Trees coloured by autumn sun

Photo: Arne Ader
Translation: Liis
Maple mirrored in water
 
This autumn has been warmer than average and the sun has pampered us too. Autumn colours are very different from year to year. We ask the experts:
 
Why are leaves more red in some years than in others?
 
University of Tartu plant physiologist Evi Padu: ”The leaves of deciduous trees lose their green colour every autumn and become many-coloured in yellow, red and violet shades. From an aeroplane it is very well visible above forests in the temperate zone how green foliage slowly retreats from day to day towards south with a clearly marked frontier. The non-green colours of autumn leaves but also of fruits and flowers are due to two groups of pigments: the carotenoids in the plastids (from yellow to red) and the anthocyanins, dissolved in the cell liquid (from pale pink to intense violet).
The appearance of the yellow colours of the carotenoids is tied to the loss of the green chlorophyll that masks them. Before the leaves fall, the chlorophyll is broken down in order not to loose the nitrogen in it. Carotenoids only contain carbon, hydrogen and oxygen – elements that are not in short supply, and that can remain in the falling leaves. The autumn colours of leaves are however not only caused by the decomposition of the green pigment but also by an increase in the content of anthocyanins.
The different intensity of autumn colours in different years is caused by variations in the amounts of anthocyanins. In the formation chain of these compounds light-activated enzymes participate, and so more anthocyanins are formed when the weather is sunny and warm but with cool nights as has characterised this autumn. On the south side of a tree the leaves colour more intensely for the same reason.
Why is the formation of anthocyanins necessary when the leaves will fall anyway?

It is believed that due to their anti-oxidant properties they reinforce the protection of the plant against free radicals formed from ultraviolet radiation and in the metabolic processes, so allowing the decomposition of chlorophyll in autumn leaves.“



 

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