Have you ever wondered why plants look green and leaves turn brown in the autumn? Well, it certainly isn’t artificial colouring, or anything like that! In order to understand this, let’s explore how we see colours.
White light, like the sunlight coming from the Sun, consists of a spectrum of colours – Red, Orange, Yellow, Green, Blue, and Violet. Sounds familiar, right? These colours are in fact the colours of the rainbow that we are all familiar with! Now, why do objects appear differently coloured to us? These objects absorb different colours in white light to different extents. The less the colour of light being absorbed, the more of it is reflected and thus, the object appears to be of that colour.
The leaves of a plant contain pigments required for photosynthesis. Photosynthesis is the process that allows plants to synthesize their food. This process requires light energy, just as the word “photo” in its name suggests. One of the most important photosynthetic pigments that are required and we are all familiar with is chlorophyll. Chlorophyll absorbs red and blue light to the greatest extent and absorbs green light to the least. Thus, a lot of green light is reflected back into our eyes. This is the reason why plants appear green!
If plants contain mainly chlorophyll, then why is it that the leaves of plants often turn yellow or brown during the autumn? Well, although chlorophyll is the main photosynthetic pigment in most plants, plants generally have other pigments too, called accessory pigments. Examples of such accessory pigments are xanthophylls and carotenoids. These pigments absorb light of different wavelengths as compared to chlorophyll, and thus appear in different colours. For instance, xanthophylls transmit more yellow light and thus appear yellow or orange. However, leaves usually still appear green throughout most of the year as the colour of the accessory pigments are hidden by the green colour of chlorophyll that predominates. During autumn, however, chlorophyll breaks down more rapidly than these accessory pigments, and thus the yellow or orange colour of the accessory pigments show through. This is the reason for the beautiful yellow, orange, and red hues of falling leaves in the autumn!