Botanists have discovered that gently rubbing the foliage between a thumb and forefinger helps to activate plants’ defence mechanisms, helping to improve their ability to resist disease.
Many gardeners, including Prince Charles, already extol the benefits of talking to their plants as a way of encouraging them to grow.
While the scientific evidence for the benefits of this are inconclusive, a bit of physical contact does seem to help.
Scientists at Fribourg University in Switzerland stimulated the leaves of thale cress plants Arabidsopsis thaliana by rubbing them with a thumb and forefinger.
Within minutes they noticed biochemical changes began to occur, with defensive genes being activated and causing the plants to become resistant to Botrytis cinerea, the fungus that causes grey mould.
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The “soft mechanical stress” this causes on the foliage mimics what happens in nature due to the wind, rain and passing animals.
Indoor plants, however, are not exposed to these sources of movement, so house plants and those in greenhouses may benefit from a bit of soft touching, according to the research.
Dr Lehcen Benikhlef, a plant biologist at Fribourg University who led the study, said previous studies had shown that damaging leaves could induce immune responses, but the new findings demonstrated a more delicate approach was all that was needed to help make plants less prone to disease.
He said: “Wounding inflicted by clamping leaves with forceps or puncturing with a needle induces a strong immunity of A. thaliana to B. cinerea.
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“In this study, we have explored the effect of softer forms of mechanical stimulation on the resistance of A. thaliana to B. cinerea.
“In particular, we have observed that a gentle mechanical stimulus applied to the surface of the leaf induced a transient and localised resistance to B. cinerea.
“Plants are known to be equipped with a sensitive and discriminative sensory system for the detection of molecular patterns generated by pathogens.
“Here we show that a gentle mechanical stress can also be perceived in a differentiated way and lead to specific plant responses that include resistance against a virulent necrotrophic fungus.”
The findings are published in the journal BMC Plant Biology.
Thale cress is a widespread weed that often grows on walls and railway sidings.
The researchers found that rubbing the thale cress leaves triggered a host of internal changes that caused genes related to mechanical stress to be activated.
Levels of reactive oxygen molecules were also increased and the protective outer layer of the leaf became more permeable, perhaps to aid the escape of biologically active molecules.
The researchers said they believed the mechanical stress is detected by mechano-sensors in the leaves that initiate the immune response.
Other plants are known to respond more visibly to touch, such as the Venus fly trap, which folds its leaves around insects that touch hairs inside and the touch me not plant Mimosa pudica.
Most plants, however, have a more discrete response. It is thought that the ability allows plants to adapt to their surrounding environment.
Trees growing on windy shorelines, for example, sometimes respond by developing shorter, thicker trunks