Thigmomorphogenesis — When I first saw this word, I thought it was a joke; but no, it’s a real response of plants to touch.
I’m not referring to the “touchy-feely” idea of hugging or caressing plants. Think instead of the active trapping mechanism of carnivorous plants or of the leaf response of the “sensitive plant” (Mimosa species). Some vine tendrils coil up when touched. However, thigmomorphogenesis refers to “more long-term changes in the appearance of a plant (‘morpho‘) in response to repeated touch (“thigmo“) (Chalker-Scott, Washington State University).
Dr. Chalker-Scott discussed this phenomenon in her article “The Myth of Stoic Trees.” She says thigmomorphogenesis “can be induced by many types of environmental mechanical perturbation (disturbance) including wind, water spray, snow load and rubbing from other plants.”
She points out that people, animals and insects may cause changes due to touch. Some of the plant changes that occur include shorter but thicker stems, shorter leaf stems (petioles), shorter needles, smaller leaves and fewer flowers. These changes can happen in annual crop plants such as beans, peas, corn or sunflowers and in many wood plants including pine, spruce, fir, poplar and elm.
When woody trees or shrubs are rubbed or brushed continually, they release ethylene gas, a plant growth regulator. This contributes to the increase of more lignin or wood on the plant. More lignin means a stockier sturdier plant that is “more resistant to breakage or windthrow” (Chalker-Scott) than an untouched plant. The short stocky trees you see at high elevations are excellent examples of wind-induced thigmomorphogenesis. Their stature makes them stronger and better able to survive the extreme conditions of their environment, than if they were thin and tall.
Thigmomorphogenesis has importance in our home landscapes in reference to how we plant and stake trees. When trees sway in the wind, rather than being tightly staked, trunk size and root growth are increased. Roots are more stable. This means a more firmly anchored tree. The thigmomorphogenesis of swaying trees also slows down crown growth. A greater root mass coupled with a smaller leaf mass means a more stable tree.
All of these factors make the compact tree more likely to survive wind stress and less likely to have the crown breakage and uprooting that a tall, thin tree might experience after being staked too tightly or too long.
Plants have amazing adaptations that allow them to fit better in their environments. Too bad we can’t use thigmomorphogenesis in scrabble!
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at email@example.com or 887-2252.