Etiolation occurs when plants are grown in either partial or complete absence of light, and is characterized by long, weak stems; smaller, sparser leaves due to longer internodes; and a pale yellow color (chlorosis). This is a mechanism to increase the likeliness that a plant will reach a light source, often from under leaf litter or underneath shade from competing plants. The growing tips are strongly attracted to light and will elongate towards it.

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 to make it ready to begin photosynthesis now that it is in sunlight.

Some of the changes that occur include:

  1. Inhibition of hypocotyl lengthening.
  2. Stimulation of cotyledon expansion.
  3. Opening of the apical hook.
  4. Stimulation of synthesis of anthocyanins.
  5. Stimulation of chloroplasts development from etioplasts.

This process is regulated by the exposure of various photoreceptors to light. Phytochrome A and phytochrome B both respond to an increasing propotion 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 reachs the surface.


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