Autumn leaves: Different this year thanks to spectroscopy

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  • Published: Nov 1, 2011
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Autumn leaves: Different this year thanks to spectroscopy

Whispering grass...

Ultraviolet spectroscopy, as well as NMR and mass spectrometry, have been used by an Austrian team to reveal the existence of a previously unknown chlorophyll decomposition product in the autumn leaves of Norway maples. The novel structure suggests an entirely different degradation route not seen in other deciduous trees is present. The work suggests that the route is more akin to the formation of bile pigment from broken down haemoglobin.

For those of us in the temperate North, Autumn is here, the Fall is upon us, deciduous leaves are changing colour and the New England tourism season is in full swing. Chemists though they knew almost everything they needed to know about what makes those verdant hues transform into the reds, yellows, oranges and browns of autumn. It was "simply" the decomposition of the green, light-trapping pigment chlorophyll, the excretion of toxic metabolites into the leaves and the withdrawal of still useful nutrients. Like I say: simple.

Don't tell the trees...

Not, so fast, say Bernhard Kräutler and his team at the University of Innsbruck, in Austria. Writing in the journal Angewandte Chemie, they describe a previously unknown chlorophyll decomposition product in the leaves of Norway maples that hints at an entirely novel biochemical degradation pathway. One that has not been seen in the biochemistry of other deciduous trees but that may have features in common with human biology where another tetrapyrrole, the haem group requires degrading from spent blood to form bile pigments.

The chemical received wisdom has it that during the summer months, green leaves carry out photosynthesis: chlorophyll converts sunlight into chemical energy. At the onset of autumn, deciduous trees reabsorb critical nutrients, such as nitrogen and minerals, from their leaves. This releases the green chlorophyll from the proteins to which it was previously bound. In this form, chlorophyll is toxic to the tree when light shines on it and so the tree, through millions of years of evolution has developed a defence mechanism, it detoxifies the chlorophyll and then sheds its leaves in perhaps the most picturesque form of biological excretion.

Kräutler points out that it is only in the last couple of decades that essential pieces of the biochemical puzzle have been solved and yet the picture was still not entirely clear. There are numerous colourless tetrapyrroles, the molecular framework containing four nitrogen-heterocyclic five-membered carbon rings, that accumulate in dying leaves as decomposition products of chlorophyll. These are the "non-fluorescent" chlorophyll catabolites (NCCs). "They are considered to be the final breakdown products of a well-controlled, 'linear' and widely common decomposition pathway," says Kräutler. However, like leaves trembling on a bow, this premise has become a little shaky of late.

The trees don't need to know

Kräutler and his co-workers have now examined the decomposition of chlorophyll in the Norway maple, a tree native to Eurasia. "We found none of the typical breakdown products in yellow-green or yellow Norway maple leaves," he explains. "Instead, the main product we found was a dioxobilane, which resembles a chlorophyll breakdown product found in barley leaves." Despite the chemical similarities, there was no known plausible route by which NCCs could degrade to form dioxobilane. "There is clearly a chlorophyll breakdown pathway occurring in Norway maple leaves that differs from those previously known," adds Kräutler.

The team points out that the structure of the newly discovered dioxobilane is similar to compounds found among the bile pigments, which suggests that they are important components of both mammalian metabolism and the light sensitivity of plants. "This supports the idea that chlorophyll breakdown is not only a detoxification process; the resulting decomposition products can also play a physiological role," asserts Kräuter. He adds that, "Chlorophyll breakdown products can act as antioxidants in the peel of ripening fruits, making the fruits less perishable. What role they play in leaves is not yet clear."


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Red maple leaf, photo by David Bradley - Ultraviolet spectroscopy, as well as NMR and mass spectrometry, have been used by an Austrian team to reveal the existence of a previously unknown chlorophyll decomposition product in the autumn leaves of Norway maples. The novel structure suggests an entirely different degradation route not seen in other deciduous trees is present.

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