17 November 2012

Better red than dead

Many people consider autumn to be the most beautiful season of the year, where the normally green leaves of trees take on striking hues of red and yellow and swathes of gossamer glitter in the morning dew. It is a season where both plants and animals brace themselves for the oncoming inclement of winter, and, as this post is written, is gripping the United Kingdom in full force.

As mentioned above, one of the of the most notable aspects of autumn is the colour change in deciduous leaves before they fall (via senescence). Whereas senescence may seem to be a waste of resources for a tree, as they will have to regrow their leaves in the following spring, it is actually a necessary stage in a clever life cycle that allows them to maximise their photosynthetic output during the summer. Simply put, their large, broad leaves are big enough to contain huge quantities of a pigment called chlorophyll, which captures the energy in sunlight and uses it to produce sugars and proteins (via photosynthesis). Thus, the leaves of deciduous trees can produce much more energy during the summer than the narrower, needle-like leaves of evergreen trees. Obviously, this is a huge advantage to a tree and will make them much more likely to survive during the spring and summer. However, to use an apt quotation from George R. R. Martin's A Song of Ice and Fire: "winter is coming", and this is where deciduous leaves hit their main problem - they are too big to defend against the cold. So, rather than allow their leaves to die and greatly increase their risk of infection, deciduous trees shed their leaves before they can be damaged by frosts; opting instead to regrow them next spring.

Autumn landscapes can be stunning, with leaves taking on a wide range of beautiful red and yellow hues.

The main problem that this tactic for survival has, is in the huge waste of the resources that a tree has invested in growing its leaves in the first place. For example, imagine how much protein and bone would be wasted if you shed your arm every year only to regrow it at a later date! Fortunately, it seems that deciduous trees became wise to this fact a very long time ago and, before they shed their leaves, they reabsorb much of the chlorophyll and useful proteins, which are then used to produce new leaves the following spring. In fact, it is this 'recycling' of chemicals and removal of leaf colouring pigments that produce the yellow colour of autumn leaves, which becomes more intense as more chemicals are stripped from its cells.

This is a very simple biological idea and has been accepted within the scientific community for decades. But, there is one aspect of senescence that is still widely disputed among plant physiologists and biologists alike - the reason behind the red colour of senescing leaves, which is produced by a class of pigment chemicals called anthocyanins. To briefly summarise the strife, many scientists are adamant that anthocyanins serve no function in senescence and believe that they are merely a waste product of reabsorbance; resulting from a complicated carbohydrate overflow process. Other scientists however, believe that is is not the case as anthocyanins are actually manufactured by trees via a very energetically expensive process; thus, arguing that they must serve a specific function or they wouldn't be produced.

This idea is supported by recent research, which indicates that anthocyanins may be crucial for the survival of a deciduous tree. Within this research, there are 2 major theories that offer explanations for the presence of anthocyanins:

  • The first, purposed by Taylor S. Feild, suggests that anthocyanins act as optical screening pigments to protect chloroplasts from being damaged by destructive ultraviolet (UV) radiation, which effectively counters the reabsorbance of normal screening pigments and allows leaves to photosynthesise for longer.
  • The second, developed by the famous geneticist William D. Hamilton, is called the 'coevolution theory' and argues that the bright red displays are produced by deciduous trees as a warning to deter destructive aphids that colonise them throughout autumn.

The first theory works using the knowledge that UV radiation is just as dangerous to trees as it is to human skin and, in accordance with this, plant physiologists discovered long ago that trees produce screening pigments in their leaves that act in the same way as sun cream. When trees begin to reabsorb these screening pigments before shedding their leaves, they expose their chloroplasts to much greater levels of UV radiation. These levels are easily high enough to destroy chlorophyll pigments and thus, stops a tree from being able to photosynthesise efficiently.

In his research, Feild found that senescing leaves produced more anthocyanins as the intensity of the UV radiation they were exposed to increased, supporting his idea that they are produced as protective screening pigments.

Feild argues that a deciduous tree counters this by investing in the production of anthocyanins, which serve as 'replacements' for the screening pigments that are reabsorbed. Thus, the presence of the red pigments allows a tree to photosynthesise for much longer than it could have ordinarily, making it much more likely to survive the winter. Furthermore, due to the relative toxicity of anthocyanins, a tree would have little incentive to reabsorb them and would have no qualms in losing the pigments when their leaves fall.

Hamilton's coevolution theory differs from Feild's hypothesis in that it considers the effects that the red colouration of foliage has, assuming that it is an honest signal to pests about a tree's defensive investment. As said above, anthocyanins are relatively toxic and are harmful to herbivorous aphids that are known to colonise trees throughout autumn. Thus, by producing more anthocyanins, which in turn makes leaves redder, a tree invests in defences that will help it to survive the winter; i.e. the more anthocyanins it produces, the less likely it is that pests can survive by living on it.

Hamilton argues that aphids have learnt to avoid trees with the brightest red foliage as they know that these will be the most difficult trees to survive on, instead colonising trees with a lower defensive commitment. Due to this active selection by aphids and other pests, producing a bright red foliage would be under a strong evolutionary pressure since such trees will be healthier in the spring and thus, will be more likely to reproduce and pass on their genes so that overtime, leaves produce more and more anthocyanins.

Hamilton noticed that the amount of anthocyanins synthesised by trees varied, evidenced by the variety of red hues that are displayed between individual trees. If the pigments were produced solely to protect against UV light, it would be expected for all of the members of a species to show the same levels of anthocyanins in their leaves, which is not the case. Furthermore, Hamilton found that the trees displaying the reddest foliage showed the lowest concentrations of aphid pests.

Both of the theories discussed above are very controversial within the scientific community at the moment and are not widely accepted. Despite this, there are many botanists and biologists that believe at least one of them to be correct, as they are unable to accept that such energetically expensive pigments as anthocyanins would be produced without a good purpose - this is not how nature works and proteins are only produced if they are needed or the resources are used to produce something else. For example, human muscle mass begins to deteriorate after about 2 weeks of inactivity as their body determines that it is not being used anymore and stops expending resources and energy to maintain it. 

Many biologists however, myself included, go even further than just believing one of these theories to be true and have noted that neither of these theories appear to contradict each other. In fact, they do not and are not mutually exclusive. Thus, it may well be that anthocyanins have a dual purpose and were originally produced by trees to extend the length of time that they could photosynthesise for and, due to their bright colour, soon developed a secondary role in deterring aphid invasion!

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