There are a number of plants that grow towards sun. This activity of plants, to bend towards light, is known as phototropism. Usually, the upper portion of the plant i.e. the stem and the leaves look for a sunny spot and try to reach out. For instance, sunflower follows the path of the sun across the sky. This characteristic of the plant enables its leaves to receive the light energy required for the process of photosynthesis. This is an extremely important process for the plants to grow and reproduce.
Scientific research in this field has resulted in the discovery of the class plant hormones, known as auxins. It was found that during the early stage of growth and development, plants demonstrate the phenomena of phototropism. The auxin hormone is responsible for expelling H+ ions, which decreases pH in the cells on the dark side of the plants. This is followed by the acidification of the cell walls, thereby activating an enzyme known as expansins. This breaks the bonds in the cell wall structure, making the walls less rigid.
The acidic environment causes disruption of hydrogen bonds in the cellulose. This leads to the decrease in cell wall strength, causing the cells to swell, exerting mechanical pressure that drives phototropic movement. Other thing worth noticing is that different plant organs exhibit different phototropic reactions, which also depends upon the wavelength of light. For sunlight, plant’s leaves and stem, grow towards it, while the roots grow away from it.
