Understanding Phototropism: How Plants Follow the Light

For plants, light radiation is essential for the performance of biological activities such as photosynthesis, growth, flowering and fruiting. In fact, plants are able to perceive the quality and quantity of the light they receive, to determine the length of the days (and therefore the seasons) and the direction from which the light comes.

The interaction that exists between light and plants comes from a mechanism called phototropism. This is nothing more than the movement made by plant organs in bending towards a light source. We can distinguish two types of phototropism: the first, called positive phototropism, leads to a bending of stems and stalks in the direction of the light, resulting in upward growth; and the second, on the other hand, is called negative phototropism and is found in roots that grow underground and away from the light source.

Light capture is determined by the presence of a series of photoreceptors that capture light particles by activating pigments. These pigments are essential for photosynthetic processes, regulation of growth and flowering, and phototropism. The most important pigments are: phytochromes, which are activated by receiving red light; cryptochromes, which are activated by blue light and ultraviolet type A (or UV-A) light; and phototropins, which are activated by blue light.

But how does phototropism work? It is generated by the reception of blue light by photoreceptors, which activate phototropins. The activation of these pigments causes the formation of a hormone called auxin. Auxin is responsible for cell growth and expansion and is activated at the point where light hits the stem or phloem of the plant. The auxin thus formed tends to move from the illuminated side to the shaded side, and it is on the latter side that the plant tends to bend toward the light, stretching upward.