The notion of phototropism is used, in the field of biology, to refer to the tropism that generates the light. Tropism, for its part, is the movement made 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 vegetable 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 the light, It’s about a negative phototropism.
The estate, for example, they have negative phototropism: they grow away from light. The stems, on the contrary, show a positive phototropism because they develop towards the light source.
It is important to mention that plants have photoreceptors, What are they molecules that perceive light. In photoreceptors there is a pigment called chromophore, which absorbs light and generates changes in proteins, responding to light stimuli. Phototropism assumes a directional response, either towards the light source or in the opposite direction.
The phototropins they are the most important photoreceptors. They have a protein bound to the chromophore which, by absorbing light, can act on the activity of other proteins. The different mode of activation of phototropins makes the auxin, a plant hormone, is directed unevenly to the different sectors of the plant. As auxin promotes cell development, plant growth varies according to the incidence of light due to the phenomenon of phototropism.
Auxins are a set of hormones which are responsible for regulating the growth of plants, in particular the elongation of their cells. Their synthesis takes place at the apex of the stem and from there they are transferred to other parts of the plant, especially towards its base, where they are highly 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 the development of plants 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 his discovery. In the year 1871, for example, Charles Darwin He 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.
That the aerial parts of the plants such as the stems grow towards the light due to the positive phototropism is key since this response favors the development of the photosynthesis: 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 first presents a positive phototropism but the second, a negative one.
While 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 as it allows them to cling to a solid object and grow around it; the gravitropism, a growth proportional to the acceleration of gravity, necessary for the roots that must penetrate the ground to function properly.
All these concepts were studied by the aforementioned scientists, throughout the many experiments they carried out to study the developing of the plants. One of them consisted of covering the coleoptiles (leaves that close over others) to observe the behavior of the Phalaris canariensis in front of the light in these conditions: the result was that it did not bend, that is, that it did not perform phototropism.