JoAnne Skelly: Thigmomorphogenesis – responding to outside influences

Trees have been stressed repeatedly since October with storm after storm, high winds, pelting rain, snow, and hail. Their roots have been flooded. Does this affect how they grow or change their physical makeup over time?

Some wind is good for trees. The swaying motion increases the girth of trunks and improves their taper allowing them to be bigger at the bottom while narrowing toward the top. This is an important strength feature in ornamental trees. Wind stress also can keep crown growth contained while increasing root production and stability.

This means trees are more firmly anchored in the soil with less crown wind resistance. When trees cannot move with wind, particularly when staked too tightly or for too long, they grow tall and thin with lots of leaves on top. This type of growth in trees makes them more likely to break or uproot when the stake is removed.

Trees adapt to outside environmental mechanical impacts. This is called thigmomorphogenesis, a response to touching. Not only does the wind, snow, ice, etc., “touch” the plant, insects, people, and animals may also “touch” plants. Insects may bore, causing trees to grow defensive tissues to resist.

Deer scrape their antlers on trunks. Bears and porcupines scratch their claws. Many animals rub up against a trunk. These mechanical impacts actually can change how a tree grows and forms.

Swaying causes girth and taper, which are thigmomorphogenic changes in response to wind.  Poor staking inhibits that change. Thigmomorphogenesis is an actual physiological (internal physical and chemical) and morphological (external form and structure) adaptation. The greater the disturbance, the greater the response.

Other thigmomorphogenic changes include decreased stem elongation and increased stem thickness, shorter leaf stem length, decreased needle elongation, smaller leaves, reduced flower number, and increased tissue death. Tree height can be reduced, while width can be increased (L. Chalker-Scott, Washington State University, https://s3.wp.wsu.edu/uploads/sites/403/2015/03/thigmomorphogenesis.pdf)

Wind can also influence photosynthesis rates. Think of the stunted shapes of high elevation alpine trees. Thigmotropism is a different plant reaction. It is a directional movement by the plant in response to an outside stimulus. Tendrils on vines will grow toward an object they touch. Stems may grow toward the sun. These are positive thigmotropic responses roots growing away something in the soil is a negative response.

Whether changing physiologically or moving in response to stimuli, plants respond to outside influences. I find these adaptive responses amazing in things that cannot actually move from place to place.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email skellyj@unr.edu.