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Published online 29 October 2007 |
Nature
| doi:10.1038/news.2007.202
Corrected online: 30 October 2007
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Why autumn leaves turn red
Colour may help trees to store up nutrients before winter.
Autumn leaves turn fiery-red in an attempt to store up as much goodness as possible from leaves and soil before a tree settles down for the winter. The worse the quality of soil, the more effort a tree will put in to recovering nutrients from its leaves, and the redder they get.
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The article states that anthocyanins are nitrogen-containing compounds, but there is no nitrogen in the anthocyanin core structure. Perhaps there has been some confusion in the interpretation of the results here? Nitrogen will be liberated when the amino acid phenylalanine is converted to cinnamic acid, which is the entrypoint into the branch of metabolism that gives rise to anthocyanins. Therefore, perhaps what was meant that, in the course of synthesising anthocyanins, large amounts of nitrogen are liberated, which the tree can then store throughout the winter. This would seem to be a better hypothesis to fit the data, than to invoke some sort of novel nitrogen-containing anthocyanin.
Having long delighted in the flaming red Maples which are typical of a Vermont Fall, I recently visited Alaska where I was surprised and delighted with the golden yellow of the Aspen and Birch trees. The only red leaves I saw were an occasional shrubby undergrowth on some slopes. I am neither a botanist nor a soil scientist, but it is hard for me to believe that the rocky Alaskan slopes covered with trees bearing bright yellow leaves were in a healthier environment than in Vermont or other Northeastern states where in the Fall I have seen bright red Maples adjacent to lowland pastures as well in highlands.
Interesting but this does NOT explain why in the same area, some trees will always turn red (to some extent) and some will never turn red but will turn a clear yellow. This appears to be species specific. Many cherries, Amelanchier, Cotinus, maples, Parrotia are all reliably red even in the UK where we have less cold weather. Birches. larches and Gingko never go red but do go yellow. Pam Hunter. Sussex UK
Statements in this article stood out as scientifically inaccurate explanations of biological phenomena. Autumn leaves turn fiery-red in an attempt to store up as much goodness as possible from leaves and soil before a tree settles down for the winter. "A plant on a nutrient-poor soil is going to be more concerned about keeping the nutrients it has," says Hoch. So it will turn red to stop its leaves dropping prematurely. Plants do not have the ability to anticipate the future. They respond to environmental cues in ways that are encoded in their genes. These responses have been honed by natural selection upon past generations. Plants cannot attempt. They have no minds to be concerned about anything. Such teleological explanations reflect a misunderstanding of cause and effect and/or scientific laziness.
When photosystems are destroyed in a senescence programme, chlorophyll derivatives are toxic, and must be degraded. Then nitrogen is released from chlorophyll molecules. That is why mutant trees that couldn't produce anthocyanins were much less efficient at storing up nitrogen for the winter. The reason was probably their inability to degrade chlorophyll, not the inability to produce anthocyanins. There is another hypothesis which is worthy to be discussed: the prevention of trees from Hemiptera insects.
Maybe this turn-to-red character is conserved in some specific species of trees, which all have some common pathways of metabolism. I think the color change is a multiple-factor-influencing process, which not only involves temperature and sunlight, but also maybe something remained to be explored.
It should be noted that the enhancement of anthocyanin in nitrogen-limited soils appears to only affect species that already have the ability to turn red. Species which are genetically predisposed to have yellow autumn leaves (e.g. birch, poplars, some species of maple etc.) will not suddenly turn red when placed in nutrient-poor soils. Furthermore, these non-red species are not lacking means for protecting nitrogen-translocation just because they lack anthocyanin. Instead, they appear to have evolved alternative strategies for protecting the translocation process. Hoch illustrates this in his study on red/non-red mutants by bringing in a naturally non-red species (Betula papyrifera) for comparison. Although non-red mutants did indeed translocate less nitrogen and senesce earlier than the red trees of the same species, the birch species was just as efficient as the wild type red species at translocating its nitrogen. All that is just to say, that depriving your ginkgos of nitrogen hoping to have a red autumn in your front yard will be a fruitless effort.
In Germany, the native trees generally turn yellow, orange or brown. This also applies to the three maple species and the two oak species. The only tree with red autumn leaves that comes into my mind is the wild cherrry (prunus avium or so) American maples or oaks, however, do turn red here as well. By the way, the autumn is usually mild and wet here.
In my opinion, there is an easiest way to explain the change of colours in leaves in autumn. It is well-known that indole-3-acetic acid (IAA), the most important growth factor in plants, originates from indole-3-pyruvic acid (IPA), a very strong free-radical scavenger whose indole ring is opened by oxygen radicals and light, giving rise to many addiction products beautifully coloured from yellow, to pink, red and brown, according to pH conditions and the links with other macromolecules. IPA is also able to interfer with iron binding to enzyme complexes and, since IAA is not used in autumn by plants, IPA concentration increases, starting the chain of radical reactions leading to colour changes in leaves.