Morgan et al. (2001) grew alfalfa for 20 days post-defoliation in growth chambers maintained at atmospheric CO2 concentrations of 355 and 700 ppm and low or high levels of soil nitrogen to see how these factors affected plant regrowth.  They determined that the plants in the elevated CO2 treatment attained total dry weights over the 20-day regrowth period that were 62% greater than those reached by the plants grown in ambient air, irrespective of soil nitrogen concentration.

De Luis et al. (1999) grew alfalfa plants in controlled environment chambers in air of 400 and 700 ppm CO2 for two weeks before imposing a two-week water treatment on them, wherein the soil in which half of the plants grew was maintained at a moisture content approaching field capacity while the soil in which the other half grew was maintained at a moisture content that was only 30% of field capacity.  Under these conditions, the CO2-enriched water-stressed plants displayed an average water-use efficiency that was 2.6 and 4.1 times greater than that of the water-stressed and well-watered plants, respectively, growing in ambient 400-ppm-CO2 air.  In addition, under ambient CO2 conditions, the water stress treatment increased the mean plant root:shoot ratio by 108%, while in the elevated CO2 treatment it increased it by 269%.  As a result, the nodule biomass on the roots of the CO2-enriched water-stressed plants was 40 and 100% greater than the nodule biomass on the roots of the well-watered and water-stressed plants, respectively, growing in ambient air.  Hence, the CO2-enriched water-stressed plants acquired 31 and 97% more total plant nitrogen than the well-watered and water-stressed plants, respectively, growing in ambient air.  The bottom line, in terms of productivity, was that the CO2-enriched water-stressed plants attained 2.6 and 2.3 times more total biomass than the water-stressed and well-watered plants, respectively, grown at 400 ppm CO2.

Luscher et al. (2000) grew effectively- and ineffectively-nodulating (good nitrogen-fixing vs. poor nitrogen-fixing) alfalfa plants in large FACE plots for multiple growing seasons at atmospheric CO2 concentrations of 350 and 600 ppm, while half of the plants in each treatment received a high supply of soil nitrogen and the other half received only minimal amounts of this essential nutrient.  The extra CO2 increased the yield of effectively-nodulating plants by about 50%, regardless of soil nitrogen supply, while it actually caused a 25% yield reduction in ineffectively-nodulating plants subjected to low soil nitrogen, yet produced an intermediate yield stimulation of 11% for the same plants under conditions of high soil nitrogen, which suggests that the ability to symbiotically fix nitrogen is an important factor in eliciting strong positive growth responses to elevated CO2 under conditions of low soil nitrogen supply.

Sgherri et al. (1998) grew alfalfa in open-top chambers at ambient (340 ppm) and enriched (600 ppm) CO2 concentrations for twenty-five days, after which water was withheld for five additional days so they could investigate the interactive effects of elevated CO2 and water stress on plant water status, leaf soluble protein and carbohydrate content, and chloroplast thylakoid membrane composition.  They found that the plants grown in elevated CO2 exhibited the best water status during the moisture deficit part of the study, as indicated by leaf water potentials that were approximately 30% higher (less negative) than those observed in plants grown in ambient CO2.  This beneficial adjustment was achieved by partial closure of leaf stomata and by greater production of nonstructural carbohydrates (a CO2-induced enhancement of 50% was observed), both of which phenomena can lead to decreases in transpirational water loss, the former by guard cells physically regulating stomatal apertures to directly control the exodus of water from leaves, and the latter by nonstructural carbohydrates influencing the amount of water available for transpiration.  This latter phenomenon occurs because many nonstructural carbohydrates are osmotically active solutes that chemically associate with water through the formation of hydrogen bonds, thereby effectively reducing the amount of unbound water available for bulk flow during transpiration.  Under water-stressed conditions, however, the CO2-induced difference in total leaf nonstructural carbohydrates disappeared.  This may have resulted from an increased mobilization of nonstructural carbohydrates to roots in the elevated CO2 treatment, which would decrease the osmotic potential in that part of the plant, thereby causing an increased influx of soil moisture into the roots.  If this did indeed occur, it would also contribute to a better overall water status of CO2-enriched plants during drought conditions.

The plants grown at elevated CO2 also maintained greater leaf chlorophyll contents and lipid to protein ratios, especially under conditions of water stress.  Leaf chlorophyll content, for example, decreased by a mere 6% at 600 ppm CO2, while it plummeted by approximately 30% at 340 ppm, when water was withheld.  Moreover, leaf lipid contents in plants grown with atmospheric CO2 enrichment were about 22 and 83% higher than those measured in plants grown at ambient CO2 during periods of ample and insufficient soil moisture supply, respectively.  Furthermore, at elevated CO2 the average amounts of unsaturation for two of the most important lipids involved in thylakoid membrane composition were approximately 20 and 37% greater than what was measured in plants grown at 340 ppm during times of adequate and inadequate soil moisture, respectively.  The greater lipid contents observed at elevated CO2, and their increased amounts of unsaturation, may allow thylakoid membranes to maintain a more fluid and stable environment, which is critical during periods of water stress in enabling plants to continue photosynthetic carbon uptake.  These effects are so important, in fact, that some researchers have suggested that adaptive plant responses such as these may allow plants to better cope with any altered environmental condition that produces stress.

In view of these many positive effects of atmospheric CO2 enrichment upon the growth and development of alfalfa plants, one can only hope that the air's CO2 content continues to rise, as we will need all the help we can get from it to be able to feed the world's still-increasing number of people in the years and decades ahead.

References
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.

Luscher, A., Hartwig, U.A., Suter, D. and Nosberger, J.  2000.  Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2.  Global Change Biology 6: 655-662.

Morgan, J.A., Skinner, R.H. and Hanson, J.D.  2001.  Nitrogen and CO2 affect regrowth and biomass partitioning differently in forages of three functional groups.  Crop Science 41: 78-86.

Sgherri, C.L.M., Quartacci, M.F., Menconi, M., Raschi, A. and Navari-Izzo, F.  1998.  Interactions between drought and elevated CO2 on alfalfa plants.  Journal of Plant Physiology 152: 118-124.