Etiolation (pronounced /iːtɪəˈleɪʃən/) is a process in flowering plants grown in partial or complete absence of light.[1] It is characterized by long, weak stems; smaller, sparser leaves due to longer internodes; and a pale yellow color (chlorosis). It increases the likelihood that a plant will reach a light source, often from under the soil, leaf litter, or shade from competing plants. The growing tips are strongly attracted to light and will elongate towards it. The pale color results from a lack of chlorophyll. The cause of the elongation is due to auxins. Auxins are not active in light; when they are active they stimulate proton pumps in the cell wall which increases the acidity of the cell wall and activates expansin (an enzyme that breaks bonds in the cell wall structure) that weaken the cell wall and allow the cell to expand.[2]
Chloroplasts that have not been exposed to light are called etioplasts (see also plastids).
De-etiolation, on the other hand, is a series of physiological and biochemical changes a plant shoot undergoes in response to sunlight. This process is also known informally as greening. The changes triggered in the plant shoot all occur in preparation for photosynthesis.[3]
Some of the changes that occur include:
This process is regulated by the exposure of various photoreceptor pigments to light. Phytochrome A and phytochrome B both respond to an increasing proportion of red light to far-red light which occurs when the shoot comes out into the open. Cryptochrome 1 responds to increasing amounts of blue light when the shoot reaches the surface.[4]