Austrian scientists unravel the secrets behind the dramatic colours of autumn.

Bernhard Kräutler and colleagues at the University of Innsbruck, have shown for the first time that a yellow breakdown product of chlorophyll contributes to the colours of autumn.

The change in autumn leaf colour is a phenomenon that affects the normally green leaves of many deciduous trees and shrubs. Every year, for a few weeks in autumn, a range of colours including intense yellows and reds shape the landscape. So far, these colours have been attributed to carotenoids and flavonoids, explains Simone Moser (a member of the research team). The colours are already present in the leaf, but are not visible due to the predominant green of chlorophyll. As autumn progresses chlorophyll disappears unmasking these hidden colours. But this is not the whole story, according to these researchers.

The breakdown of chlorophyll is a process that was considered an enigma until about 20 years ago, when the first non-green chlorophyll breakdown product was discovered, says Moser. As these breakdown products were colourless, they were thought not to contribute to the colours we see in autumn. 

These compounds were considered to be the final products of chlorophyll breakdown, but now Kräutler has shown that they may be oxidised to give a yellow-coloured compound. Using leaves from the Katsura Tree, a deciduous tree known for its beautiful autumn leaves, they successfully detected this yellow chlorophyll breakdown product, thus proving its existence. 

The similarity in structure between bilirubin, a natural compound reported to help protect cells from damage, and this oxidised breakdown product may suggest they too have important physiological properties. Moser says the team are interested in finding out just what roles, if any, these compounds play in the plant.

Sarah Corcoran

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