Why is the color of a leaf kept in the dark frequently yellow or pale green?

When a leaf is kept in the dark, it undergoes a process called etiolation, which leads to the change in its color. Etiolation occurs when a plant is deprived of light, and it triggers a series of physiological responses to optimize its growth and survival in low light conditions. One of the prominent changes during etiolation is the alteration in leaf pigmentation, resulting in a yellow or pale green coloration.

Chlorophyll degradation:
- The main reason behind the yellow or pale green color of leaves in the dark is the degradation of chlorophyll, the primary pigment responsible for the green coloration of plants.
- Chlorophyll molecules are sensitive to light and undergo constant degradation and synthesis processes. When exposed to darkness, the degradation process exceeds the synthesis, leading to a decrease in chlorophyll content.
- As the chlorophyll breaks down, the green color fades, revealing other pigments that were previously masked by chlorophyll.

Carotenoids:
- Carotenoids are another group of pigments found in leaves. They are responsible for the yellow, orange, and red colors observed in various plants.
- While chlorophyll degrades in the absence of light, carotenoids remain relatively stable. Therefore, as chlorophyll decreases, carotenoids become more visible, giving the leaf a yellow or pale green appearance.

Anthocyanins:
- In some cases, anthocyanins, another group of pigments, may be present in leaves and contribute to their coloration.
- Anthocyanins are responsible for red, purple, and blue hues in plants. However, their presence or absence depends on various factors such as genetics, environmental conditions, and the specific plant species.
- In the absence of light, anthocyanin production may be limited, resulting in a paler coloration.

Pigment stability:
- Among the pigments mentioned, carotenoids are generally more stable compared to chlorophyll and anthocyanins.
- Carotenoids have a higher resistance to degradation caused by factors like light, temperature, and pH changes, allowing them to remain visible even in low light conditions.
- Chlorophyll, on the other hand, is highly susceptible to degradation when exposed to darkness or other unfavorable conditions.
- Anthocyanins can also be less stable compared to carotenoids, as their production and stability are influenced by various environmental factors.

In conclusion, the color change of leaves to yellow or pale green in the dark is primarily due to the degradation of chlorophyll and the subsequent visibility of carotenoids. Carotenoids are more stable compared to chlorophyll and some other pigments, allowing them to maintain their coloration even in the absence of light.