Question

In response to shortened day length and cool temperatures in the fall, many trees begin a period of senescence when the breakdown of chlorophyll exceeds chlorophyll production. The leaves of these trees appear to change to yellow and orange. Using your knowledge of photosynthetic pigments, explain the source of these yellow-orange hues.

Answer 1

Carotenoids pigments are the source of the yellow-orange hues.

During fall, chlorophyll will breaks down and the leftover food in the leaf is transformed into the red colour through anthocyanin pigments.During the growing season, the plant replenishes chlorophyll, so that the supply remains high and the leaves stay green. As autumn approaches, the daylight hours shortens and temperatures become cools with decreasing amount of chlorophyll in the leavez. Often, the veins remains still green after the tissues between them have almost completely changed color. In autumn, chlorophyll degradation occurs and is followed by the breakdown of photosynthetic complexes (PS I and PSII). Chlorophylls degrade into colorless tetrapyrroles known as nonfluorescent chlorophyll catabolites and then, the hidden yellow xanthophylls and orange beta-carotene pigments are revealed. These pigments are present throughout the year, but the red anthocyanins pigments are synthesized de novo once roughly half of chlorophyll has been degraded.

Carotenoids are present through the year but the colour is usually masked by chlorophylls in the leaves. Chlorophylls gradually dwindling and the "masking" effect slowly fades away. Then other pigments present, along with the chlorophyll, in the leaf begin to show through.Then the carotenoids provide colorations of yellow, brown, orange, and the many hues in between.