Plants need light to make food, as in the case with autotrophic green plants, which are able to manufacture their own food from water, carbon dioxide, sunlight and inorganic molecules. Plants need to be in areas with available light sources, whether from direct or indirect, natural or man-made means. The four major aspects of plants affected by light are photosynthesis, chlorophyll synthesis, photoperiod and phototropism.
Photosynthesis is the process whereby plants convert light, air and water into carbohydrates and oxygen. Chlorophyll is the substance that performs photosynthesis; it absorbs blue (short wavelength) and red (long wavelength) lights. Some plants perform better when exposed to either more blue light, like that put out by fluorescent bulbs, or more red light, like that from incandescent bulbs, according to Sylvania's "Technical Information Bulletin: Light and Plants."
Plants also need energy to produce chlorophyll itself. Chlorophyll synthesis builds the plant cells that perform photosynthesis; however, like photosynthesis, chlorophyll synthesis occurs whenever the plant has enough light.
Chlorophyll serves two primary functions--absorption and transfer of light. Found in high concentrations in chloroplasts of plant cells, chlorophyll gives leaves their green color and absorbs light used in photosynthesis. Upon absorption of light, chlorophyll transfers that light energy to a specific chlorophyll pair located in the reaction center of the photosystems. The chlorophyll's selectivity regarding the wavelength of light it absorbs makes the leaf containing the molecule appears green.
Photoperiod is the plant's sensitivity to the length of day. Plants need several hours of darkness each day in order to perform other activities such as flowering. Darkness helps winter plants, also known as long-night plants, such as Christmas cactus, poinsettias, gardenias and chrysanthemums to bloom. These plants need a maximum of 10 to 13 hours of light per day to flower, with the remainder of the day spent in the dark. Even a minute of light at night is enough to prevent these plants from blooming.
Short-night plants, on the other hand, are plants that bloom in summer, where nights are shorter than days. These plants need 14 to 18 hours of light per day to flower. Examples of short-night plants include carnations and nasturtiums. During the early stages (up to first two months), plants need the opposite photoperiod; young long-night plants should have long days to encourage full growth before blooming, while young short night plants should have short days.
Night-neutral plants bloom regardless of the length of photoperiod. These include roses, carnations, geraniums and many household plants.
Phototropism is the ability of plants to move toward (or sometimes away from) a light source. It can cause plants to lean or grow unevenly when the light is uneven or one-sided. Phototropic responses depend on certain wavelengths of light just like in photosynthesis and chlorophyll synthesis.