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, the chlorophyll pigments (chlorophyll a and chlorophyll b) degrade over time. As the green colour of chlorophyll fades, the underlying yellow pigments become visible, making the leaf appear yellow or pale green.
The carotenoids and xanthophylls are more stable pigments. The context indicates that these accessory pigments protect chlorophyll a from photo-oxidation, which suggests they are more stable and persistent compared to chlorophyll. While chlorophyll breaks down in the absence of light, the yellow pigments remain intact and dominate the leaf's appearance.
Explanation
The textbook context mentions four pigments in leaves: Chlorophyll a (bright or blue green), Chlorophyll b (yellow green), xanthophylls (yellow), and carotenoids (yellow to yellow-orange). It also states that accessory pigments like xanthophylls and carotenoids protect chlorophyll a from photo-oxidation, implying they are more stable. When leaves are kept in darkness, chlorophyll degrades while the more stable yellow pigments remain, causing the colour change.