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Etiolation is a condition caused by the growth of plants in the absence of light. It is characterized by a pale yellow coloring, sparse leaves, and weak, elongated stems. The stems of a plant grown in darkness grow longer and thinner in order to reach a potential light source.
When a plant grows in sunlight, the tips of its stems move towards the source of light in order to maximize photosynthesis, the conversion of carbon dioxide into organic compounds necessary to plant life using the energy of sunlight. A plant suffering from etiolation will produce thinner and longer stems than a healthy plant, with longer internodes, in order to reach a sparse light source or to find one when it does not exist. Its stems will also grow faster than those of a plant exposed to adequate sunlight. Since leaves grow at the internodes of a plant's stems, a plant suffering from etiolation will have less leaves than a normal plant.
Photosynthesis depends largely on chloroplasts, organelles present in plant cells. Light absorbing pigments in chloroplasts, including chlorophyll, produce a green color. Chloroplasts that have never been exposed to light remain immature and non-pigmented and are known as etioplasts.
Etioplasts are more similar to leuocoplasts, another non-pigmented plant cell, than they are to chloroplasts in a healthy plant. The presence of etioplasts instead of chloroplasts in a plant grown in darkness is responsible for chlorosis, or pale yellow color, one of the major symptoms of etiolation. Chlorosis occurs when a plant lacks the necessary nutrients to synthesize chlorophyll.
When a plant suffering from etiolation is exposed to sunlight, a process known as de-etiolation occurs. In de-etiolation, a plant begins producing chloroplasts, becomes greener in color, and produces fuller and more plentiful leaves. Over time, the internodes of the stem will become a normal length. Anthocyanins, plant pigments responsible for blue, violet, and red shades, will also begin to develop in a plant undergoing de-etiolation. In addition, the plant's seeds will undergo changes contributing to the plant's fertility.