A blog post by an old friend a couple of months ago (Ancient CO2 estimates worry climatologists), at this time of all times, set me thinking about how Earth has changed over time and how it is changing today in what is often described as a new, human-influenced geological epoch – the Anthropocene. One of the biggest causes of these recent changes, (unless you believe Scott Pruitt, Donald Trump’s appointed Administrator of the Environmental Protection Agency) is increasing levels of Carbon dioxide in the atmosphere – from 2-300 parts per million in pre-industrial times to around 400 ppm today.
Terrestrial plants have tiny, variable pores called stomata on the surface of their leaves, through which they take in the carbon dioxide they need for photosynthesis and growth and through which water is also lost (see Plants get stressed too and More thirsty plants). The abundance of these stomatal pores on a leaf reflects the environment in which the plant finds itself; the logic goes that, when CO2 levels are high, the plant doesn’t need so many stomata to obtain enough CO2 for photosynthesis whereas, when levels drop, the density of stomata needs to increase. Of course many other factors affect stomatal density too (light levels, availability of water, temperature and so on) but this sensitivity to CO2 levels makes stomata useful to palaeoecologists interested in working out how much CO2 may have been in the atmosphere in the distant past. Over the years, estimates of CO2 levels based on stomatal density have been further refined by taking other factors such the shape of the stomata and carbon isotope ratios in the fossil leaves into account.
The very first plants on land evolved from green algae over 400 million years ago. In their marine home, algae had no need of stomata to obtain the CO2 they needed – CO2 can enter cells submerged in water by simple diffusion. The simplest land plants which evolved from these algae, mosses and liverworts, are small and are restricted to damp environments where they too, can obtain the CO2 they need by diffusion across cell walls and membranes. Though they don’t have stomata on their leaves, mosses have stomatal-like pores on their spore capsules which are thought to help them regulate water levels as these dry out and split to disperse the mature spores inside (Pennisi, 2017).
Moss spore capsules – green as spores develop and brown as spores mature and are ready to be shed
It wasn’t until the first fern-like plants appeared on the scene, around 350 Ma, that stomata were needed to regulate water loss from leaves. Unlike mosses and liverworts, ferns can grow big (think tree ferns). To grow to this size, ferns needed both vascular tissue (a plumbing system) to transport water and food around the plant and a way of taking up CO2 from the atmosphere.
So, how did stomata evolve? The same molecular pathways appear to regulate the formation and spacing of stomata in mosses and in the model higher plant Arabidopsis thaliana, despite their different roles – apparent evidence for a single origin in a common ancestor. Perhaps stomata evolved to aid in reproduction and were then repurposed for gas exchange? One fly in the ointment with this neat story is that, in some ferns, stomatal are not regulated by the plant hormone abscisic acid (ABA) as they are Arabidopsis and other higher plants, so they may have had a separate evolutionary origin. And we are still not really any closer to knowing how they first evolved in moss capsules….
During the Carboniferous period, when stomata were first being used for gas exchange, CO2 levels are believed to have been around twice today’s elevated levels – 800 ppm, on average. Much low-lying, swampy land was covered in tropical vegetation, with the first proper woody tissue, and many of the coal reserves which fuelled the industrial revolution were laid down at this time.
Fragments of Carboniferous plant material from mine waste dumped on the County Durham coast at Seaham
Towards the end of the period, though, an event called the Carboniferous Rainforest Collapse occurred – an extinction event, probably caused by a rapidly cooling climate. Hot humid conditions became cold and arid as a result of intense glaciation and a drop in sea levels. Atmospheric CO2 levels crashed to around 200 ppm. This would have given a great advantage to plants with higher numbers of efficient stomata.
The density of veins in the leaves is also important and is closely linked to stomatal density – after all, it’s no good having more stomata if you can’t supply leaves with enough water to allow those stomata to open and take in CO2 and if you can’t transport away from the leaves any additional sugars the plant is able to produce (Gerald et al., 2016). It has been suggested that evolution, together, of increased stomatal and leaf vein density might have allowed the flowering plants (angiosperms) to out-compete other groups and move into drier habitats as CO2 levels dropped during the Cretaceous period, around 145 million years ago. Later still, around 60 Ma, grasses appear in the fossil record – they have more efficient stomata than other plant groups, with subsidiary cells which aid the guard cells in opening faster and wider. It’s no coincidence that they appeared when the climate was becoming drier and CO2 levels were dropping further, towards the end of the Cretaceous.
Studies like these give us a fascinating insight into how plants have evolved into the well-adapted, productive organisms we know today but, arguably more importantly, they also help us to understand how plants might respond in the future to ever increasing levels of atmospheric CO2. Even Donald Trump should be thinking about this if he doesn’t want Americans to go hungry!
Gerald, C.E.F., Porter A.S., Yiotis C., Elliott-Kingston C. & McElwain J.C. (2016) Co-ordination in Morphological Leaf Traits of Early Diverging Angiosperms Is Maintained Following Exposure to Experimental Palaeo-atmospheric Conditions of Sub-ambient O2 and Elevated CO2. Frontiers in Plant Science, 7, Article 1368.
Pennisi, E. (2017) How plants learned to breathe. Science, 355, 1110-1111.