Leaves of plants are the factory units for making the food in the form of sugar from carbon dioxide, water and light by a process called photosynthesis. The process is accomplished with the help of chlorophyll which belongs to a class of organic compounds called porphyrins.
We also have noticed that plants leaves change color especially when the amount of light available changes with the change of weather.
Chlorophyll exists in two forms namely Chlorophyll A and Chlorophyll B. It’s production in the plant is dependent upon the amount of sunlight available. It also decays at a continuous rate irrespective of the sunlight availability. When the sunlight reduces on the onset of winter, the production of chlorophyll is reduced.
Chlorophyll imparts the green color to the leaves. When new leaves sprout their colour is light green and becomes darker green with the passage of the time. But in winter why the color of the leaves become brown, yellowish? This means the leaves contain other pigments also. The resultant color is a mixture of ratios of the pigments present.
There are three main pigments which impart the different colors to the leaves. These are Chlorophyll which we have discussed above, Carotenoids, and Flavonoids.
Carotenoids belong to the class of organic molecules called carotenoids. Examples of carotenoids found in leaves include lycopene, which is red, and xanthophyll, which is yellow. Light is not needed in order for a plant to produce carotenoids, therefore these pigments are always present in a living plant. Also, carotenoids decompose very slowly as compared to chlorophyll. Carotenoids are present in many vegetables also like carrots from which they derive their name, papaya and other red or yellow coloured vegetables and fruits.
Examples of flavonoids include flavone and flavol, which are yellow, and the anthocyanins, which may be red, blue, or purple, depending on pH due to the transformation of the basic structure on receiving or releasing the proton depending on the soil fluids pH.
Anthocyanins, such as cyanidin, provide a natural sunscreen for plants. Because the molecular structure of an anthocyanin includes a sugar, production of this class of pigments is dependent on the availability of carbohydrates within a plant. Anthocyanin color changes with pH, so soil acidity affects leaf color. Anthocyanin production also requires light, so sunny days are needed for the brightest fall colors
When leaves appear green, it is because they contain an abundance of chlorophyll. Chlorophyll masks other pigment colors. Anthocyanins, in turn, mask carotenoids. As summer turns to autumn, decreasing light levels cause chlorophyll production to slow.
However, the decomposition rate of chlorophyll remains constant, so the green color will fade from the leaves. At the same time, anthocyanin production in leaves increases, in response to surging sugar concentrations. Leaves containing primarily anthocyanins will appear red. Leaves with good amounts of both anthocyanins and carotenoids will appear orange. Leaves with carotenoids but little or no anthocyanins will appear yellow. In the absence of these pigments, other plant chemicals also can affect leaf color. An example includes tannins, which are responsible for the brownish color of some oak leaves.
Incidentally, some of these pigments do not undergo decomposition and remain unaltered during the decomposition of other organic material to petroleum and coal precursors. These are called biomarkers and are used to delineate the source of petroleum and coal forming organic matter in terms of terrestrial and marine.