There is an increase in CO2 emissions caused by the industrial revolution and increasing number of factories. The high amount of emission causes a rise in free CO2 concentration ([CO2]) in air. The effect of this increase on the ecosystem and plants is a subject of curiosity which has led to scientists trying to understand the effect of elevated [CO2]. The first experiment was made by Wittwer (1964) et. al. They investigated horticultural plants under elevated [CO2] in greenhouses (Wittwer and Robb 1964). Then, scientists used open-top chambers to determine the effect of elevated [CO2]. However, the problem was that inside the open-top chamber, the air was more humid, shaded, warmer, and the air movement was changing (Kimball, Pinter et al. 1997). Because of this reason, the Free-Air CO2 Enhancement (FACE) system was developed. In this system, there are vertical and horizontal pipes arranged to make a circle of different diameters. These pipes provide CO2 emission to the circle, to bring it to the desired [CO2]. The FACE system is more reliable than the open-top chamber system, so researchers have preferred to use the FACE system. They have used this system to observe the effect of elevated [CO2] on the ecosystem and plants, which will be the topic that is discussed in this essay.
Firstly, we will investigate the effect of elevated [CO2] on plants’ stomatal conductance (gs), Rubisco response, and photosynthesis response. Initially, when plants are exposed to high [CO2], there is a similar decrease in gs for both C3 and C4 plants (Ainsworth and Long 2005). Stomata give a short-term and a long-term response to make decrease in gs, which depends on duration of high [CO2] exposure. In the short term response, stomatal aperture decreases to decrease gs (Wand, Midgley et al. 1999). In the long term response, stomatal density decreases as well as stomatal aperture, to decrease gs. There is a gene called the HIC (High Carbon Dioxide) gene which is a negative regulator of stomatal development. To prove the decrease in stomatal density, Gray (2000) et al., used mutant HIC plants and control plants. When the control plants were exposed to an elevated [CO2], the decrease in stomatal development was observed . However, in mutant HIC plants the stomatal initiation increased because of disruption in the signal pathway. Another issue about stomatal conductance is its acclimation. Scientists tried to figure this out and they used an equation to determine whether or not there is an acclimation. The equation was developed by Ball et al. (1987). The equation is:
If there is any acclimation about gs, the values of the constants g0 and m must change in the equation. However, according to Medlyn et al. (2001), there wasn’t any change in these values. So, they concluded that gs didn’t show any acclimation to elevated [CO2].
Secondly, the most important issue with plants is the Rubisco response to elevated [CO2]. Almost all of the assimilated carbon passes the Rubisco active site. In the active site of Rubisco, we observe two different reactions. In one reaction, RubP (Ribulose -1,5-bisphosphate) is combined with CO2 to produce 2 molecules of 3-phosphoglyceric acid (3PGA) at the active site of Rubisco. And in other reaction, Rubisco reacts with O2 to yield one molecule of 3PGA and one molecule of 2-phosphoglycollate (2PG). The 3PGA produced by the oxygenase reaction enters the Calvin Benson Cycle, and 2PG enters the photorespiratory pathway where CO2 is released.
This pathway is observed mostly in C3 plants; conversely C4 plants prevent photorespiration by concentrating CO2 at the active site of Rubisco (Sage 2004). CO2 is converted to HCO3– to increase the concentration of CO2 at the active site of Rubisco. Thus, in C4 plants the concentration of CO2 is 15 times higher than C3 plants. This is why C3 plants use N less efficiently even though they invest a higher amount of N in Rubisco. C3 plants use a high amount of N in Rubisco and this results in a much more higher concentration of Rubisco than its substrate, CO2. This condition makes reaction substrate limited, CO2, instead of enzyme limited, Rubisco. By the end of this century, the [CO2] in the atmosphere will be between 500 and 1000 ppm. Therefore, the [CO2] will increase at the active site of Rubisco and, the rising [CO2] will increase the rate and efficiency of photosynthesis in C3 plants because of two reasons:
- Rubisco is substrate limited today, so an increase in [CO2] will increase the carboxylation reaction
- Oxygenation and carboxylation reactions show competitive behaviour. Therefore, elevated CO2 concentration will inhibit the oxygenation reaction and this will cause the reduction in CO2 releasing (Long, Ainsworth et al. 2004).
On the other hand, the effect of [CO2] rising on C4 plants is not forecasted easily because they are already CO2 saturated and they don’t have a photorespiratory effect at the current [CO2]. For this reason, it is predicted that C4 plants will lose their metabolic advantage with respect to C3 plants at elevated [CO2] (Long, Ainsworth et al. 2004).
Until now, we mentioned how Rubisco will behave at the elevated [CO2], but there is also a molecular control mechanism of Rubisco. Now, we will talk about this mechanism. To function properly, Rubisco must be activated by carbamylation of a lysine residue and by the binding of Mg2+. Rubisco activation needs the catalytic chaperone, Rubisco activase enzyme which is ATP dependent. Rubisco is sensitive to the ATP:ADP ratio because of the redox regulation of Rubisco activase. Rubisco is usually fully active under high light conditions at current [CO2]. As [CO2] increases, carbon fixation increases. So plants will need more ATP for RubP regeneration. This will make photosynthesis limited by RubP regeneration instead of being limited by Rubisco.
Thirdly, the elevated [CO2] will affect the photosynthesis response in plants. It is obvious that, elevated [CO2] stimulates photosynthesis rate (A) in C3 plants. There is a significant response of light saturated CO2 uptake (Asat) in C3 plants but it shows differences between C3 species. For example, trees show higher stimulation than legume and shrub (Ainsworth and Rogers 2007). Surprisingly in the research, an increase in photosynthesis rate of C4 plants, at elevated [CO2] was observed. To understand the reason of this increase, Leakey et al. (2006) applied water stress to maize, which is a C4 plant. They found that there isn’t any increase in A value, if there isn’t any water stress (Leakey, Uribelarrea et al. 2006). It shows that the C4 plants are stimulated by drought-stress rather than elevated [CO2]. In C3 plants, the maximum carboxylation rate (Vc,max) and the maximum rate of electron transport (Jmax) are also significantly reduced at elevated [CO2]. But the reduction in Vc,max is approximately double the reduction in Jmax (Ainsworth and Rogers 2007).
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Secondly, the effect of elevated [CO2] on ecology is another important issue. According to research, it is predicted that at the end of this century the [CO2] will be between 500-1000 ppm. So the effect of this rise on photosynthetic acclimation thus, nitrogen uptake and carbohydrates accumulation, is the subject of curiosity. According to research, the photosynthetic acclimation is achieved through the accumulation of carbohydrates, and decrease in N concentration ([N]) (Nowak, Ellsworth et al. 2004). The decrease in [N] will cause the reduction of protein content. This reduction in protein content is not observed generally in the proteins in the leaf, however there is a significant decrease in the Rubisco content. (Spreitzer and Salvucci 2002). Additionally, because of the carbohydrate accumulation there will be huge increase in the amount of sugar and starch. Thus, we will obtain high energetic plants. Another issue about the elevated [CO2] is the growth and yield of plants. It is said that elevated [CO2] will result in taller plants which have bigger stems ,and more leaves and branches; however the crop yield increase is not as much as estimated (Ainsworth and Long 2005). Furthermore, the effect of elevated O3 concentration ([O3]) with elevated [CO2] is also a field of interest in ecology. It is known that elevated [O3] causes significant decrease in growth and yield of many crops, and it is predicted that elevated [CO2] will diminish the negative effect of elevated [O3] (Ainsworth 2008). However, all of these results depend on the functional groups and where the experiments took place. They show many differences according to differences in conditions and species. Therefore there needs to be more research on FACE experiments to better understand the response of different plants to different conditions.
As a result, FACE experiments are very important to learn the effect of elevated [CO2] on plants and the ecosystem but the experiments are not enough. There needs to be more specific experiments to understand the impacts of the increase of free-air [CO2].
Ainsworth, E. A. and S. P. Long (2005). “What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2.” New Phytologist 165(2): 351-372.
Biggins, J., J. T. Ball, et al. (1987). A Model Predicting Stomatal Conductance and its Contribution to the Control of Photosynthesis under Different Environmental Conditions. Progress in Photosynthesis Research, Springer Netherlands: 221-224.
Kimball, B., P. Pinter, et al. (1997). “Comparisons of responses of vegetation to elevated carbon dioxide in free-air and open-top chamber facilities.” Advances in carbon dioxide effects research(advancesincarbo): 113-130.
Leakey, A. D. B., M. Uribelarrea, et al. (2006). “Photosynthesis, Productivity, and Yield of Maize Are Not Affected by Open-Air Elevation of CO2 Concentration in the Absence of Drought.” Plant Physiology 140(2): 779-790.
Nowak, R. S., D. S. Ellsworth, et al. (2004). “Functional responses of plants to elevated atmospheric CO2–do photosynthetic and productivity data from FACE experiments support early predictions?” New phytologist 162(2): 253-280.
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