Why is light a stimulus

The senses of the plants

To the sun, to the light

House plants on the windowsill grow towards the light. If you turn the plant, it orients its growth until its leaves turn back to the window and thus to the light.

This growth movement caused by a light stimulus is called phototropism. The plant perceives the light with the help of a light receptor in the tip of the shoot, i.e. where the growth is most intense.

This light receptor is the so-called phototropin, which is extremely sensitive to the blue components of the light.

If the sun is only shining on the plant from one side, the phototropin triggers a series of biochemical processes that cause growth to be inhibited on the exposed side and promoted on the shaded side.

The plant grows faster on the shaded side because the cells here stretch more than on the exposed side. This causes it to curve in the direction of the light source. It is believed that an increase in the concentration of the plant hormone auxin plays a role here.

This principle also applies when plants have the same genetic makeup. The Tübingen plant researcher Katja Tielbörger exposed cloned crawling cinquefoil plants to different light stimuli. The result: Despite having the same genes, the plants looked completely different from their clones after three weeks.

Against gravity

If you lay a seedling flat on its side, the root curves down and the shoot straightens up. This growth behavior is a reaction of the plants to the force of gravity and is known as gravitropism.

This plant sense of gravity ensures that the root grows into the soil and the sprout reaches the sunlight, regardless of the orientation of the seed in the soil.

In addition, the ability to react gravitropically makes it possible for the plants to grow vertically upwards even on inclined planes, for example on steep slopes.

Because the sprout grows against the gravitational pull, one speaks here of negative gravitropism. At the root, which follows gravity as it grows, of positive gravitropism.

The peanut, for example, undergoes a change from negative to positive gravitropism during its development.

Your flower stalks are initially upright, so are negative gravitropic and then turn 180 degrees after fertilization, so become positive gravitropic and grow downwards. The flower is pushed into the ground to ripen the fruit.

And how does the plant perceive gravity? The Bonn-based plant researchers Dieter Volkmann and Frantisek Baluska found small stones in the cells, so-called statoliths, in the roots of the candelabrum algae under the microscope. Thanks to gravity, the statoliths are always deposited "below", which the plant apparently registers.

Fast reaction time

The speed with which the stimulus is transmitted in some plants is remarkable. The best-known example is certainly the mimosa, in particular the species "Mimosa pudica".

The same reaction always follows on shock, touch or burn: The small leaflets of the leaf fold up and the entire leaf folds down. Next leaf after leaf, until all leaves hang down.

The signal of the stimulus moves at a speed of about one centimeter per second through the shoot of the mimosa. It is believed that chemical messengers play a role here. In addition, electrons could also be used to determine electrical impulses, so-called action potentials, which we also know from the animal kingdom.

Mechanical stimuli can also produce more general reactions in plants. The wind is such a mechanical stimulus. A tree in a very windy location is more compact than a specimen of the same species in a relatively sheltered location. Plants react to wind with a more robust growth habit.

Hearing and smelling

Plant neurobiologist Professor Stefano Mancuso from the University of Florence is convinced that plants can also hear. The experiment showed that under laboratory conditions the roots of maize plants grew at a right angle to the sound source for low tones, while they moved away from the sound source for high tones. It seems that the roots can perceive tones in the form of sound waves and also distinguish their pitch.

The seedling of the devil's twine seems to have a sense of smell. The parasite needs a host plant such as tomatoes to survive. The seedling can evidently identify their specific fragrances and tries to grow from the seed to the host plant with its scarce energy store.

When researchers put a glass over a tomato plant, the seedling left the tomato plant on the left. Apparently he couldn't find her without fragrance.

Plants can learn

Are Plants Smart? To find out, the Tübingen plant researcher Katja Tielbörger placed a mimosa in a dark test chamber. Whenever the researcher switched on the light, she pricked the mimosa with a toothpick. As a result, the mimosa closed its leaves. After a certain time, the mimosa would close its leaves even if Katja Tielbörger only switched on the light. The plant had evidently associated the light stimulus with the pique.