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Effects of Elevated CO2 and Water Stress in Alfalfa
De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M.  1999.  Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water stress.  Physiologia Plantarum 107: 84-89.

What was done
The authors grew nitrogen-fixing alfalfa plants in controlled environment chambers having atmospheric CO2 concentrations of 400 and 700 ppm for two weeks before superimposing a two-week water treatment on them, which kept half of the plants well-watered with a soil moisture content approaching field capacity and half of the plants water-stressed with a soil moisture content that was 30% of field capacity.  Thus, the authors studied the effects of elevated CO2 and water stress on plant growth and overall plant water status.

What was learned
When plants were well-watered, their leaf water potentials were not significantly different from one another, regardless of CO2 growth concentration.  When water-stressed, however, CO2-enriched plants experienced a slightly less favorable water balance than that of ambiently-grown plants.  In spite of this greater water stress, the CO2-enriched water-stressed plants attained 2.6 and 2.3 times more biomass than water-stressed and well-watered plants, respectively, grown at 400 ppm CO2, even though they consumed the same amount of, or even less, water than did the ambiently-grown plants.  Hence, the CO2-enriched water-stressed plants displayed an average water-use efficiency that was 2.6 and 4.1 times greater than that of water-stressed and well-watered plants, respectively, subjected to ambient CO2 concentrations.

Taking a closer look at the CO2-induced biomass response, the authors determined that elevated CO2 preferentially stimulated belowground biomass production, as did water stress, by significantly increasing root to shoot ratios.  Specifically, under ambient CO2 conditions, water stress increased root:shoot ratios by 108%, while at elevated CO2, water stress caused a 269% increase in this ratio.  As a result, nodule biomass in CO2-enriched water-stressed plants was 40 and 100% greater than nodule biomass of well-watered and water-stressed plants, respectively, grown at ambient CO2 concentrations.  Consequently, the CO2-enriched water-stressed plants obtained 31 and 97% more total plant nitrogen than well-watered and water-stressed plants, respectively, grown at ambient CO2 concentrations.

In conclusion, atmospheric CO2 enrichment more than alleviated the negative effects of water stress on biomass production in alfalfa, as water-stressed plants exposed to elevated CO2 produced much more total plant and nodule biomass than water-stressed or well-watered plants grown at ambient CO2.

What it means
As the atmospheric CO2 concentration rises, it is likely that nitrogen-fixing alfalfa plants will exhibit significant increases in biomass, regardless of soil water conditions.  In fact, it is conceivable that the rising CO2 content of the air will completely counterbalance the negative influences of water stress on biomass production in this important forage crop, so that water-stressed plants in a CO2-enriched environment would still produce more biomass than well-watered plants do now.

Reviewed 15 May 2000