Negative phototropism is the tendency for certain parts of plants to grow away from light. Roots often demonstrate this characteristic, causing them to grow deeper into the soil so that they are able to tap into the moisture and nutrients that support the plant.
Plant parts respond with both positive and negative phototropism. Just as the roots grow away from light, leaves grow toward it. Leaves display positive phototropism. If nature disrupts the orientation of the plant, these characteristic helps it to re-establish itself, within certain limits, in a way that allows the leaves to collect the most sunlight for photosynthesis and the roots to continue to provide the needs of the plant, increasing the chances of survival.
Plants respond to light using molecules within the plant cells. These receptors respond to blue light changing the direction of plant growth through a series of steps. The production of the hormone auxin takes part in the steps. In positive phototropism, auxin moves to the darker side of the plant stem and cause the cells there to elongate, turning the stem toward the light.
Gravitropism also affects plant growth. Most plant roots grow toward gravity with positive gravitropism while their leaves grow away from gravity with negative gravitropism.