What are plant tropisms?

What is tropisms? There growth responses that result in curvature of plant organs towards or away from stimuli due to differnt rates of elongation.

A plant’s shape is greatly influenced by its environment. The growth response that results in curvatures of plant organs towards or away from the stimuli is known as tropisms. The tropisms mechanism is the differential rate of elongation of cells on opposite side of the organ. There are three types of stimuli that include tropisms that change the shape of the body. These are light-phototropism, touch-thigmotropism, and gravity-gravitropism.

Phototropism is the growth of a plant shoot towards or away from the light. Most common theories have stated that grass grows towards the light, because the darker side of the stem cells would elongate faster than the light side because of a symmetrical distribution of auxin moving down from the tip. This theory has been disproved more and more by testing other organs. One example that is commonly seen is there is no evidence that unilateral light causes an asymmetrical distribution of auxin in the stem of sunflowers. Yet a growth inhibitor from a symmetrical distribution of certain substances, when the substances are in a heavier concentration on the lighted side of the stem.

The main conclusion from researchers is that the shoot tip triggers the growth response. Unlike the two theories of phototropism being a result form auxin stimulating cell elongation on the darker side of the stem or that the chemical messengers inhibit elongation on the lighter side. The photoreceptors are pigment molecules that are more sensitive to blue light. Yet it is most likely that the yellow pigment that is related to the vitamin riboflavin. Also the same receptor may be involved in the stomatal openings and some of the plant responses to light.

When you see vines or other climbing plants you can see the tendrils that coil around for support. Most of these organs can grow straight until they touch something. Then when contact happens this stimulates the coiling response, which is caused by differential growth of cells on opposite sides of the tendril. Thigmotropism is the direction growth that is a response to touch.

Much more directional growth can occur from mechanical stimulation. One experiment showed that plants manipulated by rubbing sticks a few times made the plants grow shorter and thicker than plants that have not been manipulated at all. This experiment can be seen in nature from the wind blowing plants; they end up thicker and shorter trying to hold on to the ground. This natural response to mechanical perturbation is called thigmomorphognests. This is usually a result from an increase of production in ethylene, which is created in response to chronic mechanical stimulation.

Gravitropism is a response to gravity. When a seed is placed on its side, it naturally adjusts its growth so the roots grow down and the shoots bend up. The roots display a positive gravitropism, while the shoots exhibit negative gravitropism. When a seed germinates the gravitropism functions, this ensures that no matter how the seed landed the root will go down and the shoot will go up. Some plants can tell up and down by the settling of their statoliths. Statoliths are specialized plastids that contain dense starch grains to the low points of the cell. These are located in certain cells on the root cap. There is a hypothesis that aggression of statoliths at the low points of the cells trigger the redistribution of calcium. This in turn causes the lateral transport of auxin within the root. On the lower side of the root’s zone for elongation is where the calcium and auxin have accumulated. These chemicals are moved by active transportation from one side to the other, because the chemicals have been dissolved. The cell elongation is inhibited when auxin is at a high concentration. This slows down the growth in the lower side of the root. When the elongation speeds up on the upper side the root curves as it grows. This tropism will continue to make the root grow straight down.

Written by Chelsea Palmer