Why leaves turn yellow or red
Looking out of the window, I see trees with yellow and reddish leaves in front of the house. It’s autumn and year after year it’s the same again: the trees’ green foliage turns yellow and red until the leaves fall off, giving me the back-breaking task of raking them together and disposing of them. What’s the point of their changing colour?
Since branches closer to the street-lamp retain their green leaves much longer, shedding of the leaves and their change in colour are obviously connected phenomena. But then again, there are naturally red-leaved species, which possess red leaves throughout summer and they, too, lose their leaves in autumn. Let’s investigate.
There can be no doubt that the lack of leaves in winter is advantageous: it reduces the risk of physical damage through winter storms and heavy loads of snow, it virtually eliminates the need to transport water from the (sometimes frozen) ground to the leaves, and it prevents frost-induced leaf tissue injuries. But what does a plant lose by shedding its leaves?
After all, for many months photosynthesis cannot take place. The main coloured chemical components of the leaves are the chlorophylls (reflecting green light, they mainly absorb red and blue lights). Then there are the green-absorbing and, thus, yellow-appearing xanthophylls, and finally there are the green and yellow absorbing anthocyanins, which give a leaf its red colour. When in autumn temperatures decrease, daylight wanes, and herbivorous insects (those still around) become more desperate for green food, then the energetically costly chlorophyll synthesis winds down, unmasking existing yellow xanthophylls. This costs the plant nothing, but increasing the amounts of red anthocyanins does. So, why then in times of stress spend energy at all to boost anthocyanin levels?
An answer may come from those plants that naturally possess red leaves throughout their lives. Anthocyanins are powerful antioxidants, slowing down ageing processes, speeding up healing of torn and punctured leaves, providing leaves with greater frost-resistance, slowing down photosynthesis, and giving the plant greater protection against harmful UV-radiation and bright light. Although disputed by some, it has also been suggested that reddish leaves are less attractive to herbivorous insects like aphids and leaf-cutting ants, but have the opposite effect on ‘friendly’ animals like birds, which consume injurious insects. Some of these features of the red pigment may explain its existence in autumn leaves; others don’t. If anthocyanins protect a leaf against bright light, harmful UV radiation, and insects, what’s the point of having them in autumn, when day lengths become shorter, UV radiation is decreasing and insect numbers are falling?
A recent theory claims that the explanation for the dominance of red autumn leaves in North America and yellow autumn leaves in Europe is, indeed, related to herbivores, but in connection with the North-South direction of mountains in North America, facilitating the spread of plants and animals, rather than the East-West direction of mountains in Europe, creating obstacles. During glaciation trees and herbivores in North America could reach southern refuge more easily than their European counterparts could. Assuming that glaciation in Europe led to the extinction of many of the herbivorous insects, it is argued that the main ‘driving force’ for red autumn colouration then had disappeared, leaving only trees with yellow-coloured autumn leaves. Looking out of the window and taking a sip of red wine from my glass, I’m left wondering.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2015.
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