Nature is a great chemist. It is playing with chemical pigments to present vivid colors. Even a single leaf is a piece of art. There are many classes of pigments present inside it but their amount and times of breakdown and synthesis decides the resultant color. The different colors are on display during autumn season. The leaves begin to look less and less green. They can take yellow, orange and red hues depending upon the ratios of the amounts of different pigments present in the leaves.
Most important pigment in the leaves is the chlorophyll. It is this pigment which imparts the green color to the leaf. It’s amount is dictated by the warmth and amount of sunlight the plant receives. It’s presence is the indication that plant is alive and carrying out photosynthesis to convert carbondioxide and water into sugars and oxygen. Sugars contain energy from the sun which is harvested by tree or plant during photosynthesis.
What happens when it is not sunny. We see a kaleidoscope of different colors in leaves. There are yellow, orange and red hues. These colors are attributed to other pigments present inside the leaves. These were there throughout the life of the leaf but there colors were masked by the strong green color.
In the autumn, when sunlight is not available in plenty, the production of chlorophyll is halted. On the other hand, the chlorophyll present begins to breakdown. At this time, color contribution from other pigments begin to show up.
Chlorophyll is a type of chlorine with magnesium as the central metallic ion. There are 4 nitrogen atoms which are Lewis bases and thus trap the positively charged magnesium ion. Chlorophyll is synthesized in the warm and sunny conditions by the plants. It’s green color dominates the color in the leaves. During autumn, the sunlight is not fully available and hence the production of chlorophyll halts and since it is not required the already present chlorophyll in the leaves begins to breakdown and hence result is the decrease in green color of the leaves.
Carotenoids and flavonoids are pigments which are always present in the leaves but there color is masked by the green color of the chlorophyll. When during autumn, the chlorophyll begins to breakdown, the color of these two classes of compounds begins to show up.
Xanthophylls which are oxygenated carotenoids are responsible for the yellow color of leaves. They do not require light for synthesis, so that xanthophylls are present in all young leaves as well as in etiolated leaves.
A class of carotenoids known as beta carotene is responsible for the orange color in leaves. It absorb light of green and blue wavelengths and reflects red and yellow wavelengths light thus causing the orange color in leaves during autumn. Beta-carotene are also responsible for this color in carrots. They begin to degrade at the same time as chlorophyll but at a slower rate thus showing up the orange color gradually.
There is another class of compounds called anthocyanins which begins to get synthesized in the mature leaves due to the high amount of sugars in them. These are red in color. These are thought to prolong the falling of leaves.