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Effects of Elevated CO2 and Water Stress on Soybean
Serraj, R., Allen, L.H., Jr., Sinclair, T.R.  1999.  Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment.  Global Change Biology 5: 283-291.

What was done
Soybeans were grown from seed in pots within a glasshouse until they were four weeks old.  At this time, half of the plants were subjected to an ambient atmospheric CO2 concentration of 360 ppm, while the other half were exposed to an elevated concentration of 700 ppm.  Additionally, at each CO2 concentration, a water stress treatment was imposed to keep half of the plants well-watered and half of them water-stressed for a period of 18 days to study the effects of elevated CO2 and water stress on photosynthesis and growth.

What was learned
Short-term exposure of soybeans to elevated CO2 for 18 days significantly decreased daily and cumulative transpirational water losses compared to plants grown at 360 ppm CO2, regardless of water treatment.  In fact, elevated CO2 reduced total water loss by 25 and 10% in well-watered and water-stressed plants, respectively, relative to their ambiently-grown counterparts.

Drought stress significantly reduced rates of net photosynthesis for plants grown in both CO2 concentrations.  However, plants grown in elevated CO2 consistently exhibited higher photosynthetic rates than those of plants grown at ambient CO2, regardless of water treatment.

At final harvest, elevated CO2 had little effect on the total dry weight of plants grown with optimal soil moisture.  On the other hand, elevated CO2 significantly increased the total dry weight of water-stressed plants by about 33%.  It is interesting to note that while root dry weight declined for plants grown under conditions of water stress and ambient CO2 concentration, no such decline was exhibited by those subjected to atmospheric CO2 enrichment and water stress.

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that agricultural crops, like soybean, will display concurrent increases in photosynthesis and decreases in water loss and usage.  Thus, in the future, it is conceivable that the water-use efficiency of soybean will be significantly enhanced, thereby allowing plants to tolerate less frequent rains.

Reviewed 15 August 1999