Etiolation (ee-tee-oh-lation) is a phenomenon that occurs in plants that grow in limited or absent light. This triggers the elongation of cells at the growth tip to help the plant reach a potential light source (skotomorphogenesis). The word etiolate is derived from the French word étiolé, which means to blanche, referring to the pale colour of the plant. Have you ever lifted an object from the ground to find white plants beneath it? This is a perfect example of etiolated plants.
Auxins are plant hormones responsible for etiolation which causes the cells at the tip of the plant to elongate. Auxin is transported down the tip of the growing plant. Proton pumps in the cell walls are stimulated by auxins, which increases the acidity of the cell wall, resulting in expansin, cell wall proteins that allow them to expand.
Light, water and carbon dioxide are essential for photosynthesis, allowing the plant to create oxygen and energy in the form of sugar. If any of these are not present, the plant will not survive for long. Plants have evolved evolutionary adaptations to increase their chance of survival.
When is etiolation normal?
Skotomorphogenesis is a type of growth that occurs in seeds planted beneath the soil. These seedlings rely on limited energy stores within the seed, therefore it is critical the shoot emerges out of the soil as quickly as possible by hypocotyl elongation, to reach a light source. You may have noticed some newly emerged seedlings are often pale. Once the seedling perceives light, hypocotyl elongation is arrested and photomorphogenic (light-mediated) development begins. This entails the opening and growth of petioles (the stalk that supports a leaf and attaches it to the stem) and cotyledons (the first leaves of a plant), root elongation and the differentiation of proplastids into chloroplasts. This is known as de-etiolation, or greening.
If the emerged seedling is in poor light, etiolation continues, producing the characteristic long-stemmed, pale appearance. In etiolated seedlings, proplastids (an organelle found in the meristematic regions of the plant) develop into etioplasts (an intermediate type of plastid that develop from proplastids that have not been exposed to light) instead of chloroplasts (an organelle that conducts photosynthesis).
Signs of etiolation in plants
Etiolation can happen if seedlings are raised indoors, houseplants that don’t have an adequate light source or sun-loving outdoor plants that have been planted in the shade.
- A seedling or plant with uncharacteristically long and thin internodes (the space between nodes), commonly referred to as ‘legginess‘. The long and fragile stem means that they are more likely to fall over.
- Fewer, smaller and cream/yellow leaves that would ordinarily be green.
- Rosette-shaped succulents such as Echeveria begin to grow downwards to increase the surface area of the leaves.
- Plants that lean towards a light source.
Some fruit and vegetables are intentionally etiolated to prevent bitterness. Belgian endive, champagne rhubarb, seakale, white asparagus and celery are deliberately grown in the dark to prevent the development of chlorophyll which has a somewhat bitter taste and prevents the development of fibre, which produces a more tender plant.
How to prevent etiolation
Seedlings can recover from etiolation, as long as they are provided more light. Unfortunately, etiolation in mature plants is irreversible. It may be possible to salvage some plants (especially succulents) by taking cuttings, re-potting and providing more light.
Gardeners in the northern hemisphere have a considerably shorter growing season and generally start seedlings indoors to protect them from frost and get a head start. Unfortunately, available daylight hours may not be long enough. A growlight can help to rectify this problem.
Ensure you provide an adequate light source for plants. Provide seedlings raised indoors with an adequate light source, close to a window or use heat lamps.