Why Plants Need Light
Electromagnetic energy, particularly in the red and blue visible light wavelengths, is readily absorbed by specific photoreceptive proteins in plant cells to use in chemically converting carbon dioxide into sugars, a process called photosynthesis. Besides its role in creating food for the plant, light also stimulates other important functions like forming flowers, bending toward light, germinating (in some species) and growing foliage.
Wavelengths of light that are visible as being blue in color best facilitate foliar growth. Red light facilitates flowering and the ripening of fruit. In the spring, as the Earth begins tilting closer to the sun, more blue light shines into the atmosphere. As the summer progresses, more red light penetrates to the ground, naturally giving plants exactly what they need to fully mature. In short, all plants need at least some light to survive and thrive.
Why Plants Need Darkness
Plants need periods of darkness, too; the amount varies by plant species. Photoperiodism describes how specific types of plants react to or even require specific day and night lengths to trigger biological functions. Some plants require short days and longer nights in order to flower. Chrysanthemums, poinsettias and strawberries are examples of short-day plants. Other plants have opposite needs; they depend on fourteen or more hours of daylight before they can be induced to flower. Long-day plants in this category include carnations, oats and clover. While some plants require very specific periods of light and darkness to trigger hormonal activity or gene expression, others can still function without these optimal conditions, just not as readily. Photoperiodism in plants, particularly in those that are most viable from a commercial standpoint, is a subject of intense study in universities and agricultural laboratories.
Darkness also stimulates growth and germination in some types of vegetation. In most cases, the absence of light, at least at night, is not total; photosynthesis can still take place even in moonlight and starlight, though at a much slower rate.
Plants Without Light
Some plants will germinate without light and, using the stored food from their seeds, will produce white sprouts. If they can't find light, chlorophyll to process photosynthesis will not form, so the seedlings will remain white. When the sprouts run out of reserve energy, they'll wither and die.
Sure signs that plants aren't receiving sufficient light include "legginess," that is, long segments of stem between leaves, extreme bending of the stalk toward the light source and yellow or light green leaves due to the lack of chlorophyll formed in them. Most "full sun" plants need a minimum of six to eight hours of direct sunlight.