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!

"I spy with my AMAZING eye..."

Human vision is incredible, with our eye being one of the most sophisticated structures for capturing light that has ever evolved. Our eyes are capable of detecting a single photon of light at night and can create complex and unbelievably definite images during the day, being able to make out structures 1/10th of a hair-span wide. Our eyes allow us to see the world in a detail that almost no other mammal (or animal) can imagine and, since humans rely mostly on our vision to interact with and perceive the world, are extremely important to our survival and quality of life.

Although light is detected by the eye, and is essential for it to function, too much light can be very damaging to our vision. Thus the size of the pupil, the black hole in its centre where light enters, can be controlled by the iris that surrounds it. The iris contains light absorbing pigments and determines how much light can enter the eye. The eye ball itself is protected by being shrunk back into its socket, with our brow protruding over it to reduce the likelihood of it being physically struck. In addition, our eyelids have eyelashes that help to stop dust and debris from falling onto its surface.

The human eye is classified as a 'lens eye', since it forms images using a biological lens that is suspended behind the pupil. Lens eyes are the most complex form of visually perceiving light and have only evolved in primates, birds and some Cephalopods (squid and octopuses), being unique in their ability to alter their focus to produce crisp images of close up objects and those that are much further away. Most organisms have a 'fixed focus' system where any object that is not at a certain and specific distance from their eye will appear blurred. The lens makes this focus possible by bending the light that passes through it so that it is refracted neatly onto the fovea (which is at the centre of the macula), at the back of the eye. The fovea is a small area of the retina where the light receptor cells that make make up the retina are particularly dense, and produces the most detailed image of our surroundings. Thus, by becoming thicker for closer up objects and thinner for those further away, the lens can refract light so that most of it lands on the fovea and a sharp, clear image is formed.

To focus on close up objects the lens is made thicker so that its refractive power is increased. This is accomplished by a contraction of the ciliary bodies so that the tension on the suspensory ligaments is reduced. This means that they pull the lens less taught and it contracts. Likewise, to focus on far away objects the converse is true: the ciliary bodies relax, causing the suspensory ligaments to tighten and the lens is pulled upon, stretching it out. This then decreases the refractory power of the lens and light is bent less.

The mammalian visual system is made up from 2 different types of photoreceptor cell: cones, which are responsible for seeing colour during the day (or in other conditions of high light intensity); and rods, which work in 'black and white' and are responsible for our night vision. These photoreceptors contain 4 different photopigments that split when photons of light hit them, producing an electrical charge. This charge is then magnified into a nerve impulse and is sent to the optic chiasm in the brain (via the optic nerve), where it is collated with other impulses from the eyes and processed to form an image. The type of photopigments present in the photoreceptor depends upon its type. Rod cells only contain rhodopsin, which is made from vitamin A (the reason why carrots, which are high in vitamin A, can improve your night vision). Rhodopsin is very sensitive and can detect a single photon of light, responding best to light at 498nm. Although this isn't enough to form an image, it shows just how sensitive human eyes are and explains why our eyes sting when we go from the dark into the light: the sudden increase in light intensity splits all of the photopigment and prevents it from being reconstructed. This is known as 'bleaching' and takes about 15 minutes to be reversed, which is why it takes your eyes a while to adjust to seeing in the dark. Rod cells show the opposite distribution to cone cells and are less concentrated in the fovea, becoming more abundant towards the edge of the retina. Thus, human night vision is at its best in its periphery and objects often become less clear to us in the dark when we look at them directly! This fact is how many scientists explain those incidences where you seem to see something out of corner of your eye that vanishes when you look to see what is was...

The remaining 3 photopigments then, are involved in colour vision and work together to form the 'pallet' of colours that humans can see. This complimentary system is called a trichromatic system and each photopigment responds best at a different wavelength of light, so that most of the electromagnetic spectrum is covered. Long Wave Sensitive (LWS) opsin responds best to wavelengths of 564nm and sees red light; Medium Wave Sensitive (MWS) opsin responds best to wavelengths of 533nm and sees green light; and Short Wave Sensitive (SWS) opsin responds best to wavelengths of 433nm and sees blue light. The brain mixes the signals coming from the 4.5 million cone cells in the retina of each eye and produces colour. Interestingly, this is the same system that early (tube) colour television sets used to form colour picture! They used thousands of units of 3 triangles placed side-by-side that were coloured red, green and blue respectively.

Many nocturnal predators, such as canids and felids (like the cat in this picture), possess a reflective layer of cells beneath their retina called the tapetum lucidum. These cells reflect light back through the retina so that each photon is detected twice. This greatly improves their night vision and explains why their eyes seem to glow in the dark.

However, despite human vision being one of the best visual systems in the world that allows us to see with a clarity experienced by very few other organisms, it is fundamentally limited. This is believed to be due to a phenomenon called 'nocturnal bottlenecking', which occurred over millions of years during the rein of the dinosaurs. Due to the size and ferocity of the dinosaurs the mammals present in this era remained very small and were only active at night to avoid predation. This meant that many genes for colour photoreception were lost, since they were not needed and were not selected for. Thus, the mammalian colour visual system had to be 'rebuilt' from only the 3 photopigments that we had left when the dinosaurs had died out and we began to display diurnal activity. This unfortunately means that our perception of the electromagnetic spectrum is very limited and, as a result, mammals cannot see infra-red or ultraviolet light (UV) light like many of the organisms in the other classes of animals.

However our evolution on the flat African savannah plains has helped to compensate for this and we have  developed a fantastic visual system, which evolved as our primary sense. Our visual system is far superior in terms of quality to that of most other organisms, even if we cannot perceive as much of the electromagnetic spectrum as them; and personally speaking, I would much rather be able to see in higher quality than in more colours so our bottlenecking may actually have worked out for the best!