50 Shades of Autumn

by Leanne Massie

Photo Credit: Bert Kaufmann

Photo Credit: Bert Kaufmann

Ever wonder why some trees turn stunning shades of red and yellow this time of year while others stay a bright green year round? It’s all about three important things plants need to survive: warmth, light and water.

Evergreens, which are mainly conifers, tend to live in regions that get very cold in the winter and have quite short summers as in the Boreal forests that ring the Arctic Circle. They experience shortages of all three necessary ingredients in the Arctic winter, no sun for months means temperatures of as low as 80 degrees Celsius below zero and any available water being locked away, unreachable to plants, in the form of ice.

Deciduous trees, on the other hand, lead a much more comfy life. They still have chilly winters but temperatures tend to hover around or dip slightly below the zero mark. They also get lovely, long, warm summers with plenty of time to grow and barrels of rainwater to spare. This allows them to store up enough energy during the growing season so they can afford to drop their leaves when it gets cold and have enough energy to regrow the following spring.

But why bother dropping their leaves and re-growing them? The answer is in the third element in our list: water. Leaves have very large surface areas so represent a lot of potential area to lose water from. When it’s winter and all the ground water is frozen, losing any water through the leaves can be very damaging to the tree that need to hang to on as much liquid as possible. By dropping their leaves and thus reducing their surface area, deciduous trees are able to avoid a large amount of water loss. Of course they don’t just fall off, the trees recover as much nutrients as possible from their leaves before dropping them; it is this relocation of the green, nutrient rich chlorophyll in particular that is seen since this exposes the vivid reds and yellows of the anthocyanin and carotenoid pigments that are always present in the leaves but usually masked by the much more abundant green chlorophyll.

Going back to evergreens though, they still have the problem of water loss since they don’t drop their leaves in the winter. They have developed another solution to reduce water loss which is a typical characteristic of conifers; they have reduced their leaves to needles. This again reduces their surface area to minimize water loss while not incurring the huge cost of dropping then re-growing their leaves. Dropping them entirely is not feasibly because the low temperatures slows the actions of the microbial community to practically nothing with the result that nutrient cycling is very slow so the soil is poor quality, thus the trees don’t have the resources to regrow their needles every year.

What you might not know is that cold places are not the only place where evergreens are found. Non-conifer evergreens exist around the equator due to a lack of seasonal change. With good growing conditions all year round there is no need to drop their leaves or reduce them to needles so we find broad-leaf evergreen trees in equatorial regions. Another side effect of the lack of seasons is that the trees don’t lay down the characteristic annual rings in their trunks but have a more or less consistent grain throughout.

So there you have it! Trees that change color do so not to delight us and give us material to make leaf piles with, but because it makes evolutionary sense for them to do so. It allows them to conserve nutrients while avoiding the worst of the winter drought caused by plummeting temperatures and wait for the sun to come back. That certainly doesn’t mean we can’t enjoy the brilliant colors this produces as we always do!

Plants in science fiction and fantasy

By Lilian Halstead

When it comes to plants in real life, many people seem to think that because they don’t move, they can’t be very interesting. This is hardly the case in science fiction, where plants (and plant-animal hybrids) are much more active, dangerous things. If you think of plants in science fiction the thing you are mostly likely to come up with first would be the triffids, which first appear in The Day of the Triffids. These intelligent flowering plants originally kept for their oil escape and take over, killing with a venomous sting and wondering around on three stumpy legs. Another well known dangerous plant is Audrey II from the Little Shop of Horrors, which although it remains rooted in place manages to persuade Seymour to bring it the flesh it needs to grow.

These man eaters are by no means the first though— there is a long history of claims of trees that eat people during the exploration of Africa by the Europeans, although they tended to be depicted as masses of tentacle vines which would lash out and grab the unwary, much like the bloodoak and tarryvine in the Edge Chronicles. Some aspects of their design are echoed in the more benign whomping willow in Harry Potter, which is more grumpy than hungry. Many of the plants in the Harry Potter books are drawn from folklore: the devil’s snare was a bloodsucking vine believed to grow in central America, while the mandrake is a real plant which has fleshy roots that was said to scream to kill those who uprooted it.

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Some Arthurian legends say Merlin was turned into an Oak tree by Niviane
(Credit Rob Young)

Another common theme is for plants to resemble people. In Invasion of the Body Snatchers people are gradually replaced by replicas grown in pods. But others are more friendly, one of the main characters in Farscape is a humanoid plant, although it’s not possible to tell from looking at her. Ents are more typically what you’d expect from plant people, and the Cactacae in the world of Bas-Lag are also more plantlike, having wooden bones and being covered in spines. In mythology there were the dryads, tree spirits in humanoid form, although in myths people also had a habit of turning into trees. In some versions of the Arthurian legends Merlin suffers this fate, doomed to live as an oak by the actions of Niviane.

As well as plant people, plant-animal hybrids tend to spring up all over the place. The Sarlacc from Star Wars is one of these, some expanded universe depictions describe it as having roots as well as tentacles and teeth. The best hybrid though has to be the vegetable lamb of Tartary, which is exactly what it sounds like—a plant shaped like a sheep attached to the ground by an umbilical cord-like stalk.

Despite all these plant monsters and hybrids, the most common use of plants is as a MacGuffin: they are variously able to cure diseases, kill the invincible monsters, lift curses and even, in some folk tales, open locked doors. So while animals may get most of the glory in most of science fiction and fantasy, there is also no shortage of plants.

But baby it’s not that cold outside…

By Charlie Whittaker

We all get a bit chilly around Christmas, but unfortunately the plants amongst us don’t have the luxury of being able to up-root, put on a jumper and snuggle up by the fireside. So are plants condemned to wither and perish, rooted in the cold, freezing their (metaphorical) socks off under a covering of snow?

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Credit: Sakaori

The answer is unequivocally no. Some plants are able to engage in a process called thermogenesis, by which a vast amount of heat gets produced through metabolic processes such as respiration. Using an alternative pathway, the majority of the energy released by respiration gets emitted as heat, as opposed to being converted to chemical energy.

Being warm has its benefits. In areas that see snowfall during the winter, the process of thermogenesis (which can raise the plant’s temperature by up to 30C!) helps to melt the snow covering the plant. This allows the plant to start photosynthesizing earlier than its competitors and, in the case of seedlings, germinate and sprout earlier than other plants — giving them a competitive advantage!

Alien invaders or peaceful co-existors? The matter of non-native species in Britain

By Lilian Halstead

With the world becoming ever more connected, people are not the only things taking advantage of planes and boats to get around. Plant species are constantly being introduced, intentionally or otherwise, into places far outside their original range. The term ‘invasive species’ is sometimes used to refer to any of these non-native species. However, it more usually refers to species that are not native to the country they are growing in but that are able to propagate of their own accord and cause both environmental and economic problems. Not all species that aren’t native are invasive: in fact, it is estimated that out of all the new species arriving by human influence, only ten percent of those will survive in their new environment, and only ten percent of those will go on to have any measurable impact. But if you think of the number of plant species in gardens across the UK, that’s still a substantial number.

BeechAttribMost invasive plants in Britain start off as garden plants that spread to the wild either by releasing seeds or through being dumped as garden waste. Surprisingly, many invasive species are still being sold as garden plants despite their known detrimental effects on biodiversity. Parrot’s Feather is a water plant often sold as an oxygenator for ponds that forms thick mats of vegetation, blocking out light and choking waterways so much that they dramatically increase the risk of flooding. Ironically, Japanese Knotweed, now a notorious invasive species that would cost over £1.5 billion to eliminate from the UK, won a gold medal for being “the most interesting new ornamental plant of the year” when it was first introduced.

So introducing species for gardens can lead to invasive species escaping and causing ecological damage by forming monocultures that shade everything else out, and economic impacts such as exacerbating flooding or growing through tarmac. For this reason there are many schemes that act to monitor the species introduced to see if they become a problem and to hopefully prevent them from doing so in the first place.

But given Britain’s geological past, deciding what is native and what is not is a lot harder than it first appears. Many European species either did not make it into Britain before the English Channel cut Britain off, or went extinct soon afterwards, and humans then subsequently introduced many of them. Species that were introduced by humans a long time ago are known as archeophytes, and one such plant is beech, which was probably introduced 2000 years ago because the nuts are edible. For many species it is hard to tell whether they were there to start with or whether they were introduced. For species that were here once but went extinct before humans arrived, and which were then subsequently reintroduced, it’s not clear whether they should be counted as native or not.

KnotweedAttribIn addition to this, it has to be remembered that ecosystems are not static – succession and evolution are constantly changing the species composition, so trying to exclude species to keep things as they are is sometimes in the best interests of biodiversity, but it isn’t natural. Perhaps hybrid ecosystems altered by non-natives may have their own value, if given the chance to develop properly. Especially if further flow from the source location is halted, then the non-natives may speciate to some degree, which would make them worthy of protection in their own right.

Invasive species are a threat to biodiversity and do need to be controlled if native species are to survive. But the impacts of those that are less damaging can be positive, and given that species transfer is unlikely to stop anytime soon, seeing the value of newly emerging hybrid ecosystems may be a better strategy than attempting to maintain a ‘pure’ state, especially here in Britain, where most of what we think of as natural has been heavily shaped by humanity.

For more information, check out the GB non-native species secretariat, which has a database of most of the invasive species in Britain at the moment.

 

The little alga that could: Algal photosynthesis and its potential for incorporation into crops

By Charlie Whittaker

Algae are pretty cool. And when I say pretty cool, I mean ridiculously cool. They’re involved in everything from potential biofuel synthesis to novel metabolic pathway generation, but they’re also pretty special because of the unique way in which they fix carbon and generate new biomass.

Algae

Image of the algae Scenedesmus quadricauda. The pyrenoid is visible in the middle of each of the cells as the distinct circular object.

The main site of carbon fixation (the way plants and most other photosynthetic organisms, including algae, incorporate carbon dioxide to produce molecules that will eventually become new biomass) is at an enzyme called Ribulose Bisphosphate Carboxylase Oxygenase, or Rubisco for short. Rubisco catalyses the addition of CO2 to Ribulose Bisphosphate, producing a precursor that will eventually go on to generate glucose, sucrose, cellulose, and other sugars. These in turn can be respired to generate chemical energy for the cell or be polymerised to make the macromolecules constituting new algal biomass. However, as well as having the capability to interact with CO2, Rubisco can also catalyse another pathway, by which O2 is added instead. Known as the oxygenation reaction, this results in a net loss of carbon, which is highly problematic for the plant.

Algae tend to be aquatic, and this presents a number of challenges with regards to getting sufficient CO2 to supply and meet the cell’s demand. Diffusion is very slow in water, and thus it takes a long time for CO2 to enter the cell. As well as this, CO2 equilibrates with water to form bicarbonate   (HCO3) on a pH dependent basis. The pH of seawater is such that CO2 is mainly available in the form of bicarbonate, potentially representing another barrier to CO2 uptake.

In response to the challenges associated with living in an aqueous environment, algae employ what is known as a biophysical carbon concentrating mechanism to ensure CO2 supply to Rubisco, and hence carbon fixation, is not compromised. They possess a cellular microcompartment within their chloroplast called the pyrenoid, where all of the Rubisco contained within the cell is stored. In most photosynthetic organisms, plants included, Rubisco is spread throughout the entire chloroplast. By localising Rubisco to this single area, CO2 extracted from the surrounding environment can be concentrated in a single, small area.

Chlamydomonas

Stylised cross section of a Chlamydomonas reinhardtii cell showing the pyrenoid and other subcellular components.

As well as this dense aggregation of Rubisco, the carbon concentrating mechanism involves a number of other proteins. CO2 is taken up from the extracellular environment into the algal cell in the form of bicarbonate (HCO3). From there it is shuttled via a series of transporters into the chloroplast, whereupon it gets converted back to CO2 in the thylakoids by an enzyme called carbonic anhydrase, and is then subsequently delivered to the pyrenoid. The idea behind this is that in doing so, the algal cell is able to effectively exclude O2 from the pyrenoid, due to the specific nature of this CO2 delivery, and also ensure a continuous supply of CO2 to Rubisco, given the ubiquity of HCO3 in seawater. This increases the efficiency of photosynthesis, and maximises CO2 fixation.

At the moment, efforts are being made to engineer some aspects of this system into higher plants. The idea behind this is if something resembling a pyrenoid was developed in crop plants, they would be able to better exclude oxygen from the site of carbon fixation (i.e. Rubisco) and increase photosynthetic efficiency. This would translate to substantially increased yields, which is important for food security the world over, particularly in the face of increasing climactic variability and increasing global temperatures, as well as a rapidly increasing population. With more mouths to feed globally, and no concomitant increase in farmable land (if anything, a decrease due to changing weather patterns) increasing yields of key crop species such as rice, maize, cassava and millet represents an important objective for ensuring supply can meet demand globally, as well as making sure that small scale farmholders have the necessary tolerance built into their yields to allow for extreme climactic fluctuations, something that will become increasingly common in the face of climate change and global warming.

Ash Dieback: What’s causing the rise in plant diseases?

by Nathan Smith

Ash Dieback is the latest craze sweeping the nation, and by ‘craze’ I of course mean ‘plant disease’. Like Dutch Elm disease before it, it threatens to destroy thousands of iconic trees and restructure the shape of British woodland.

Caused by the fungus Chalara Fraxinea, Ash Dieback was first noticed in Poland in 1992, though it is thought to have originated somewhere in Asia. It affects the crown of the tree (the bushy top bit) and causes it to die back, although it may not kill a mature tree for a number of years. Even then it is often the case that when a tree is killed it is through an opportunistic infection. C. Fraxinea may not kill, but it does significantly weaken the tree.

AshAttributedSo what’s to blame? Despite fears that the fungus may have come to the UK via infected plants in nurseries, the current view is that it came in the wind from Europe (at least in the majority of cases of the disease). Whilst this may seem good news (particularly for the people running the nurseries), it causes us to reach a depressing conclusionwe cannot ‘stop’ the disease. The reasoning behind this is that most plant diseases can be controlled in the early stages of an outbreak via selective removal of plants. These techniques will probably not work now: we are in the middle of a full blown Europe-wide pandemic and even if we could remove the disease from the UK, it could still come back on the wind from across the seas. This may all sound rather despairing, and work is being done to try and reduce the ecological damage, but the truth is the models still predict that in 10-20 years time the majority of ash trees will be infected, if not already dead.

In a world of increasing globalisation, ensuring plant security from biological threats is almost impossible. Whether from the soil on a backpacker’s shoe or on a tree imported from afar, new microbes will always be brought into environments they have not come across before. Ash Dieback is not the first major tree disease to affect the UK and it’s probably safe to say it will not be the last.

For all those interested in some light reading, please find the link for the Government’s plan on tackling Ash Dieback: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69620/pb13843-chalara-control-plan-121206.pdf