The University of Southampton

Author: Alexanda Hewitt

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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