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Plant Responses To Increased Atmospheric Carbon Dioxide Environmental Sciences Essay

With atmospheric carbon dioxide likely to double in the next century the effects of this increased CO2 extend beyond simply global warming. This essay addresses the potential effects of increased atmospheric CO2 on the plants and the way they photosynthesise. It addresses whether the C3 and C4 types of photosynthesis for plant will be advantages or disadvantaged. Consideration is also given to how CAM plants will respond to increase CO2,

Different types of photosyntheis

Photosynthesis is a process that converts carbon dioxide (co2) into organic compounds, especially sugars, using the energy from sunlight.[1] Photosynthesis occurs in plants, algae, and many species of bacteria. There are three types of photosynthetic path ways; c3, c4 and its variant Crassulacean Acid Metabolism (CAM).( http://wc.pima.edu/Bfiero/tucsonecology/plants/plants_photosynthesis.htm )

C3 plant Photosynthesis takes place throughout the leaf using the enzyme RUBISCO this is involved in photosynthesis as well as the uptake of the CO2.( http://wc.pima.edu/Bfiero/tucsonecology/plants/plants_photosynthesis.htm ) Plants using the c3 pathway make up 95% of earth's plant species, some of these include wheat, barley, potatoes and sugar beet. (facing the inevitable: plants and increasing atmospheric co2)

C4 plants use the enzyme Phosphoenolpyruvate carboxylase (PEP Carboxylase) to assist in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photosynthesis, this takes place in the inner cells.(http://wc.pima.edu/Bfiero/tucsonecology/plants/plants_photosynthesis.htm ) Plants using c4 pathways make less the 1% of the earth’s plant species some of these include fourwing saltbush and corn. (facing the inevitable: plants and increasing atmospheric co2)

CAM plants open their stomata at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis (http://wc.pima.edu/Bfiero/tucsonecology/plants/plants_photosynthesis.htm ). CAM plants make up about 4% of the earth’s plant species some of these include many succulents such as cactii and agaves, some orchids and bromeliads. (facing the inevitable: plants and increasing atmospheric co2)

Impact of increased co2 in a controlled environment on photosynthesis in c3 and c4 plants:

Plants need co2 to photosynthesise. An increase in the atmospheric co2 will increase productivity of both c3 and c4 plant types although C3 plants typically respond better to atmospheric CO2 enrichment than do C4 plants in terms of increasing their rates of photosynthesis and biomass production. According to (Definitions good) Photosynthetic rates in C3 plants increase by 25-75% for a doubling of CO2. For C4 plants the data are less conclusive and range from no response to an increase of 10-25%. Results likely are temperature dependent. (Definitions good) ,

It has been suggested that in a world of rising atmospheric CO2 concentration, C3 plants may out-compete C4 plants and displace them, decreasing the biodiversity of certain ecosystems. 

Likely impact of increased atmospheric co2 in the predicted climate changed environment on c3 and c4 plants:

The impact of increased co2 on c3 and c4 plants is much more complex then this simple scenario based on increased co2 levels. In the next centry it is predicted that the average temperature will rise by about 2.8 degrees Celsius.( http://en.wikipedia.org/wiki/IPCC_Fourth_Assessment_Report) Without factoring in any other changes such a temperature, sunlight intensity, precipitation levels and lack of nutrients, it is imposible to ascertain whether c3 or c4 plants have an advantages environment(Biodiversity (C3 vs C4 Plants) – Summary)

Although c3 plants respond well to an increase in atmospheric co2 they require a lot more water to make use of it. For example When C3 and C4 grasses are grown in the same environment, at 30°C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277 water molecules per CO2 molecule fixed. This increased water use efficiency of C4 grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.[7]

C3 plants cannot grow in hot areas because RuBisCO incorporates more oxygen into RuBP as temperatures increase. This leads to photorespiration, which leads to a net loss of carbon and nitrogen from the plant and can, therefore, limit growth. In dry areas, C3 plants shut their stomata to reduce water loss, but this stops CO2 from entering the leaves and, therefore, reduces the concentration of CO2 in the leaves. This lowers the CO2:O2 ratio and, therefore, also increases photorespiration. C4 and CAM plants have adaptations that allow them to survive in hot and dry areas, and they can, therefore, out compete C3 plants.

Other impacts and effects

Summary

Further support for this conclusion comes from the study of Campbell et al. (2000), who reviewed research work done between 1994 and 1999 by a worldwide network of 83 scientists associated with the Global Change and Terrestrial Ecosystems (GCTE) Pastures and Rangelands Core Research Project 1, which resulted in the publication of over 165 peer-reviewed scientific journal articles.  After analyzing this great body of research, they concluded that the "growth of C4 species is about as responsive to CO2 concentration as [is that of] C3 species when water supply restricts growth, as is usual in grasslands containing C4 species."  Hence, the work of this group of scientists also provides no evidence for the suggestion that C3 plants may out-compete C4 plants and thereby replace them in a high-CO2 world of the future.

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