Plants subjected to thigmomorphogenesis, or physical disturbances such as wind and touch, generally respond through reduction in the rate of stem elongation and shoot height, and they increase in stem diameter. All of these features result in the formation of short, stocky plants.
This response is purely adaptive and allows individual plants to compensate for the different levels of stress that occur in their natural environment. The advantage of this is that shorter and stronger plants are less easily damaged by natural mechanical stresses (especially wind) than their taller, more slender counterparts.
Inhibitory effects of mechanical stress on flowering of a few species have also been observed. Thigmomorphogenesis is common and may be as important to plants as their responses to light, temperature, water, and gravity.
The indoor plant environment (greenhouses, sheds, and homes) influences the plants’ hardiness with regard to mechanical stimuli. Unless deliberately altered, the indoor space is typically a calm, windless environment.
Moving air causes a plant to lose moisture faster; in contrast, a windless environment encourages the development of a thin cuticle (waxy layer on outer epidermal walls, such as leaf surfaces).
The absence of physical disturbance also promotes formation of long cells with thin cell walls. These modifications result in the development of slender stems, which are not adapted to the buffeting provided by the wind outside.
Tall plants usually occur under indoor conditions where there is a relatively low light level and fewphysical disturbances. Thesemorphologies are simply a response to the environment in which the plants are grown and, unless conditions change, these plants are well adapted to such an environment.
Problems occur once the plants are transplanted outside because of the dramatic change in environment. On the other hand, plants in the wild are hardened by the wind, bright sunlight, lack of nutrients, and fluctuations in soil moisture and therefore show little further response to mechanical stress.
Response in Nature
|Withstanding high levels of wind disturbances by modifying their structures|
Wind-tolerant and wind-intolerant genotypes exist within any population of plants. The plants that are capable of withstanding high levels of physical disturbances will respond by modifying their structures. They respond by growing more slowly and changing theway they build their cells.
The cells that are produced are short and thick-walled, thereby making for short, sturdy plants. The decrease in elasticity also provides plants with a means of absorbing the strain within their structures.
In trees, the response is usually increased taper, which is the result of a reduction in height or increase in radial growth. Increased radial growth confers bending stiffness and maintains a tree’s vertical orientation in windy environments.
Location of Response
It has been shown that plants’ response to mechanical stimuli is localized and not whole-plant-based, as was previously thought. The more highly stressed areas of the plants, such as the base of the stem, are the areas that exhibit the greatest response.
Most information available deals with the effects on aerial components of plants. Knowledge of the effects on roots is quite limited. To examine this, researchers have compared corn and sunflower plants that have been flexed with still ones.
It was found that both species were able to change the morphologies and mechanical properties of their roots in response to wind. Furthermore, mechanically stimulated plants were found to have more numerous, thicker, and stiffer roots than still plants.
Perception of Stimuli
|Giving plants some mechanical stimuli by rubber strips|
to encourage growth and remove pests.
How do plants perceive mechanical stimuli? Calcium ions have been implicated in mediating various growth responses including thigmotropism and thigmonasty (discussed below).
Calcium may also be involved in thigmomorphogenesis. Differentially expressed genes have been isolated and are being identified to increase knowledge of the molecular and physiological responses of plants to increased mechanical stimuli.
In Arabidopsis, mechanical stimuli appear to induce the expression of certain genes that encode proteins related to calmodulin, a calcium-binding protein. Calcium levels have also been reported to increase in stressed cells.
Deformation of stressed cells may result in the opening of calcium channels in the cell membrane. Ethylene, a gaseous plant growth regulator, has also been implicated because mechanical stimulation results in increased ethylene production.
Thigmomorphogenesis is especially important for vegetable producers who use automated crop transplanters in the field and where robustness of the seedlings is important.
Scientists are aware that plants grown in greenhouses tend to be thin and limp as a result of high temperatures, low light levels, abundance of nutrients, and low wind speeds. Automated machinery easily damages these kinds of plants.
To counter this problem, commercial agriculturists often resort to the use of chemical growth regulators, high concentrations of fertilizers, salts, and water-absorbent gels. They also reduce the amounts of nitrogen and phosphorus available to the seedlings.
By a simple technique of brushing or stroking plants several times a day, these problems may not only be overcome, but crop quality may also be significantly improved. Several kinds of plants have been shown to respond very well to mechanical stimulation.
The technique of brushing or stroking seedlings has been shown to work on a variety of vegetable plants, such as cabbage, lettuce, tomatoes, cucumbers, and bedding plants including petunias, fuchsias, marigolds, and salvia.
The effect of thigmomorphogenesis can also be used when planting trees. When short stakes are used instead of a long ones, the unsupported part of the stem can flex, making it stronger and tougher.
Other Plant Responses
Thigmomorphogenesis may be confused with thigmotropism and thigmonasty. In thigmotropism, the plant responds directly to the direction of the source of the stimulus. For example, contact of tendrils stimulates the coiling response caused by differential growth of cells on opposite sides of the tendril.
In thigmonasty, the response is unrelated to the direction of the source of stimulus. An example of thigmonasty is the movement of the leaves of sensitive plants due to the rapid change in turgor pressure in specific cells at the base of leaflets.
Thigmomorphogenesis is related to thigmonasty because the response is also not in the direction of the stimulus. However, thigmomorphogenesis involves alteration of growth pattern and is irreversible.