How is Climate Change Affecting Stomatal Conductance?

Meghan Lepel. Edited by Autumn Berlied

Photosynthesis 

Photosynthesis is the process by which plants use the energy from the sun to convert carbon dioxide and water into oxygen and sugars. The chlorophyll in the leaves of green plants absorb the sunlight and carbon dioxide and combines these with water molecules and then converts them into glucose and oxygen. This process is shown in figure 1. The process of photosynthesis can be represented by this equation: 6CO2 + 6H2O → C6H12O6 + 6O2. Photosynthesis is absolutely essential for life on earth as this process is the number one producer of oxygen in the atmosphere. Without photosynthesis, there would be very little to no oxygen in our atmosphere (2). 

Figure 1: The photosynthesis process. "File:Photosynthesis en.svg" by At09kg : original Wattcle : vector graphics is licensed under CC BY-SA 4.0 

Transpiration 

Many people do not realize that 98% of the energy created during the process of photosynthesis is used in transpiration (1). In figure 2, you can see that transpiration occurs when water moves up the plant via the roots and vascular system. 

Figure 2: Transpiration overview. In image 1, you can see the water from the ground being take up by the roots. In image 2, you can see how turgor pressure is pulling up the water molecules through the xylem. In image 3, you can see the water evaporating through the stoma. Source: Wikimedia Commons 

 The water will evaporate from the surface of the leaf via tiny pore-like openings on the underside of the leaf called stomata (shown in figure 3). Transpiration serves two main functions: pumping water and nutrients throughout the plant and regulating the temperature of the plant (1). Transpiration is also a key component in the water cycle and therefore, it’s important to know and predict whether transpiration rates will be affected in the future from climate change (Kirschbaum & McMillan, 2018). 

Figure 3: Stoma in a tomato leaf. Source: By Photohound - http://remf.dartmouth.edu/images/botanicalLeafSEM/source/16.htmlLicense on site: http://remf.dartmouth.edu/imagesindex.htmlThis JPG graphic was created with GIMP., Public Domain, https://commons.wikimedia.org/w/index.php?curid=3248530  

Photosynthesis & Transpiration: How are They Connected? 

One physical factor of the plant that connects both photosynthesis and transpiration are stoma or stomata. During photosynthesis, the stomata will close themselves for the purpose of controlling water loss so that this process can be done in the most efficient way possible (4). Stomatal conductance is a central factor in the transpiration process due to their ability to open and close to allow water evaporation from the leaves (1). Stomatal conductance is highly affected by three factors: light, temperature, and intracellular CO2 concentration through the process of photosynthesis (6). Furthermore, since the stomata play such an important role in both photosynthesis and transpiration processes and now that we know that these processes are also essential for life on earth, how will climate change fit into these scenarios? How will rising global temperatures and increased atmospheric CO2 levels effect stomatal conductance? 

The Rising Factors of Climate Change and Stomatal Conductance 

Climate change and other anthropogenic factors has provided the world with higher concentration of atmospheric carbon dioxide and rising global temperatures. It is estimated from the Intergovernmental Panel on Climate Change (3) that atmospheric CO2 has been increasing at an annual rate of 1.8 ppm per year over the last four decades (shown in figure 4). 

Figure 4: The keeling curve. This graph is showing the rising carbon dioxide concentrations from 1960 to 2010. Source: Wikimedia Commons. 

 While we know that increased CO2 in the atmosphere will increase the rate of photosynthesis in plants (5), transpiration rates will decrease and will become more efficient by partial stomatal closure which will result in the decrease of water loss out of the leaves (Kirschbaum & McMillan, 2018). Long story short, as carbon dioxide levels increase, the stomata will not open as wide and both processes of photosynthesis and transpiration will increase the energy efficiency of the plant itself.  

The global average temperatures have been rising at a rate of 1.7 degrees Celsius per century since 1970 (3). A comparison of the CO2 concentration in the atmosphere and the average surface temperature of the earth is shown in figure 5.  

Figure 5: A graph showing the carbon dioxide concentration and average surface temperature from 1880 to 2000. Source: mattlemmon via flickr 

Transpiration rates and stomatal conductance are expected to increase as the temperature increases because warmer air can hold more water (Kirschbaum & McMillan, 2018). Therefore, as the transpiration process increases and becomes more frequent, the overall energy efficiency of the plant will decrease. On the other hand, increased temperatures on the stomatal conductance via the process of photosynthesis and is an indirect relationship. Warmer temperatures will increase stomatal conduction due to the leaf temperature increasing which causes the stomata to open more frequently (7). If the stomata open more frequently, the rate of photosynthesis will decrease because water concentration will be lost. Since water is a key contributor in the process of photosynthesis, the rate of photosynthesis will decrease. However, since transpiration rates are increasing along with higher temperatures, this will cool the plant down so the stomata will be able to remain closed which will increase the efficiency of the two processes and balance out the whole system.  

References Cited:  

Brawner, M.. Transpiration and Why it Matters. Harlequins Gardens. https://harlequinsgardens.com/transpiration-and-why-it-matters/#:~:text=The%20water%2C%20warmed%20by%20the,to%20the%20leaves%20for%20photosynthesis 

Brenner, L. (2019, November 22). Why Is Photosynthesis Important for All Organisms? Sciencing. https://sciencing.com/photosynthesis-important-organisms-6389083.html  

IPCC, 2018: Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. 

Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)]. In Press. 

Jones, H. G. (1998). Stomatal control of photosynthesis and transpiration. Journal of Experimental Botany, 49(90001), 387–398. https://doi.org/10.1093/jexbot/49.suppl_1.387  

Robinson-Brown, A. (2017). The Effect of Concentration of CO2 on the Average Rate of Photosynthesis in Spinach Leaf Disks. Journal of the South Carolina Academy of Science, 15(2). 

Tuzet, A., Perrier, A., & Leuning, R. (2003). A coupled model of stomatal conductance, photosynthesis and transpiration. Plant, Cell & Environment, 26(7), 1097–1116. https://doi.org/10.1046/j.1365-3040.2003.01035.x  

Urban, J., Ingwers, M. W., McGuire, M. A., & Teskey, R. O. (2017). Increase in leaf temperature opens stomata and decouples net photosynthesis from stomatal conductance in Pinus taeda and Populus deltoides x nigra. Journal of Experimental Botany, 68(7), 1757–1767. https://doi.org/10.1093/jxb/erx052