Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?

When a leaf is kept in the dark for an extended period, it undergoes changes in its pigmentation and overall physiology due to the absence of light, which is essential for photosynthesis. The yellowing or pale green coloration of leaves in darkness is primarily attributed to the breakdown of chlorophyll and changes in pigment composition. Here's why:

1. Chlorophyll Degradation:

   • Chlorophyll is the primary pigment responsible for the green coloration of leaves and is crucial for photosynthesis. In the absence of light, chlorophyll molecules are not continuously regenerated through the process of photosynthesis.
   • Without the replenishment of chlorophyll, its existing molecules begin to degrade. This degradation process is accelerated in darkness, leading to the breakdown of chlorophyll molecules into colorless or pale compounds.

2. Formation of Anthocyanins:

   • In some cases, the breakdown of chlorophyll in dark conditions may also trigger the production of other pigments, such as anthocyanins. Anthocyanins are responsible for the red, purple, or blue coloration observed in some leaves, fruits, and flowers.
   • However, the formation of anthocyanins in response to darkness is not as common as the degradation of chlorophyll, and it may depend on various factors, including the plant species and environmental conditions.

3. Stability of Pigments:

   • Among plant pigments, carotenoids are generally more stable than chlorophyll, especially in the absence of light. Carotenoids, such as beta-carotene and xanthophylls, contribute to the yellow, orange, and red hues observed in leaves and fruits.
   • While chlorophyll is susceptible to degradation in darkness, carotenoids are relatively stable and may remain present in the leaf tissue, contributing to the yellow or pale green coloration observed when chlorophyll breaks down.

In summary, the yellowing or pale green coloration of leaves kept in darkness is primarily due to the breakdown of chlorophyll, the primary green pigment, in the absence of light. While some plants may exhibit changes in pigment composition leading to the formation of other pigments like anthocyanins, carotenoids generally remain more stable and may contribute to the observed coloration.