04 November 2013

Puzzling colours

This blog post was written by postgraduate student Elleni Vendras as part of the course, Research Methods in Ecology (Ecol608). Elleni revisits a Lincoln University research area that look at why leaves change colour in autumn published in 2002.

Travelling in early autumn through the South Island of New Zealand, made me wonder whether the changing trees I see are native. Most trees with autumn leaf colors seem strikingly familiar to the ones in Europe — an almost unnatural contrast to the green natives here. I found that, indeed, very few natives lose their leaves, and the majority of New Zealand native trees are evergreen.

Eurasian aspen at Lake Tekapo
Photo by Elleni Vendras

Deciduous trees lose their leaves in preparation for a cold winter (or dry season in the tropics). They don’t photosynthesize over winter when there is not enough water or sunlight for this oxygen and glucose generating process. The chlorophyll in the leaves breaks down and the components move to the stem and branches of the tree.

What happens next is the reason why so many people travel to places like Wanaka, Mackenzie Country, Waitaki Valley, and Central Otago (and of course New England in the United States as the most popular place for autumn coloration). They take delight in watching the beautiful autumn coloration: carotenoids – yellow and orange pigments, become visible once all the green chlorophyll is gone. These light-absorbing pigments play a role in photosynthesis and act as a photoprotector for the chlorophyll. Another pigment anthocyanin is responsible for the red autumn coloration. It is produced from glucose during autumn and as a result, leaves from maple, aspens and many others turn into a beautiful red.

But, hold on. Why would a tree use energy in producing color pigments in leaves that eventually fall off and wither away? To please Homo sapiens? Certainly not. Scientists believe that these pretty pigments might keep the leaves alive longer by lowering the freezing point and protecting the chlorophyll from photo damage. In that way the leaves might stay longer at the tree and valuable components like glucose or nitrogen can get removed from them and be transported into branches and stem, rather than getting lost with the falling leaves. Others say that anthocyanin may prevent other plant species from growing where the leaves have fallen and decayed.

Besides the physiological meaning of autumn colors, Hamilton and Brown advanced in 2001 a hypothesis on the adaptive significance of autumn leaf colors. They proposed that the yellow and red colors in leafs are a signal of the defensive commitment against autumn colonizing aphid species. This theory aroused scientific argument and was refuted a year after publication by David Wilkinson and colleagues, including the Lincoln University-based author, Steven Wratten.
They stated that the time of the signal is inappropriate since aphids tend to colonize their host BEFORE leaves fall, so while they are still green and have no anthocyanins produced yet. So why hasn't evolution allowed for leaves to defend themselves just in time? Other studies argue that yellow color actually attracts aphids; some insect traps are even yellow colored.  Also it is found that aphids tend to colonize at varying levels on red and yellow leaves depending on the time of the year.

Another argument they made was that the changing of leaf colors are strongly environmentally regulated and not linked to insect colonization. Hamilton and Brown’s theory that bright autumn coloration serves as an honest signal of the trees defensive abilities does not convince the criticizing authors especially when it comes to the cost of honesty. This term which is often used in animal behavior research certainly can be used for plants as well. It describes that communication signals need to be costly to be honest and so honest signals have to be handicaps. Otherwise, there’s no reason not to cheat. But, in fact, there is no extra cost for a tree to reveal yellow leaf color.

So how about red autumn coloration, since this signal is produced in autumn? Are red pigments with their antioxidant abilities just used as another sunscreen complementing the already available yellow carotenoids? Another alternative explanation for their presence offered by the authors is that red leaves absorb more light and energy allowing photosynthesis to happen under harsh conditions.
Red anthocyanin in an leaf of trident maple (Acer buergerianum)
Photo by Elleni Vendras
It’s amazing that something simple that we look at each year is still not well understood. The jury is still out on aphids and red leaves. Marco Archetti found out that aphids tend to avoid red-leaved apple trees and are less fit than those on green leaves. London-based scientists revealed that although aphids lack a red photoreceptor, they may be able to differentiate between green and red leaves and certainly find yellow leaves the most attractive. While in carnivorous plant species the red anthocyanins tend to attract insects. What a confusing mess of contradictions! But that’s science for you. The journey to the truth rarely follows a straight line.
And so scientists continue to do what they have always done: research and debate. Either way, trees in New Zealand can count themselves lucky. While they have to cope with 122 aphid species, trees in Poland have to fight 500 more. By the way, in those latitudes autumn colors are much more striking as well.

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