Watch That Wheat

It wasn’t so long ago that many folks were fixated on Century’s End and the New Millennium. But an extraordinarily important thing happened approximately 10,000 years ago in that part of world we today call Iraq – humans domesticated wheat. Ever since, Homo sapiens have become remarkably dependent upon this once-humble grass. Wheat sustains humans throughout the world whether it takes the form of breakfast cereals, a seemingly infinite variety of breads, or is brewed into some of the world’s greatest beer.

So it’s not surprising that many researchers are curious about wheat’s future in a world where atmospheric CO2 concentrations are rising inexorably. Anything that might be bad for wheat could be bad for humans. We’ll risk stepping on our punch line: Wheat seems to win in a world of higher atmospheric CO2.

A recent entry in the long list of articles on this subject comes from a team of Swedish and Estonian scientists. Pleijel et al. grew spring wheat for three years in open-top chambers 50 km northeast of Göteborg, Sweden. Atmospheric CO2 levels maintained at 350 ppm and 680 ppm. Atmospheric ozone (O3) concentrations were maintained at ambient and double-ambient levels. The team anticipated a growth enhancement due to elevated CO2, but harm associated with elevated O3. With their knowledge of past research, though, they also expected that elevated CO2 would reduce the negative effects of higher O3 levels because of reduced stomatal conductance (which could also be expected to increase the water-use efficiency of the plants).

Neither the researchers nor the wheat were disappointed by the results. Pleijel et al. state, "Doubling the CO2 concentration had a positive effect on grain yield in all three years (+21, +7, and +11, respectively)". The overall enhancement was lower than results from other experiments, probably due to the cool and wet summer climate of southern Sweden. In 1996, measurements of their experiment were made to more carefully examine water interactions with the soil and plants. The team reports, "The less negative soil water potential in elevated CO2 in 1996 suggests that an elevation of the CO2 concentration reduced the water consumption of the plants."

Their results were consistent with experiments in other parts of the world where wheat has been observed to increase its water-use efficiency under higher CO2 levels. The Swedes and Estonians also found that the water-use efficiency increase would be greatest in warm and dry years. Such timing would be impeccable as that is just when more efficient use of water is most critical. In their own words, "In years when the beneficial effect of the CO2-induced improved water-use efficiency on leaf area duration is expressed, i.e. warm and dry years, the yield is likely to be significantly increased. Under wetter and cooler conditions, typical for southwest Sweden in many summers, the yield increase will be smaller."

Pleijel et al. also were interested in how elevated O3 could harm the growth characteristics of the spring wheat plants. Once again, they noted tremendous benefits with higher levels of atmospheric CO2. The team found, "The 1000-grain weight was negatively affected by O3 in low CO2, but remained unaffected in the high CO2 treatment." Later, they conclude, "Thus in climatic conditions that allow the CO2 effect to be expressed to a significant extent, elevated CO2 will probably compensate for yield losses due to O3."

You add it up. Elevated CO2 (a) substantially increases grain yield of wheat, (b) increases plant water-use efficiency, particularly in warm dry years, and (c) reduces or eliminates the negative effects of higher ozone levels. Is the result a negative sum?

Pleijel, H., Gelang, J., Sild, E., Danielsson, H., Younis, S., Karlsson, P.-E., Wallin, G., Skärby, L., and Selldén, G. 2000. Effects of elevated carbon dioxide, ozone, and water availability on spring wheat growth and yield Physiologia Plantarum, 108, 61-70.