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Date: 25 May 2018
Cereal grains use their awns to "swim" into the soil  

Topic Name: Cereal grains use their awns to "swim" into the soil
Category: Nanobiotechnology
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Research persons: Rivka Elbaum, Liron Zaltzman, Ingo Burgert, Peter Fratzl

Location: Wissenschaftspark Golm, Am Mühlenberg 1,14476 Golm,Munich, Germany


Cereal grains use their awns to

A grain of wild wheat has everything required for plant
propagation - even tools for drilling into the soil. It uses its two awns for
this: in the dry daytime air, these bristles bend outwards. At night, dampened
by the dew, they straighten. Over several days, this movement, similar to the
swimming strokes of a frog, pushes the grain into the soil. This discovery was
made by scientists from the Max Planck Institute of Colloids and Interfaces. The
fine, barb-like silica hairs on the outside of the awn ensure that the seed can
only move downwards. A similar mechanism could use fluctuating humidity levels
to drive micromachines
The awns of the wild wheat are both steering mechanism and
engine at the same time. They guide a ripe grain to the earth with the pointed
end downwards by providing it with the correct balance as it falls. Once the
grain is sticking in the earth, the two bristles transform themselves into a
drill and drive the grain into the tilth. This action is powered solely by the
air which in the habitat of the wild wheat plant is dry during the day and damp
during the night. However, domesticated wheat has lost the ability to perform
this trick

During the day when it is dry, the two awns bend outwards; in the dampness of
the night, they bend towards each other. This is because the cap of the awn, the
side facing towards the other awn, reacts to humidity in a different way from
the side facing outwards, the ridge. This is due to the construction of its
cellulose fibres, which biologists call fibrils. In the cap, the cellulose
fibrils are all parallel to the awn. In the lower section of the ridge of the
awn, they are arranged randomly. That not only makes the ridge ten times as
rigid as the cap, but also makes the awn into a simple drill. Under damp
conditions, all the fibrils swell widthways. This means however that the awn cap
only swells on the side where all the fibres are arranged lengthways. The ridge
of the awn on the other hand stretches as some of the fibres are also at right
angles to the bristle, thus making the whole awn stand up.

Rivka Elbaum, a scientist who was involved in the project and who is a Humboldt
fellow at the Max Planck Institute of Colloids and Interfaces explains:
"The mechanism is similar to that with which pine cones open. The central
area of the ridge functions like a muscle, bending and straightening the
awns." However, on its own, the muscle is not sufficient to allow the
grains to burrow into the soil. That only happens with the help of the fine
silica hairs on their outer side. The hairs act as barbs which we can also feel.
If you run your finger along the awn away from the grain, the awn feels smooth;
towards the grain, the barbs offer tangible resistance.

These tiny silica hairs prevent the awns from pushing themselves out of the soil
when the bristles straighten during the night. They can only move into the earth
and thus push the grain a little deeper every night. The scientists discovered
this by wrapping a grain of wheat and the lower section of its awns in a cloth.
The silica hairs caught on the cloth. When the researchers alternately raised
and lowered the humidity, the grain moved a little deeper into the cloth with
every cycle.

"Wild wheat uses this mechanism to disperse itself," says Professor
Peter Fratzl, Director at the Max Planck Institute in Potsdam and Head of the
Research Group. The seed uses its swimming movements to propel itself across the
ground, as well as into the soil. "We have already built simple machines
and muscles modelled on the awn mechanism to convert variations in humidity to
movement." Fratzl sees this as a potential contribution to the use of
renewable energy. "I’m fascinated by the possibility of converting solar
energy to movement in this way." After all, it is the heat of the sun that
dries the air which the dew dampens during the night.

About Researchers:
Prof. Peter Fratzl (Director, Department of Biomaterials)
Planck Institute of Colloids and Interfaces
, Potsdam
Tel.: +49 331 567-9401
Fax: +49 331 567-9402
E-mail: Peter.Fratzl@mpikg-golm.mpg.de

Dr. Ingo Burgert

Max-Planck Institute of Colloids and Interfaces
Department of Biomaterials
14424 Potsdam, Germany
phone: +49-331-567-9432
fax: +49-331-567-9402
email: Ingo.Burgert@mpikg.mpg.de
room 1.213

Rivka Elbaum

Wissenschaftspark Golm, Am Mühlenberg 1
14476 Golm,Tel: +49 (331) 567 - 90,Fax: +49 (331) 567 - 9102

Planck Society,

In The Images:
1. Prof. Peter Fratz
2. A seed drill: I The seed and part of the
awn in the soil (the red arrow is pointing to a silica hair). II When humidity
rises during the night, the awns become erect and push the grain into the soil,
because the hairs prevent any movement out of the soil. III As the airs dries
the next day, the awns bend apart again. This tensions the drill that will push
the seed further into the ground during the following night.

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