In order to truly thrive, plants need three things: minerals, water and sunlight. But the amount of sunlight each plant needs varies widely. Some plants need direct sun for up to 10 hours in order to do well. Others would prefer to live in the shade of taller plants and receive no direct sunlight at all. But no plant can survive in total darkness. That's because light controls the essential processes that go on inside each and every plant.
The biggest need that plants have for sunlight is to provide the energy for photosynthesis. Photosynthesis is how plants create sugar from carbon dioxide and water. The sugar that plants create during photosynthesis is converted to adenosine triphosphate (ATP), which is the fuel that every living being uses to power its cells. The sunlight is absorbed into the plant through a green pigment called chlorophyll. Chlorophyll absorbs all light along the color spectrum except for green, which is useless to them. This is why plants appear green when we look at them.
Plants are not able to absorb a significant quantity of sunlight for photosynthesis. Most plants absorb less than 5 percent of the incident light energy that they are exposed to. This may be why trees produce such a large quantity of leaves in the summer, when the plant is most active. Plants that utilize more incident light are classified among the varieties that prefer to be planted in full sunlight. Plants that use less light are grouped among shade-tolerant plants.
When a plant is placed in a low light situation, it will often grow so that it appears to bend toward the nearest strong light source. If a plant is placed in a situation where it has no available light source, it will neglect the development of shoots and leaves, and instead put all of its' energy into developing roots. Once the roots are well established, they can collect nutrients from the ground and put them into developing shoots quickly to search for sunlight. This phenomenon, which is known as etiolation, is most obvious in bulbs and tubers such as potatoes.
Light is also essential in assisting a plant in developing pigments such as Anthocyanins. These anthocyanins are responsible for the red color in some apples, and in the tips of maize kernels. Apples that are shaded from the sun by leaves will show an afterimage of the leaf pattern in a lighter shade. Some fruit growers have taken advantage of this by sticking a sticker onto the surface of an apple to prevent the development of pigment under the sticker. When the sticker is later removed, the apple contains a reverse image of the sticker logo on its skin.
Some seeds require sunlight to begin germination. These seeds are typically planted at a shallow depth, or pressed into the surface of the soil rather than placed in furrows. These seeds are referred to as being positively photoblastic. Many self-seeding weeds exhibit this phenomena.