Germination and Seedling Development

• When germination begins, the root emerges first, holding the seed in place so the stem and primitive leaves can unfurl and push toward the soil surface. Plants require light in order to manufacture energy for themselves; although the seed contains a food source from the mother plant, this food will last only a short time. Once the seedling reaches the surface, if exposed to light, it develops chlorophyll, turns green and begins photosynthesis, the process of turning light energy into sugar. Most of its growth is devoted to leaf development so it can maximize energy production from light. The stems between the leaves remain relatively short.
  
  

Biochemistry of Etiolation

• Etiolated plants, on the other hand, are pale in color, with long stems and small leaves. Seedlings naturally produce proteins called phytochrome-interacting factors, or PIFs. In the dark, these PIFs lock into the plant's genes, turning on a gene that causes the plant's cells to elongate, giving its stems the stretched appearance typical of etiolation. In the light, however, something else happens. Plants still produce PIFs, but they also produce a second protein called a DELLA protein that is triggered by exposure to light. DELLA proteins combine with PIFs, changing the structure of the PIF protein so that it will no longer be able to switch on the gene that causes cell elongation. Cells remain normal-sized, and etiolation does not occur.
  
  

Function of Etiolation

• Because the seed provides only a limited food source, seedlings need to move into the light and begin photosynthesis as quickly as possible. Etiolation helps them to do that by producing added length that will, ideally, move the plant out of a shady patch and into the sunlight. Once it's exposed to light, etiolation stops, and the plant grows normally. Etiolated plants that never reach a light source eventually die.
  
  

Shade Avoidance Effect

• Plants grown under the shade of other plants receive primarily light that falls on the red end of the light spectrum. This wavelength of light is not strong enough for plants to use to produce energy through photosynthesis. Plants growing in such a situation, therefore, benefit from growing in a direction that moves them toward a brighter light source. Similar to etiolation, plants grown in the shade will elongate and grow in the direction of a brighter light source. A plant hormone called auxin concentrates on the dark side of a plant's stem, speeding up growth on that side and causing the plant to bend and grow rapidly in the direction of a light source.