'Stunted' pot plants cannot reach full potential

MRI scans of potted sugarbeet and barley (c) Hendrik Poorter The MRI scans show how barley (left) and sugarbeet (right) plants arrange their roots

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Plants grown in pots never reach their full potential, images of their roots show.

A medical imaging technique called magnetic resonance imaging (MRI) has been used by researchers to capture plant pot root snapshots.

The pictures reveal that the roots "sense the size of the pot" and restrict the growth of the plant.

The findings have been presented at the Society for Experimental Biology's annual meeting in Salzburg, Austria.

House plant heaven

A flamingo lily flower
  • House plants, such as flamingo lilies and red-edged dracaena, are known to remove indoor pollutants that are contained in paint, detergents and synthetic furnishings
  • The light levels inside a house are similar to those under a tropical or sub-tropical forest canopy from where many house plants evolved
  • If a house plant has grown too large, it is possible to root prune plants using the bonsai method
  • Plants grown in pots that are too large are susceptible to root disease because of excess moisture retained in the soil

Lead researcher Hendrik Poorter, from the Julich research institute in Germany, told BBC Nature that as soon as he saw the results, he re-potted all of his houseplants.

"I thought, you poor guys, what have I done to you?" he recalled.

For the imaging study the research team focused on two species - sugarbeet and barley.

Dr Poorter's colleague Dagmar van Dusschoten produced the MRI scans. The technique, used widely in medicine, reveals the water molecules within the plant roots.

The resulting 3D map of the roots' structure stretching to the outer limits of the pot shows, for the first time, exactly how restricted potted plants are.

In their experiments on 80 different species, the team found that doubling a pot's size caused a plant to grow almost half as big again.

"The most surprising thing is that there seems to be no end to the pot limitation," explained Dr Poorter.

"For every plant species we looked at, pot size was the factor limiting its growth."

'Happy' plants

Within as little as two weeks of seeds being sown, the scientist explained, a plant's roots would stretch to the edge of the pot and then, "the trouble starts".

"When they reach the edge, they send some kind of signal to the shoots to say, 'there's a problem - stop growing'."

Each plant appeared to be trying to escape its pot; more than three quarters of the root system was in the outer half of the container.

"The inside of the pot is hardly used," explained Dr Poorter.

Research in this subject has, in the past, focused on pot size from the perspective of how small a container plants can be grown in, as the aim is to grow as many plants as possible per square metre in a commercial setting.

But Dr Poorter said: "We want to make plants as happy as possible."

Although this may sound sentimental, understanding a plant's full potential is crucial for the researchers that study them; it reveals how much of a parallel can be drawn between studies carried out in the lab and how plants would grow in nature.

"Even the largest pot was not large enough not to limit growth."

Prof Andrew Fleming from the University of Sheffield said the use of MRI was a "neat approach" to studying plants.

He told BBC Nature: "It shows how novel (live) imaging can be used to provide new insights into how plants actually grow."

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