wait until shade really is a problem, it may be too late, because that means a competitor is already taller than they are. So plants have evolved to respond not to the quantity of light, but to its quality. Plants do not use the whole visible spectrum for photosynthesis, they mainly use red light, which is why they look green (if you take the red out of daylight, you are mostly left with green). However, the light just along the spectrum from red, but not quite infra-red, known as far-red, is not used at all.

Ordinary sunlight contains lots of red, but light reflected from leaves contains little red and thus relatively more far-red light, so the red/far-red ratio acts as an early-warning system for the detection of other plants. To make use of this property, all plants contain a chemical that is switched between two different forms by red and far-red light.

Thus plants are able to detect the presence of other plants nearby, long before they cast any serious shade, by measuring the spectrum of the light reflected from their leaves. In one classic experiment, purslane seedlings grew away from a strip of green plastic only 2cm high, far too small to cause any real shade, because the light reflected from it was low in red.

During evolution, light with the red taken out has proved to be a good guide to trouble ahead. To the extent that, faced with a choice between full-spectrum shade and shade with a high far-red/red ratio (ie light filtered through leaves), purslane continues to grow away from the green shade until you make the ordinary shade 50 times deeper than the green shade.

Light can't always be used to spot an encroaching neighbour. Before they flower, rosettes of the garden weed thale cress are basically flat (too flat to cast any shade or affect the red/far-red ratio), so what happens when they meet? New research in the journal Proceedings of the National Academy of Sciences (PNAS) shows that here touch is crucial. When the leaf edges touch they begin to grow up, away from the soil, in an effort to shade their neighbour. Touching any obstruction makes the leaves do this, and only the touching leaves grow upwards – the rest of the plant stays flat.

Although plants like to keep up with their neighbours, they don't necessarily want to grow much taller, because plants that stick up too far risk losing too much water or being damaged by wind. In an experiment reported in the journal New Phytologist, pots containing the weed fat hen were arranged in a block. If a pot was lowered below the general level, the plant in it grew faster, but if it was raised up, it slowed down, so after a while the tops of all the plants ended up at more or less the same level. Further experiments showed that the plants use light quality and touch to keep tabs on their neighbours, and only showed this "converging" behaviour if they could use both kinds of information.

The plants in your garden may not look like they're doing much, but they're constantly monitoring their place in the horticultural rat-race, and adjusting their behaviour to keep up with the Joneses.

Ken Thompson is a plant biologist with an interest in the science of gardening. He writes and lectures extensively and has written four gardening books, including 'Compost' and 'No Nettles Required'. His latest is 'Do We Need Pandas? The Uncomfortable Truth About Biodiversity'.

More from Telegraph Gardening

How we moderate
blog comments powered by Disqus

More from the web


More from the web

More from the web

Back to top

© Copyright of Telegraph Media Group Limited 2016

Terms and Conditions

Today's News


Style Book

Weather Forecast