How does sunlight affect plants?
Sunlight is essential for the life of plants and since only plants can use sunlight to produce oxygen, all life on earth ultimately depends on this property of plants. The process whereby plants use sunlight is called "photosynthesis". In this process the energy produced is used to produce sugars and also, as a by-product, oxygen. Specifically, the light energy causes carbon dioxide from the air to combine with water to produce sugars and oxygen in a series of reactions that may be simplified as:
6CO2 + 6H2O → C6H12O6 + 6O2 (where C6H12O6 represents glucose)
In actual fact, the main product is not glucose but three carbon (C3) or four carbon (C4) sugars which are converted directly to sucrose and starch which may be stored by the plant. These processes are called "carbon-fixation" as they attach carbon, from the carbon dioxide, to stable compounds making it available for use by the plant.
In plants, the sunlight is absorbed mainly by chlorophyll. Some light is also absorbed by carotenoids. Chlorophyll is responsible for the green color of plants whereas as carotenoids are red, yellow and orange (and usually are visible only in the fall when the chlorophyll has disappeared). The light absorbed produces the energy required for the photosynthetic process.
As gardeners, we know that different plant species require different amounts of sunlight to grow and flower. Thus, roses do not thrive in the shade whereas yews will grow quite well in a shady location. Too much light can also be harmful to plants. Plants will not survive in 24 hours of light, although the long days in the short arctic summer enable some plants to grow well in these regions even though the sunlight is not very strong. Many plants that grow in the intense sunlight of the tropics use a different mechanism to produce sugars (called the C4 pathway) from other plants (which use the C3 pathway). Many succulents, which also grow in strong sunlight, use another mechanism, known as the crassulacean acid metabolic (CAM) pathway to produce sugars.
Sunlight also affects the ability of plants to flower. Most of the plants that we grow in the garden require full sun to grow well, although, of course, there are many exceptions, e. g impatiens or begonias. The color of flowers, of course, depends on the variety of pigments in the flowers and their ability to absorb or reflect light of different wavelengths (including ultraviolet). In many plants the duration of the light determines their flowering schedule. Thus, so called “short-day” plants, e.g. chrysanthemums, require long nights before they will flower. Conversely, “long day plants”, e. g. cone flowers, need short nights to flower.
Another way that sunlight affects plants is by influencing the movement of their stems and leaves, such as when shoots tip towards the light source. This process is known as “phototropism” and is controlled mainly by the hormone auxin. A good example of this is a field of sunflowers with all their blooms facing the sun.
P.H. Raven, R.F Evert and S.E. Eichhorn. Biology of Plants. 7th Ed. W. H. Freeman and Co., New York, 2005. pp. 115-139.
Courtesy of the NYBG Plant Information Service