Climate Change and Grasses with C4 photosynthetic pathway

Climate Change and Grasses with C₄ photosynthetic pathway

By: FiorellaEduardo-Palomino

After expending the last 5-years learning and working with high Andean grasslands of Perú, that are mainly composed by grasses with the C₃ photosynthetic pathway such as Festucas (ex: Festuca dolichophylla – “chilligua”) or Calamagrostis (that some are now Deyeuxia), was for me a nice surprise learned in my class of Grassland Ecology at SIUC, that grasslands of the Great Plains in the United States are mainly formed by grasses with the C₄ photosynthetic pathway such as Andropogon gerardii (Big bluestem). This fact had sense because grasses in those areas should be very water efficient. I was working at that time with data of seasonally dry forests in Perú and I found that grasses in the herbaceous strata of those forests also have mainly the C₄ photosynthetic pathway.

Grasses with C₃ photosynthetic pathway show an ecological advantage in response to climate change because they can use the high amount of CO₂ to increase the photosynthesis process, in consequence, they may increase the biomass production. On the other side, grasses that evolved in the C₄ photosynthetic pathway did not show an ecological advantage, yet. Part of my midterm exam was to find any advantage using the knowledge we acquire in class and new publications, so I decided to share my answer with you. In this post, I will narrate what I found.

The carbon dioxide (CO₂) starvation hypothesis could explain the origin of the C₄ pathway approx. 30 million years ago (Oligocene) due to the reduced CO₂ concentration in the atmosphere that also occasioned aridity in tropical areas (Pinto, 2014; Sage, 2012; Gibson, 2009). In a climate change scenario, it is expected the opposite an increase of carbon dioxide (CO₂) in the atmosphere. Experiments with grasses such as Poa pratensis (C₃) and Andropogon gerardii (C₄) showed that the C₃ photosynthetic pathway grass increased its photosynthesis rate in high concentrations of CO₂ at the same rate as the C₄ pathway grass in normal conditions (Nie et al. 1992 in Kirkham, 2011). As it is presented in figure 1, the C₄ pathway did not show any change in photosynthesis rates in the presence of high concentrations of CO₂. However, the protein content of grass with the C₃ pathway decreased in a high concentration of CO₂ (Morgan et al., 2004) while the grass with the C₄ pathway maintained protein content. This behavior could be explained by the photosynthetic nitrogen use efficiency that some C₄ grasses exhibited as in the example of Panicum milioides (Pinto et al. 2014) shown in figure 2.

	Figure 1. The graphic shows the variation in the photosynthetic rate of Poa pratensis (C3) and Andropogon gerardii (C4) in low (354 µmol/mol) and high (715 µmol/mol) atmospheric carbon dioxide concentrations and low (no water) and high (field capacity) concentration of water in the soil. Means of 8-16 measurements (Nie et al. 1992 in Kirkham, 2011). In low or high presence of water in the soil the photosynthesis rate of Poa pratensis (C3) increase in high concentrations of CO2 and Andropogon gerardii (C4) remain the same in both environments.
Figure 2. Growth parameters. Light-satured photosynthesis, Asat (A), stomatal conductance, gs (B), photosynthetic water use efficiency, PWUE (C), leaf N per unit dry mass, [N]mass (D), photosynthetic nitrogen use efficiency, PNUE (E), and plant dry mass, PDM (F) of 10 grass species belonging to C3, C3–C4, and C4 (NAD-ME, PCK, NADP-ME) photosynthetic types grown at glacial (180 μl l–1, open columns) or ambient (400 μl L–1, filled columns) [CO2].

Values are means ±SE of species within each photosynthetic type (Pinto et al. 2014). Panicum milioides (C₃–C₄) exhibited elevated photosynthetic nitrogen use efficiency compared with Panicum bisulcatum (C₃) in ambient CO₂ and less in Glacial CO₂.

The protein content of plants with the C₄ pathway may be a comparative advantage because it will be more digestible by mammals than plants with the C3 pathway. Mammals that feed on C₄ pathway grasses might take advantage of the protein content. One example is presented in the article of Griffith et al. (2017), they intended an approximation on the historic relation between herbivores and vegetation on the grasslands of USA over the years using the carbon isotope C δ¹³. They found that bison (Figure 3) might have a strong preference for grass C₄ over the years, mentioning the impressive level of stability between the C₄ grassland and the community of bison when they measure the carbon isotope (C δ¹³) concentrations in bison tissue (1739 CE) and C₃ as well as C₄ grasses composition in North American grasslands.

 

Figure 3. A nursery group of bison cows and calves makes its way through Lamar Valley. Due to high rates of survival and reproduction, the bison population increases by 10 to 17% every year: ten times faster than the human population grows worldwide. Photo: NPS  / Neal Herbert. 

Source: https://www.nps.gov/yell/learn/management/bison-management.htm

Another study which includes in vitro digestibility between Stipa comata (C₃) and Bouteloua gracilis (C₄) was presented by Morgan et al. (2004). They found that the digestibility in vitro of Stipa comata (C₃) that is also the dominant grass of the Central Plains Experimental Range in Colorado USA, exposed to an elevated CO₂ environment was lowest than Bouteloua gracilis (C₄) (Figure 4). This result implies a reduction of the forage quality in grassland dominated by C₃ grasses. Grazing animals tend to select high-quality species and leave on the field low-quality species. This behavior might cause concern due to grasslands used for grazing are dominated by C₃ species, might become a less palatable state under climate change (Morgan et al. 2004).

Figure 4. In vitro dry-matter digestibility of Pascopyrum smithii, Stipa comata and Bouteloua gracilis (averages over two years) in ambient and elevated CO2 (360 and 720 µmol/mol) open-top chambers and in unchambered controls on the shortgrass steppe of eastern Colorado. Means with different letters are significantly different using Tukey’s HSD test (P<0.05); error bars represent ± 1 SE (Morgan et al. 2004).

More studies using digestibility in vivo should be developed to support the idea of the ecological advantage that might have the C₄ pathway grasses in response to climate change. Studies that consider a long-term evaluation of grasses to characterize the species response to climate change could be useful too because they can help develop predictions of the future behavior of C₄ grasses (Zelikova et al, 2014).

This topic is very interesting for me so I would like to know What do you think about my answer and If you know any advantage that C₄ grasses might have in a scenario of Climate change. Please feel free to leave your comments in the section comment and I hope you follow my blog to read the next post. I want to thank also Dr. David Gibson for edit my exam, so actually, he edited part of this post.

References

Gibson, D. (2009) Grasses and Grassland Ecology. Oxford University press.  Pg. 58-73.

Griffith DM, Cotton JM, Powell RL, Sheldon ND, Still CJ. (2017). Multi-century stasis in C₃ and C₄ grass distributions across the contiguous United States since the industrial revolution. J Biogeogr. 44 (11): 2564–2574.

Kirkham, M.B. (2016). Elevated Carbon Dioxide Impacts on soils and water relations. C.R.C. Press. Pg. 7, 8.

Morgan, J. A., Mosier, A. R., Milchunas, D. G., LeCain, D. R., Nelson, J. A. and Parton, W. J. (2004), CO2 Enhances Productivity, Alters Species Composition, and Reduces Digestibility of Shortgrass Steppe Vegetation. Ecological Applications, 14: 208–219.

Pinto, H. Sharwood, R. Tissue, D. Ghannoum, O. (2014). Photosynthesis of C₃, C₃-C₄, and C₄ grasses al glacial CO₂. Journal of Experimental Botany, 65(13): 3669-3681.

Sage RF, Sage TL, Kocacinar F. (2012). Photorespiration and the evolution of C4 photosynthesis. Annual Review of Plant Biology 63, 19–47.

Zelikova, T. J., Blumenthal, D. M., Williams, D. G., Souza, L., LeCain, D. R., Morgan, J., & Pendall, E. (2014). Long-term exposure to elevated CO₂ enhances plant community stability by suppressing dominant plant species in a mixed-grass prairie. Proceedings of the National Academy of Sciences of the United States of America, 111(43), 15456–15461.