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Tag: Stomata

Stomata – an ancient insight into a modern problem

Posted on by Anonymous

Findings from a pharaoh

What can an Egyptian pharaoh from 1300 BC tell us about how plants will respond to 21st century environmental change?

Olive leaves in Tutankhamun’s tomb allowed Beerling & Chaloner to show that the number of stomata declined in response to increasing CO2. Photo credit – Discovery Times Square Museum (Available: http://www.discoverytsx.com/exhibitions/kingtut).
Olive leaves in Tutankhamun’s tomb allowed Beerling & Chaloner to show that the number of stomata declines in response to increasing levels of carbon dioxide.
Photo credit – Discovery Times Square Museum (Available: http://www.discoverytsx.com/exhibitions/kingtut).

 

When Howard Carter discovered King Tutankhamun’s tomb in 1922, the boy pharaoh’s treasures captivated the world. Amidst such splendor, olive leaves found in the burial chamber were overlooked. But, in 1993, two botanists, David Beerling and William Chaloner decided to compare these leaves with their modern counterparts. They found that olive leaves grown today have 33% fewer stomata (tiny ‘pores’ on the leaf surface) than they did in 1300 BC. Beerling & Chaloner linked this decline in stomata to increasing carbon dioxide (CO2) levels.

Stomata – microscopic pores on the surface of a leaf let plants ‘breathe’.
Photo credit – University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).

 

Powerful pores

Stomata (singular – stoma) are vital to plant function. Plants use these pores to ‘breathe’. Stomata allow CO2 to enter a leaf, where it is used in photosynthesis to generate sugars, which the plant needs to survive and grow. The trade-off is stomata also let water escape. Plants, therefore, need to balance CO2 gain and water loss.

Stomata control a vital trade-off between carbon dioxide gain and water loss.
Photo credit – University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).

 

Rising CO2 – a certainty in an uncertain world

CO2 is the most well known of the greenhouse gases. These gases trap the Sun’s heat within Earth’s atmosphere, causing our planet to warm up. We produce CO2 every day when we burn oil, gas and coal to drive our cars, heat our homes and power our factories.

This has caused CO2 levels to increase by 40% since the industrial revolution started in 1750 (IPCC, 2014). CO2 levels are predicted to increase more rapidly in the future as populations and economies grow (IPCC, 2014).

With some exceptions (Bettarini et al., 1998), most scientists agree that this rise in CO2 will cause a decline in stomatal density (Woodward & Kelly, 1995; Lin et al., 2001). Paoletti et al. (1998) showed that stomata numbers will decline until CO2 levels reach 750 μmol mol-1.

But, how will this change impact plant function?

Some plants naturally have few stomata because they have a change in the gene that controls stomatal development (Gray et al., 2000). As CO2 levels rise, each stoma will be able to bring in more CO2. So, a leaf will need fewer stomata to bring in the same amount of CO2.

A leaf with fewer stomata will lose less water. As a result, the plant will have a higher water use efficiency and so has an advantage over others (Drake et al., 1997). With time, CO2 will act as a selection pressure and these plants will become more common.

Stomatal density will decline as CO2 levels rise. Photo credit - University of California Museum of Paleontology's Understanding Evolution (http://evolution.berkeley.edu).
Stomatal density will decline as carbon dioxide levels rise.
Photo credit – University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).

 

Future plants will be better able to conserve water, making them more tolerant to periods of water stress (Woodward, 1987). This is just as well as droughts are predicted to become more common (IPCC, 2014). It will also mean we will be able to grow the same amount of crops, but using less water (Drake et al., 1997).

Stomata transfer water from the land to the atmosphere. If plants have fewer stomata, less water will be transferred, which will cause increased air temperatures and runoff (Kürschner et al., 1997).

The olive leaves from King Tut’s tomb showed that stomata respond to CO2 levels. With rises in CO2 predicted for the 21st century, perhaps the insights provided by these leaves are the boy pharaoh’s most valuable treasure.

 

References

Beerling, D.J. & Chaloner, W.G. (1993) Stomatal density responses of Egyptian Olea europaea L. leaves to CO2 change since 1327 BC. Annals of Botany. 71(5), 431-435.

Bettarini, I., Vaccari, F.P. & Miglietta, F. (1998) Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy. Global Change Biology. 4(1), 17-22.

Drake, B.G., Gonzàlez-Meler, M.A. & Long, S.P. (1997) More efficient plants: a consequence of rising atmospheric CO2? Annual review of plant biology. 48(1), 609-639.

Gray, J.E., Holroyd, G.H., Van Der Lee, F.M., Bahrami, A.R., Sijmons, P.C., Woodward, F.I., Schuch, W. & Hetherington, A.M. (2000) The HIC signalling pathway links CO2 perception to stomatal development. Nature. 408(6813), 713-716.

IPCC (2014) Summary for Policymakers. In: Pachauri. R.K. & Meyer, L.A. (eds.) Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Geneva, Switzerland, pp. 1-31.

Kürschner, W.M., Wagner, F., Visscher, E.H. & Visscher, H. (1997) Predicting the response of leaf stomatal frequency to a future CO2-enriched atmosphere: constraints from historical observations. Geologische Rundschau. 86(2), 512-517.

Lin, J., Jach, M.E. & Ceulemans, R. (2001) Stomatal density and needle anatomy of Scots pine (Pinus sylvestris) are affected by elevated CO2. New Phytologist. 150(3), 665-674.

Paoletti, E., Nourrisson, G., Garrec, J.P. & Raschi, A. (1998) Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments. Plant, Cell & Environment. 21(10), 1071-1075.

Woodward, F.I. & Kelly, C.K. (1995) The influence of CO2 concentration on stomatal density. New Phytologist. 131(3), 311-327.

Woodward, F.I. (1987) Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels. Nature. 327(6123), 617-618.

 

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What can an Egyptian King tell us about climate change?

Posted on by Carla Broom

Our plants are hugely affected by carbon dioxide levels in our atmosphere. Green plants absorb CO2 to produce sugars for growth (and oxygen for us!) in a process known as photosynthesis, meaning CO2 levels can hugely affect plant function.

All green flowers take in carbon dioxide and convert it to sugars and oxygen
All green plants absorb CO2 and produce sugars and oxygen

CO2 levels are currently at 406 parts per million (ppm), which may not seem high, but have risen 21% over the last 230 years (Woodward, 1987). This fluctuated constantly throughout time, varying from 280 to 370ppm over the last 24 million years (Van Der Burgh et al, 1993), but now for the first time exceeds 400ppm (Khatiwala et al, 2009).

CO2, oxygen and water are absorbed and released through small pores on the underside of leaves known as stomata, protected by guard cells which open and close the pore. This is affected by conditions such as hormones, light, and- you guessed it- atmospheric CO2 concentration. Stomata allow plants to survive stress, for example retaining water during extreme heat and drought (Hetherington & Woodward, 2003).

Microscope image of two kinds of stomata on the underside of leaves
Microscope image of stomata on the underside of leaves (Hetherington & Woodward, 2003)

How do we investigate change in stomata over time? We can find out what plants were like thousands of years ago- that’s where the Egyptian King Tutankhamun comes in. Olive leaves from his tomb (from 1327 BC) were taken, and the amount of stomata on the underside counted and compared to Egyptian olive samples from 332BC, 1818AD, 1978AD and 1991AD (Beerling & Chaloner, 1993a).

The tomb of King Tutankhamun, with ancient olive leaves around the headpiece
The tomb of King Tutankhamun, with olive leaves around the headpiece (Beerling & Chaloner, 1993a).

As CO2 increases, the number of stomata decrease (Beerling & Chaloner, 1993a). This is also seen in other species, such as oak (Van der Burgh et al, 1993) and pine (Van de Water et al, 2007), but their methods weren’t as creative! The first ever study to show this change found a 40% decrease in number of stomata over the last 230 years (Woodward, 1987).

Why does this happen? CO2 affects the genetic make-up of plants, reducing the number of cells developing into stomata. Plants are likely adapting to rising temperatures and CO2 levels by decreasing the amount of pores to reduce water loss, improving their water use efficiency (Beerling & Chaloner, 1993b).

Is this change a problem for plants? The short term effects of more CO2 can be beneficial, increasing photosynthesis and growth and therefore  yield (Osborne et al, 1997), however the long term effects are not so good. Fewer stomata decreases CO2 uptake, reducing growth and leading to higher atmospheric CO2 (Reddy et al, 2004). Although the plants retain more water, if CO2 levels and temperature decrease again, plants with fewer stomata will have reduced water use efficiency (Woodward, 1987) as they will not be able to exchange as well. Less sugar production also reduces metabolism, therefore CO2 intake is inhibited even further (Flexas & Medrano, 2002) in a cycle known as negative feedback; a reduction in one factor causes a further reduction in something else.

This shows us that plants have changed greatly over thousands of years to adapt to the increasing carbon dioxide levels around them, which could have long-term negative effects on both plants and other life. The environment around us is changing with the atmosphere, and human-caused CO2 increase should not be ignored as it affects many things, no matter how small.

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References

Beerling, D. J., Chaloner, W. G. 1993 a. Stomatal Density Responses of Egyptian Olea europaea L. Leaves to CO2 Change to 1327BC. Annals of Botany, 71: 431-435

Beerling, D. J., Chaloner, W. G. 1993 b. Evolutionary Responses of Stomatal Density to Global CO2 Change. Biological Journal of the Linnean Society, 48: 343-353

Flexas, J., Medrano, H. 2002. Drought Inhibition of Photosynthesis in C3 Plants: Stomatal and non-Stomatal Limitations Revised. Annals of Botany, 89(2): 183-189

Hetherington, A. M., Woodward, F. I. 2003. The Role of Stomata in Sensing and Driving Environmental Change. Nature, 424: 901-908

Khatiwala, S., Primeau, F., Hall, T. 2009. Reconstruction of the History of Anthropogenic CO2 Concentrations in the Ocean. Nature, 462: 346-350

Osborne, C. P., Drake, B. G., LaRoche, J., Long, S. P. 1997. Does Long Term Elevation of CO2 Concentration Increase Photosynthesis in Forest Floor Vegetation? (Indiana Strawberry in a Maryland Forest). Plant Physiology, 114(1): 337-344

Reddy, A. R., Chaitanya, K. V., Vivekanandan, M. 2004. Drought-Induced Responses of Photosynthesis and Antioxidant Metabolism in Higher Plants. Journal of Plant Physiology, 161(11): 1189-1202

Van Der Burgh, J., Visscher, H., Dilcher, D., Kürschner, W. M. 1993. Paleoatmospheric Signatures in Neogene Fossil Leaves. Science, 260(5115): 1788-1790

Van de Water, P. K., Leavitt, S. W., Betancourt, J. L. 2007. Trends in Stomatal Density and 13C/12C Ratios of Pinus flexilis Needles During the last Glacial-Interglacial Cycle. Science, 264: 239-243

Woodward, F. I. 1987. Stomatal Numbers are Sensitive to Increases in CO2 from pre-Industrial Levels. Nature, 327: 617-618



Plant Pores: How Carbon Dioxide Changes Stomata

Posted on by Alexanda Hewitt

 

Humans change the world around them. From farms to factories, that’s all on us. But what about a deeper level of change, happening to the parts of arguably our most important friends on this planet. The plants.

All types of plant have small holes, or pores, on their leaves called stomata. Each individual stoma is bound by a pair of cells called guard cells (see Figure 1), that help to control the uptake and release of gases (most importantly carbon dioxide (CO2) and water vapour) between the inside of the leaf and the atmosphere(1). This gas exchange as it were is regulated by the number of stomata that form on the leaf (the stomatal density) and by how open (the aperture) the stomatal pores are kept by the guard cells. The stomatal density and aperture are themselves influenced by environmental conditions such as light intensity and CO2 concentration(1).

Figure 1: A microscope image of a stoma. The pore is visible in the centre of the image, whilst the two guard cells (although they look like one circular cell surrounding the pore) can be seen either side of the pore(2).
Figure 1: A microscope image of a stoma. The pore is visible in the centre of the image, whilst the two guard cells (although they look like one circular cell surrounding the pore) can be seen either side of the pore(2).

CO2 concentration in the atmosphere is particularly important for modern day plants as although CO2 levels have fluctuated considerably over the last 400 million years(3), in the last 250 years they have risen by nearly 40%, a significant increase at a fast rate(4). Plants have thus had to go from living in some relatively low CO2 environments to living in a higher CO2 one(3). Experimental CO2 increases have shown to change the stomatal density by different amounts in different types of plant, but with an average of an 11% reduction with a doubling of the CO2 concentration, regardless of the starting density of the stomata(1). Furthermore, higher than normal CO2 levels in the atmosphere result in the closure of stomatal pores in plants(5).

These changes generally lead to a decrease (of between 21% and 40% in some studies(6)) in the amount of gas exchange between the plant and the atmosphere(1) but even this has other influences acting on it. The response to CO2 changes has been shown to be significantly stronger in younger trees, in non-evergreen trees and in trees that do not have enough water compared to those that do not have enough nutrients(6). This however, seems to be affected by the length of time that the leaves are in the higher than normal CO2 conditions with some leaves returning to a “normal” stomatal density after 2 years in a higher CO2 environment(7).

What does any of this actually do to the plant though? In some cases there has been an increased maximum rate of photosynthesis (the process by which plants make sugar from CO2 and water) at these higher CO2 levels(1). However, other studies have shown plants with the highest stomatal densities obtained the highest gas exchange rate and rate of photosynthesis, contrary to the previous results(8). Effectively we need more experiments to take place to get an accurate answer. What we do know is that we are changing the stomata on plants, be it for better or worse is yet to be decided.

 

References

  1. Hetherington AM, Woodward FI. The role of stomata in sensing and driving environmental change. Nature. 2003 Aug 21;424(6951):901-8.
  2. Ferry RJ. Stomata, Subsidiary Cells, and Implications. North American native orchid journal. 2008:168.
  3. Woodward FI. Do plants really need stomata?. Journal of Experimental Botany. 1998 Mar 1:471-80.
  4. Singh UB, Ahluwalia AS. Microalgae: a promising tool for carbon sequestration. Mitigation and Adaptation Strategies for Global Change. 2013 Jan 1;18(1):73-95.
  5. Engineer CB, Hashimoto-Sugimoto M, Negi J, Israelsson-Nordström M, Azoulay-Shemer T, Rappel WJ, Iba K, Schroeder JI. CO2 sensing and CO2 regulation of stomatal conductance: advances and open questions. Trends in plant science. 2016 Jan 31;21(1):16-30.
  6. Medlyn BE, Barton CV, Broadmeadow MS, Ceulemans R, De Angelis P, Forstreuter M, Freeman M, Jackson SB, Kellomäki S, Laitat E, Rey A. Stomatal conductance of forest species after long‐term exposure to elevated CO2 concentration: A synthesis. New Phytologist. 2001 Feb 1;149(2):247-64.
  7. Ainsworth EA, Rogers A. The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions. Plant, cell & environment. 2007 Mar 1;30(3):258-70.
  8. Woodward FI, Lake JA, Quick WP. Stomatal development and CO2: ecological consequences. New Phytologist. 2002 Mar 1;153(3):477-84.

 

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