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Interactive Effects of Water Stress and CO2 on Alfalfa
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.

What was done
The authors grew alfalfa in open-top chambers at ambient (340 ppm) and enriched (600 ppm) CO2 concentrations for twenty-five days, at which time 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 thylakoid membrane composition.

What was learned
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.

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 for periods of ample and insufficient soil moisture, 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.

What it means
As the concentration of CO2 in the atmosphere rises ever higher, alfalfa should increase production of leaf nonstructural carbohydrates, which should allow plants to maintain a better internal water status, especially during periods of limited water supply.  Likewise, with greater amounts of CO2 in the air, changes in thylakoid membrane composition and lipid unsaturation may provide greater membrane stability and integrity, thereby allowing proper functioning in times of drought or other adverse environmental conditions.  In certain extreme situations, in fact, these membrane adaptations may mean the difference between a plant's perishing or surviving.

Reviewed 1 March 1999