December 3, 2021

I, Science

The science magazine of Imperial College

The Colour of Leaves

Across the temperate regions of the northern hemisphere, deciduous trees are pushing out bright green new leaves that unfurl from buds like verdant moths thrust slowly from winter cocoons. The green colour of these new leaves is, of course, due to the pigmentation of chlorophyll. In green plants, chlorophyll molecules absorb light and channel that energy for the conversion of carbon dioxide into nutrients via photosynthesis.

Leaves appear green because they reflect green light, absorbing radiation from the upper and lower parts of the spectrum. But it’s not certain why the majority of plants have evolved to be green – black plants may have been more efficient photo-synthesisers. Indeed, this Nature article on what colours alien plants might be covers all possibilities (except blue).

What is certain, though, is that in roughly six months time, the production of chlorophyll will dwindle and the green colouring with it. With the chlorophyll absent, leaves may appear orange or yellow due to the carotenoid and xanthophyll pigments that are left behind. Red autumn leaves, however, get their colour from pigments called anthocyanins, which are synthesised only once the chlorophyll starts to be broken down. Anthocyanins are a type of flavinoid, that class of organic compound more often mentioned in the context of the antioxidant properties they give to tea, red wine and chocolate.

Again, it’s not certain why some trees lose their leaves and others do not. Both strategies to surviving winter seem to involve trade-offs. Shutting off photosynthesis and dropping leaves probably conserves energy, minimising water loss and insect damage, but then energy must be used to start the cycle going again each spring. Regrowing leaves annually may simply be less costly than repairing and maintaining them throughout the year. Still, whatever the reasons, those cheery green wings emerging each April make me happy.

Image: Peter Larkin