Atmospheric CO2 Levels

As we look towards the future in crop production practices we will need to examine the changing atmospheric conditions and adjust our traditional cropping methods accordingly. It has been well documented that atmospheric levels of Co, have increased over the last 130 years, and that increased co, levels are now accelerating (Stuiver, 1978; Wong, 1979; Gardner et al., 1985). Because of the physiological anatomy of wheat and other C-3 plants, increased levels of co, have been shown to increase dry matter production and yield of these crops (Garbaud et al., 1980; Kavalyshin et al., 1985; Musgrave & Strain, 1987; Sionet et al. 1979 & 1980). Further consequences of the increasing atmospheric CO, concentration are increasing temperatures, increasing water stress, and decreasing 02 levels. It becomes necessary to consider these environmental changes and examine their impact on the production of wheat since this is such an important crop world-wide.

Atmospheric CO2 Levels

Today, dry air contains 78% nitrogen, 21% oxygen, 0.43% argon, 0.034% CO2 and traces of other gasses. Although the CO2 concentration seems to be relatively low, it is very important since it has been estimated that 90% of a plant's dry weight comes from the incorporation of CO, within the plant tissue during photosynthesis. In 1850 the atmosphere was thought to contain 0.0268% CO2, or 268 ppm (Stuiver, 1978). In the last 130 years the atmospheric co, concentration has increased 72ppm. It has been shown that the rate of Atmospheric CO, increase is becoming exponential, and it is predicted that by the year 2030 we will have reached concentrations approaching 600ppm, over twice the content in 1850 (Wong, 1979).

The primary causes for the huge increase in atmospheric CO2 concentration are thought to be the burning of fossil fuels and the cutting and burning of the world's forests. It has generally been predicted that these two activities will only increase in the near future, which helps to explain the predicted increases in atmospheric CO, concentration. These predictions are being backed up by today's headlines, where we read of massive oil spills, record automobile sales, and the deforestation of the Amazon basin. The resulting high levels of CO2 from these activities are thought to create a "Greenhouse" effect as CO, absorbs infrared bands of light which reflect and reradiate from the earth's surface. The atmosphere will thus retain more heat with the result the we may experience global warming trends and perhaps a shift in rainfall patterns. These side-effects must also be considered when analyzing the future of wheat production.

CO, Pathway of Wheat

Wheat is a C-3 plant. This means that the first measurable compound formed in the leaf chloroplast after adding radioactive co, is the three carbon compound, 3-Phosphoglyceric acid (3-PGA). All C-3 plants have similar co, fixation pathways, so this will be explained generally and can be applied to wheat specifically.

The carbon pathway in photosynthesis was worked out by Calvin and his associates in 1957. For the purposes of this paper we will look only at the portion of the photosynthetic pathway which deals with the initial fixation of CO2. The fixation of coz in C-3 plants occurs in the mesophyi cells and is catalyzed by the enzyme ribulose bis-phosphate (RuBP) carboxylase. This reaction is driven by energy produced during the light reactions of photosynthesis. After co, combines with RuBP, a temporary 6 carbon compound is formed which in turn breaks down into two 3 carbon compounds, 3 PGA. In this way, carbon is "fixed" into the plant and will later converted to CHO and further broken down to release energy in respiration.

Some co, never gets a chance to be fixed in this way. While RuBP carboxylase has an affinity for CO2, it also has a strong affinity for oxygen, Oz. Because of this, RuBP carboxylase can fix Oy as well as co2, an event that begins the process of photorespiration. In this way we can see that O2 competes strongly with CO, for the RuBP enzyme. In fact, O2 may competitively inhibit the fixation of co, and the resulting production of 3PGA. The more 02 is present at the sight of the RuBP enzyme the more on will be fixed, starting photorespiration, and the less CH20 will be produced. The process of photorespiration thus brings a decline of available potential energy to the plant. In practical terms, this means that the plant is not as vigorous under conditions of low CO, concentrations, so in this situation dry matter yields decrease.

It is not yet clear what purpose photorespiration serves for the plant. It is known that coz is evolved without coupling the acquired energy to any useful purpose. However, it is also believed that photrespiration provides amination for amino acid synthesis and this may be beneficial at low temperatures. Nevertheless, the competitive inhibition of co, by 0, does decrease yields of wheat since the byproducts of photorespiration do not produce usable energy. Increased levels of CO, can at least partly overcome this inhibition since in this enriched environment there will be more con to be fixed by the RuBP enzyme, and 0will be less inhibitory.

The more tropical crops such as maize are termed C-4 plants because the first measurable compound formed after the addition of radioactive carbon is the 4 carbon compound Oxaloacetic Acid, OAA. The C-4 plants have an anatomical make-up which allows them to fix co, at much lower concentrations than C-3 plants, and consequently they have much higher photosynthetic rates at present atmospheric co, levels. The C-4 anatomy is called Kranz anatomy. The C-4 species have chloroplasts in the vascular bundle-sheath cell while the C-3 species do not. Since C-4 plants also have chloroplasts in the mesophyl cells, they are able to fix CO2 there once and again in the vascular sheath cell. Further, C-4 plants initially fix co, with PEP carboxylase rather than with RuBP carboxylase. Since PEP has a much greater affinity for Co, than does RuBP, it is a much more efficient enzyme for fixing Co,. Once the carbon has been fixed in the mesophyl cell and OAA produced, it moves to the Bundle sheath

cell where co, is liberated and then re-fixed by RuBP carboxylase and fed into the Calvin cycle as with the C-3 plants,

This co, "shuttle" moving carbon from the mesophyl to the bundle sheath cells of C-4 plants raises the partial pressure of CO, within the vascular bundle sheath cell to almost 1000 ppm, or over 3 times the ambient atmospheric partial pressure of Co,. Because of the high level of co, within the bundle sheath cell, fixation of O, by RuBP carboxylase and the resulting photorespiration is inhibited. Thus, more CO, is fixed and less photorespiration takes place in C-4 plants, with the result that at present levels of CO2 C-4 plants have a higher photosynthetic rate than C-3 plants. Because the C-4 plants have internally increased their partial pressure of CO2, they do not appear to increase photosynthesis if atmospheric levels of coz are experimentally increased. In contrast, the C-3 plants have been shown to increase their photosynthetic capacity with increase of atmospheric CO2 since this directly increases the CO2 concentrations within the mesophyl cells and so the competitive inhibition of Co, fixation by 0, is overcome.

Responses of Wheat to Increased CO, levels

Many experiments have been conducted showing the response of wheat to increased concentrations of co2 (Gerbaud & Andre, 1980; Musgrave & Strain, 1987; Sionit et al., 1980; Kavalyshin & Lenina, 1985; sionit et al., 1979). Without exception, the results of these studies have shown that by increasing the co, concentration in the environment around wheat, the photosynthetic rate of the plant increased. Sionit, Hellmers, and Strain (1979) put it concisely when they say, "The high co, plants produced significantly greater grain yields and total dry matter than did the low CO2 plants. on the average, grain production under 1,000 ppm co, in the controlled environment was increased 62.1% and grain size was increased 16.2%". Fischer and Aquillar (1976) reported a 23% in the grain yield of wheat plants by increasing ambient CO2 concentration from 340 ppm to 759 ppm. Similar experiments were also conducted using maize, a C-4 plant. Results of the experiments conducted by Wong (1979) showed that Maize was not as effected by increases in co, as was cotton, a C-3 plant. As wong notes, "in contrast (to cotton), maize is relatively insensitive to CO, enrichment". Wong further attributed the differing responses of the C-3 and C-4 plants to differences of leaf anatomy and carboxylase between the two species, as stated above. Such results indicate that in the future, assuming that atmospheric Co, levels do continue to increase, we will see an increase in the yields of wheat if all other factors remain constant. As we have discussed, the greenhouse effect will carry side effects other than just increases in atmospheric CO2 content, however. These side effects may include global warming and drought.

Sionit, Hellmers, and Strain (1980) conducted an experiment where they examined yields of wheat under CO2 enrichment and water stress. They found that the leaf water potentials of the low CO, plants decreased more rapidly and reached a lower value

at the end of the stress period than the leaf water potentials of the high co, plants. Yields of the water stressed high Co, plants were greater than yields of the water stressed low COplants.

In fact, they found that the "high CO, plants under water stress conditions produced yields equal to the nonstressed low CO, plants." This suggests that as atmospheric CO, levels continue to increase, the resulting beneficial yields of wheat may be offset by changing rainfall patterns resulting in water stress

(drought) conditions.

Conclusion

As long as deforestation and the burning of fossil fuels continue, atmospheric Co, levels will correspondingly increase. While a higher atmospheric co, concentration does not appear to increase production of C-4 plants, C-3 plants such as wheat do seem to increase dry matter production in this environment. Such responses to fluctuations in the atmospheric CO2 concentration are due to differences in C-3 vs. C-4 anatomies. While wheat yields may increase as atmospheric CO, levels go up, we should not start celebrating. Increased global temperatures and changing rainfall patterns will compensate for any increases in production, and further global changes could turn forests into deserts. It is the hope of this author that the new advances in atomic fusion will someday make that a cheap, safe, and limitless supply of energy so that the greenhouse effect will come to an end.

Bibliography

Gardner, F. P., R. B. Pearce, and R.L. Mitchell. 1985. Physiology

of Crop Plants. Iowa State University Press.

Gerbaud, Alain and M. Andre. 1980. Effect of CO2, O2, and Light

on Photosynthesis and Photorespiration in Wheat. Plant Physiol. 66. 1032-1036.

Kavalyshkin, B. M. and A. Yu. Lenina. 1985. Spring Wheat

Productivity in an Artificial Climate as a function of the CO2 Concentration and the Irradiation Level. Doklady Vsesoyuznoi Akademii. No.1. pp.1-10.

Musgrave, Mary E., and B. R. Strain. 1988. Response of Two Wheat

cultivars to co, Enrichment Under Subambient 0. Conditions. Plant Physiol. 87. 346-350.

Sionet, N., H. Hellmers, and B. R. Strain. 1979. Growth and

Yield of Wheat Under Co, Enrichment and Water Stress, Crop Sci. Vol.20. 687-690.

Sionet, N., D. A. Mortensen, B. R. Strain, and H. Hellmers. 1980.

Growth Response of Wheat to CO, Enrichment and different Levels of Mineral Nutrition. Agronomy Journal. Vol.73. 10231026.

Stuiver, minze. 1978. Atmospheric Carbon Dioxide and Carbon

Resevoir Changes. Scince. vol.199. 253-258.

Wong, S. C. 1979. Elevated Atmospheric Partial Pressure of CO2

and Plant Growth. Oecologia. 44. 68-74.