It’s the season of mists and mellow fruitfulness, the days are balmy, the nights are crisp. Down the road from where I live the liquidamber has changed from forest green to russet red, the London planes from soft green to dull brown .. the leaves are falling, winter is on its way.
So why do the leaves turn different colours at this time of the year and why do they fall?
When leaves appear green, it is because they contain an abundance of chlorophyll. Chlorophyll refers to a green pigment found in most plants, algae and cyanobacteria. It absorbs sunlight and uses its energy to synthesise carbohydrates from carbon dioxide and water. This process is called photosynthesis and is the source for sustaining the life processes of all plants. There is so much chlorophyll in an active leaf that the green masks other pigment colours. Light regulates chlorophyll production, so as autumn days grow shorter, less chlorophyll is produced. The decomposition rate of chlorophyll remains constant, so the green colour starts to fade from leaves.
At the same time, surging sugar concentrations cause increased production of anthocyanin pigments. Leaves containing primarily anthocyanins will appear red. Carotenoids are another class of pigments found in some leaves. Carotenoid production is not dependent on light, so levels aren’t diminished by shortened days. Carotenoids can be orange, yellow, or red, but most of these pigments found in leaves are yellow. Leaves with good amounts of both anthocyanins and carotenoids will appear orange.
Leaves with carotenoids with little or no anthocyanin will appear yellow. In the absence of these pigments, other plant chemicals also can affect leaf colour. An example includes tannins, which are responsible for the brownish colour of some oak leaves.
Temperature affects the rate of chemical reactions, including those in leaves, so it plays a part in leaf colour. However, it’s mainly light levels that are responsible for fall foliage colors. Sunny autumn days are needed for the brightest colour displays, since anthocyanins require light. Overcast days will lead to more yellows and browns.