Phototropism is a type of tropism.
The notion of phototropism is used, in the field of biology, to refer to the tropism generated by light . Tropism, for its part, is the movement carried out by a sessile organism (subject to a substrate) in response to a stimulus.
It can be said, therefore, that phototropism is a reaction of a plant to a light stimulus . When the plant grows towards the source of said light stimulus, we speak of positive phototropism . On the other hand, if the plant develops in the opposite direction to that of light , it is negative phototropism .
Roots , for example, have negative phototropism: they grow away from light. The stems , on the other hand, show positive phototropism because they develop towards the light source.
Photoreceptors and phototropism
It is important to mention that plants have photoreceptors , which are molecules that perceive light. In the photoreceptors there is a pigment called chromophore , which absorbs light and generates changes in proteins, responding to the light stimulus. Phototropism involves a directional response, either toward the light source or away from it.
Phototropins are the most important photoreceptors. They have a protein attached to the chromophore that, by absorbing light, can act on the activity of other proteins. The different mode of activation of phototropins causes auxin , a plant hormone, to be directed unequally to the different sectors of the plant. As auxin promotes cell development, plant growth varies depending on the incidence of light due to the phenomenon of phototropism.
Phototropism involves a reaction of a plant to a light stimulus.
The role of auxin
Auxins are a set of hormones that are responsible for regulating plant growth, particularly the elongation of their cells. Their synthesis takes place at the apex of the stem and from there they move to other parts of the plant, especially towards its base, where they are largely concentrated. It is thanks to the parenchyma of the vascular bundles that this displacement becomes possible.
Although the first formal description of auxins and their role in plant development corresponds to Frits Warmolt Went , a mycologist and botanist of Dutch origin, it was thanks to the work of the Polish scientist Theophil Ciesielski that their discovery took place. In 1871, for example, Charles Darwin relied on Ciesielski's doctoral thesis to speak of an "influence" that was transmitted from the end of the stem and that was responsible for a form of tropism.
Importance of phototropism and other tropisms
That the aerial parts of plants such as the stems grow towards the light due to positive phototropism is key since this response favors the development of photosynthesis : the leaves have better access to light energy. In fact, the stem grows towards the light while the root goes in the opposite direction and for this reason it can be said that the former presents a positive phototropism but the latter a negative one.
Although phototropism is one of the most obvious tropisms in plants, it is not the only one. The two most important besides this are the following: thigmotropism , which is of vital importance for climbing plants since it allows them to cling to a solid object and grow around it; gravitropism , a growth proportional to the acceleration of gravity, necessary for roots that must penetrate the soil to function properly.
All of these concepts were studied by the aforementioned scientists, throughout the many experiments they carried out to study the development of plants. One of them consisted of covering the coleoptiles (leaves that close on each other) to observe the behavior of Phalaris canariensis against light under these conditions: the result was that it did not bend, that is, it did not perform phototropism.