Why leaves turn colour in autumn.

The beautiful colours of the leaves in September and October are
triggered by shortening days and lower temperatures. The green
chlorophyll is degraded to reveal the yellow, red and orange colours
that were masked by the overpowering green chlorophyll during the
summer season.

In addition, there are two parts to photosynthesis: light-dependent
and light-independent. When the temperatures turn cooler, the
light-independent part of photosynthesis slows down (carbon fixation
portion which is tightly controlled by enzymes---very sensitive to
changes in temperature). However, the light-dependent portion does not
slow down to the same extent. The light-dependent portion continues to
receive light (photons) which split water to produce electrons which
drive the photosystems of the light-dependent portion of
photosynthesis. Since the light-dependent portion flows into the
light-independent portion, there is a back-log created when the
light-independent portion slows down. This back-log causes excess
electrons which induce free radical formation in the light-dependent
portion. In order to protect itself, the leaf synthesizes anthocyanins
(red pigments) which absorb the free radicals (same pigments in red
wine and red fruits). In addition, the anthocyanins are also located in
the upper epidermal layers and essentially serve as a sun block to
reduce excess photons from entering the leaf. In this way, the reduced
amount of light received will be in more balance with the reduced level
of activity in the light-independent portion of photosynthesis and the
number of free radicals will also decrease.

By Dr Karen Tanino