Higher CO2 and Draught Resistance

When elevated CO2 makes plants grow faster and bigger, and that growth enhancement is combined with decreased stomatal conductance brought on by higher CO2 (thereby reducing water loss), plants become substantially more water-use efficient and far more resistant to drought.

A large team of Arizona researchers grew sorghum outdoors under ambient (360 ppm) and elevated (ambient plus 200 ppm) atmospheric CO2 concentrations. The grain received both ample and reduced water supplies over two growing seasons. Wall et al. report that the elevated CO2 causes well-watered sorghum to reduce its stomatal conductance by 37 percent. Water-stressed plants reduced their stomatal conductance by 32 percent.

The extra CO2 caused the well-watered sorghum to increase its assimilation of CO2 (a measure of photosynthesis and growth) by 9 percent. Their water-stressed counterparts increased net assimilation by 23 percent. When combined, these results demonstrate how water-use efficiency increases by approximately 60 percent for both treatments due to elevated CO2. They researchers conclude, "By ameliorating the adverse effects of drought, elevated atmospheric CO2 improved plant water status, which indirectly caused an increase in carbon gain."

In other research, Yale University’s Jeffrey Amthor at Oak Ridge National Laboratory examined fifty different studies on how wheat growth and yields are impacted by varying levels of atmospheric CO2. He divided the studies into those that used laboratory chambers, and those using glasshouses, closed-top field chambers, open-top chambers, and free-air field CO2 enrichment systems. He found that yields increased with elevated levels of CO2 "with a maximum effect (+37%) at about 890 ppm CO2 ". He also reports, "On average, doubling [CO2] from 350 to 700 ppm increased yield about 31%." On top of that he found that elevated CO2 stimulated the yield of water-stressed wheat.

Further focusing on the relationship of elevated CO2 and water-use efficiency, Blaschke et al. selected mature (30 to 50 year old) oak trees that grow near CO2 -emitting springs in central Italy. At that location, the atmospheric CO2 concentrations vary along a gradient from 370 ppm to over 700 ppm. The researchers discovered that the rate of photosynthesis was 26 percent to 69 percent higher for the trees growing close to the source of higher CO2, the springs. The trees growing near the spring experienced stomatal conductance nearly 25 percent lower than that of trees growing under conditions of near-ambient CO2. This combination of higher photosynthetic rate and lower stomatal conductance again yields a substantial increase in water-use efficiency and drought tolerance.

A team of scientists from across the United States grew a variety of common grass species during a four-year study where atmospheric CO2 was maintained between 200 ppm and 550 ppm. Anderson et al. conclude that elevated CO2 linearly increases photosynthesis in all species while stomatal conductance generally decreases. They report, "Plant water relations have already changed significantly [given the increase in CO2 since the Industrial Revolution]."

Dr. Robert C. Balling Jr.
Arizona State University

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