Why Plants Bend Towards Light

Plants have a tendency to bend towards light. This phenomenon is known as phototropism. Generally, the upper portion of the plant i.e. the stem and the leaves follow the direction of sunlight. This phenomenon is explained as the activity that helps the plants receive light energy required for the process of photosynthesis. Photosynthesis is the process by which the plants produce their food in the presence of water, sunlight and oxygen. Plants require sunlight not only for photosynthesis, but also for sensing the seasonal changes in the climate. They detect the direction of light by transforming the light signal into a chemical signal. This is followed by the execution of the photo response, which involves a series of multiple steps performed by the coordinated tissues and organs. This phenomenon about how the plant detects the direction of light is yet to be fully discovered and explained by the scientists.

 

The phenomenon of phototropism in the plants was first investigated by Charles Darwin. Darwin used grass and green seedling to explain this phenomenon. He proved that the cells present in the darker side of the plant, which received no light, elongate. This expansion of cells caused the seedling to bend in the direction of the light source. The theory was expanded and investigated further to form the modern explanation for the phenomenon. Scientific research in this field has resulted in the discovery of the class plant hormones, known as auxin (also called indolacetic acid). This hormone is found to be responsible for phototropism.

 

It has been found out 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.