In this essay we will briefly describe the concepts of Global warming and the Greenhouse effect, photosynthesis and plants various carbon fixation pathways. We will then answer the question, does increased atmospheric CO2 concentrations have any effect on a plants ability to photosynthesize and will this give plants using the C3 pathway any advantage over plants using the C4 pathway.
We will put forward a thesis that: Increased atmospheric CO2 does enhance a C3 plants photosynthetic ability, but have negligible effects on C4. However a plants competiveness is primarily determined by environmental parameters: Therefore C3 plants advantage in an CO2 enriched atmosphere may be countered by Climate changes varying future conditions and that no one C3 or C4 pathway will have complete dominance but competiveness is ecosystem and species specific.
C4 ability to withstand Heat and direct Sunlight and less vigorous water requirements may come to dominate mid latitude ecosystems. as temperature rises and increasing destruction of rainforests and altered fire regimes by human influence, grasslands, open plains and savannas may start to dominate the mid latitudes encroaching towards the high latitudes.
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With the increased CO2 in the atmosphere and C3 plants enhanced photosynthetic ability they may come to dominate the higher latitudes and temperate zones where temperature, light, and water availability is less of an issue.
No one carbon fixation pathway has complete advantage as a whole, but rather competiveness between C3/C4 is entirely species and environmental condition specific.
That ecosystem and communities composition will alter is undoubtable, but where and how much is a matter of speculation as not all is known about the effects climate change will have on the earth.
Raven, J.A.; Edwards, D. (2001). "Roots: evolutionary origins and biogeochemical significance". Journal of Experimental Botany 52 (90001): 381-401. doi:10.1093
Global Warming and the increasing atmospheric CO2
Throughout the earth's geological past, climate has fluctuated, going through glacial and interglacial periods. For roughly 10 000 years the planet has been in an interglacial period characterized by a fairly stable climate and global temperature. However, during the past century, as people began clearing more forests, and the industrial revolution brought a large increase in the burning of fossil fuels, there has been soaring levels of atmospheric CO2 and a corresponding rise in temperature. (Miller & Spoolman, 2009).
The concept of global warming, caused by increased atmospheric CO2, is currently a hot topic and source of much debate in today's society - it needs little introduction. The general consensus of the scientific community however, projects that atmospheric CO2 may be doubled by the end of the century if current CO2 emission rates continue. What is yet an unknown, is how severely this will alter the current environment, and to what degree it will effect rainfall patterns, rising sea levels, light, temperatures and many other environmental factors. (Millar & Spoolman 2009)
Consequences of increased atmospheric CO2, and how it is warming the global climate, is often refered to as the "Greenhouse effect". This is caused by the trapping of long-wavelength radiation by the earth's atmosphere. (Miller & Spoolman, 2009, Taiz & Zeiger, 2006). This is similar to that of a greenhouse, which transmits light in short wavelengths, through the glass roof. The light is absorbed by plants and surfaces, then converted to heat, and partly re-mitted as long-wave radiation. As glass transmits this wavelength poorly, the result is a warmed greenhouse.
Although the prospect of a doubling of atmospheric CO2 seems extreme, from a geologic and biologic perspective, it is the rapidity of change that is more significant than its degree. Trees, 200 years old, have already encountered a 28% increase in CO2, and are facing the prospect of over a 100% increase, during their life-span. (Bowes, 1993). Amid the ongoing political and scientific debate over the consequences of the "greenhouse effect," most concern is focused on the prospect of changes in the global climate. Less attention is given to the fact, that the elevated CO2 will directly affect plants in natural and agricultural systems, though such an effect is an integral part of the issue (King et al 1992).
Photosynthesis a brief overview
Photosynthesis is a complicated biochemical process whereby plants use sunlight to convert carbon dioxide and water into various carbohydrates. This chemical reduction process is mediated by a number of factors, namely water, mineral nutrients, carbon dioxide concentrations and ambient temperature (Campbell, Reece & Meyers, 2006). As with all chemical reactions, the conversion of substrate to product is impacted by any variations in factors that govern the overall reaction.
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Photosynthesis is also a temperature dependent process where there is a link between CO2 uptake and H2O loss through transpiration as a consequence of stomatal opening. The heat load of a leaf exposed to direct sunlight is very high and heat loss is required to avoid damage to the photosynthetic apparatus. This can be achieved by means such as radiative, sensible and latent heat loss (Taiz & Zeiger, 2006, Campbell 1977)
Light of wave lengths from 400 to 700 nm are utilized in photosynthesis. Leaf anatomy and orientation control the absorption of light. When leaves are exposed to more light than they can utilize, the photosynthetic apparatus can be damaged in a phenomenon called photoinhibition ().
The substrate water, whilst delivered by the root system, exists as a vapour around the photosynthetic cells, has additional physical roles in the photosynthetic process. Water evaporation cools cells and osmotic pressure controls the opening and closing of stomata cells that restrict the uptake of gases.
Minerals have an important role in the maintenance of enzymes that govern the photosynthetic reactions and the production of the reduction molecule NADPH and the energy generation molecule ATP.
The C3, C4 and CAM Pathways
Approximately 95% of terrestrial plants are C3 species, while about 1% are C4, and 4% use the CAM pathway, they all have various ways of coping with levels of CO2 concentrations.
C4 plants have ways of concentrating CO2 in environments where plant leaves are exposed to direct light. The CO2 is converted to a C4 intermediary in mesophyll cells that have a more direct exposure to the atmosphere and transferred to bundle sheath cells that house the cellular apparatus for the Calvin cycle (C3). The heat generated by exposure to light can cause a lowering of CO2 concentrations around the mesophyll cells; hence, the need for a substrate that can be easily made from atmospheric CO2 and released in appropriate concentrations when required by the chemical infrastructure in the bundle sheath cells. C4 plants belong to approximately 18 families of monocotyledons and dicotyledons that are particularly prominent in the Gramineae (corn, millet, sorghum and sugarcane); all plants that grow best in open fields or plains and some important agricultural species. (Edwards & Walker 1983).
The C3 plants, those that solely fix CO2 by the Calvin Cycle, tend to thrive in areas where sunlight intensity and temperature is moderate and sufficient amount of water is available. C3 plants cannot grow in hot areas because RuBisco (the first enzyme in the calvin cycle) incorporates more O2 into RuBP as temperatures increase. This leads to photorespiration where they shut their stomata to reduce water loss but also reduce the concentration of CO2 in the leaves (Raven & Ewards, 2001).
The Crassulacean Acid Metabolism (CAM) Plants inhabit arid environments with seasonal availability of water and have a third method of concentrating CO2 They store a 4 carbon acid in vacuoles. During the day, the stomata are closed and they capture the CO2 at night. During daylight, the malic acid (C4) is converted to CO2 and the energy generated from the sunlight converts the CO2 to carbohydrates. (Taiz & Zeiger, 2006).
The ancestral photosynthetic pathway is C3 photosynthesis while the C4 is a derived pathway. It is believed that throughout geological history, time periods exhibited much higher levels of CO2 in the atmosphere. CO2 diffusion through stomata into C3 leaves would have been much higher than that of today's resulting in higher photosynthetic rates. The evolution of C4 is believed to have happened as recently as 10 to 15 million years ago as an adaptation to cope with a CO2 limited atmosphere. Although C4 make up 1% of total terrestial species, their close affinity to humans and the fact they have many agricultural uses, mean they make up a comparitively large percentage of the worlds net primary productivity. C3 plants however, still make up nearly 70% of the NPP. ( 1 )
Plant responses to increased atmospheric CO2, Global warming and Environmental constraints.
The photosynthetic mechanism of a plant determines how it will respond to changes in CO2. However, as well as CO2, environmental factors such Light, Temperature, Water availability, Nutrients, and Human influence, all play a role and may be considered the primary factor in regulating growth (Bowes 1993).
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It is generally agreed in the scientific community, as a result of a large number of studies, that CO2 enrichment enhances the photosynthesis and growth of C3 plants. However, whether such enhancement can occur when factors other than CO2 limit growth is often questioned. There are differing opinions about the degree to which enhanced photosynthesis translates into improved growth and yield, and about whether positive responses at the plant level can be related to communities and ecosystems as a whole, or whether initially high photosynthetic rates are sustained. (Bowes 1993).
In nature, photosynthesis often takes place in a wide range of light and/or temperatures and nutrient availiablity. However, a large number of CO2 enrichment studies on photosynthesis (mostly those done by the agricultural industry), have used optimum levels of sunlight, temperature and nutrients. In these studies, CO2 supply or rubisco capacity were the major plant limitations and under these conditions several observations indicate that CO2 enrichment enhances photosynthesis and growth 2, 38, 50, 119). These results may not necessarily be extrapolated to the natural environment.
Competition between C3 and C4 Plants
Under the present CO2 /O2 ratios in the atmosphere, there are photorespiratory loses of approximately 25% for C3 plants. (71). With projections that atmospheric CO2 concentration will be doubled by end of the century, it can be expected that the negative effect of O2 on C3 plants will be at least halved. There will be very little effect on C4 plants as a result of this projection. (Bowes 1993).
The ability of C3 and C4 plants to be competitive within an ecosystem is determined by a number of environmental factors such as light, water, temperature, nutrients, and human influence. As mentioned earlier, C3, C4 and CAM plants have adapted to differing environments. C3 tend to thrive in areas where sunlight and temperature is moderate, while C4 is more able to tolerate higher temperature. Although studies show that an enriched CO2 atmosphere, favor C3 plant growth, the result of this enriched atmosphere, means that climate is likely to be warmer. This suggests there may be an advantage for C4 plants. The outcome of these changes, cannot be fully determined at this time as both seem to have some advantage.
In studies done by (15, 16, 33, 134) the competitive interactions of species of C3 and C4 plants were investigated. C3 plants photosynthesis and growth rates were enhanced by the CO2 enrichment. However, conclusions were made that CO2 was not the most important factor affecting competition, as under some conditions C4 plants were more competitive. In addition, not all species respond the same way to various factors. The competitiveness of species within a community is affected by its response to various factors. This leads to changes in the composition of plant communities.
Ecosystems and Communities: The big picture
Although it is relatively easy to conduct studies on CO2 enrichment at plant level, the same cannot be said for a whole ecosystem, and with the speculative nature of global warming, further problems arise.
The effect of CO2 enrichment on a whole ecosystem has not been extensively studied to date. (Bowes 1993). Two ecosystems were studied for a period of four years- a mesohaline salt marsh, and arctic tundra. (43 118) The salt marsh study showed that the gain in carbon accumulation was maintained, whereas that was not the case in the tundra (43, 118).
The salt marsh contained both C3 and C4 species but was dominated by the perienal C3 sedge scirpus olnyei . Upon doubling the CO2 concentration, the salt marsh showed an increase in photosynthesis, biomass and net primary productivity. This was because of the greater number of shoots, higher photosynthesis capacity, and a longer growing season of these plants. Although both C3 and C4 plants responded to CO2 enrichment, the C4 plants did not respond to the same extent and had a lower salt tolerance. This suggests that the C3 plants may have a competitive advantage in the future in some ecosystems. (43)(Bowes 1993)
The Tundra ecosystem was dominated by the arctic sedge eriophorum vaginatum. Upon doubling the CO2 concentration, the tundra did not show an increase photosynthesis, biomass and productivity because of low nutrients or low temperature constraints. (118)
These studies show that although an increased CO2 concentration can induce plant growth, photosynthesis and productivity, the ecosystems environmental constraints play a major part in the result.
Whether the responses of the world's biomes will resemble the salt marsh or the tundra, is a matter of speculation. The Tundra scenario shows the terrestrial biosphere, even though with increased CO2, is near its limit due to environmental constraints. (74, 118). The other scenario anticipates increased global vegetation as CO2 limitations on photosynthesis eases (43, 50). As these two ecosystems represent two extremes in the worlds different biomes, it is difficult to estimate the
where and when the atmospheric CO2 compensation point will stabilize. As CO2 increases, so does growth, but at some point environmental factors may regulate this effect.
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