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Elevated CO2 Mitigates Negative Drought Effects on Barley Plants
Robredo, A., Perez-Lopez, U., Miranda-Apodaca, J., Lacuesta, M., Mena-Petite, A. and Munoz-Rueda, A. 2011. Elevated CO2 reduces the drought effect on nitrogen metabolism in barley plants during drought and subsequent recovery. Environmental and Experimental Botany 71: 399-408.

The authors write that "barley, an economically important and extensively cultivated cereal worldwide, increases its yield in parallel with an increase in CO2," but they indicate that it "responds to drought stress through altered nitrogen metabolism and reduced productivity," which complexities they thus set out to explore.

What was done
Barley (Hordeum vulgare L. cv. Iranis) seedlings were grown in 2.5-L pots containing a 3:1 mix of perlite:vermiclite in a controlled-environment growth chamber, first at ambient and then at elevated atmospheric CO2 concentrations (350 and 700 ppm, respectively). Initially, the pots were watered twice a week with a complete Hoagland solution and with deionized water between each Hoagland solution application. Then, drought was initiated when the seedlings were 18 days old by withholding water for intervals of 9, 13 and 16 days, while the effects of these actions were analyzed at the end of each drought period, and water recovery was analyzed three days after re-watering the 13-day droughted plants, with each complete experiment being replicated three times.

What was learned
First of all, the six Spanish scientists say their barley plants showed a reduction in water use, even though under elevated CO2 the plants had a larger leaf area that could have transpired more water per plant and offset the lower transpiration per unit leaf area, much as others have also found (Owensby et al., 1997; Niklaus et al., 1998). In addition, they report that "during the period of drought, the depletion of soil water content was delayed by 3-4 days in plants grown under elevated CO2 conditions," and they state that in the CO2-enriched plants "water stress also developed more slowly than at ambient CO2 because of a slower rate of water depletion." And as a result, they report that "leaf water potential in plants subjected to drought but grown at elevated CO2 was less negative than in their ambient CO2 grown counterparts."

With respect to nitrogen issues, Robredo et al. determined that "absolute values for nitrogen uptake by barley plants were higher under elevated CO2 compared to ambient CO2." And they say they "observed high nitrate reductase activity in plants grown at elevated CO2, which should parallel an increase in photosynthesis (Robredo et al., 2007) and sugar content (Perez-Lopez et al., 2010)." In addition, they say that "under ambient CO2 conditions, protein content decreased as the water stress progressed," but that "when plants grew under elevated CO2 conditions, the rate of photosynthesis was higher [and] drought had less effect on the protein content." In fact, they report that the barley plants "showed a greater content of proteins under elevated CO2," in harmony with the findings of Geiger et al. (1999), who they say "reported a similar outcome in tobacco with the same supra-optimal nitrogen concentration." And they remark that these findings also mesh with the results of studies reviewed by Idso and Idso (2001), who concluded that any negative effects of elevated CO2 on crop protein content "could be ameliorated by increased use of nitrogen fertilizer."

What it means
In the simplest of terms, Robredo et al. conclude that "elevated CO2 mitigates many of the effects of drought on nitrogen metabolism and allows more rapid recovery following water stress."

Geiger, M., Haake, V., Ludewig, F., Sonnewald, U. and Stitt, M. 1999. The nitrate and ammonium nitrate supply have a major influence on the response of photosynthesis, carbon metabolism, nitrogen metabolism, and growth to elevated carbon dioxide in tobacco. Plant, Cell and Environment 22: 1177-1199.

Idso, S.B. and Idso, K.E. 2001. Effects of atmospheric CO2 enrichment on plant constituents related to animal and human health. Environmental and Experimental Botany 45: 179-199.

Niklaus, P.A., Spinnler, D. and Korner, C. 1998. Soil moisture dynamics of calcareous grassland under elevated CO2. Oecologia 117: 201-208.

Owensby, C.E., Ham, J.M., Knapp, A.K., Breemer, D. and Auen, L.M. 1997. Water vapour fluxes and their impact under elevated CO2 in a C4-tallgrass prairie. Global Change Biology 3: 189-195.

Perez-Lopez, U., Robredo, A., Lacuesta, M., Munoz-Rueda, A. and Mena-Petite, A. 2010. Atmospheric CO2 concentration influences the contributions of osmolyte accumulation and cell wall elasticity to salt tolerance in barley cultivars. Journal of Plant Physiology 167: 15-22.

Robredo, A., Perez-Lopez, U., Sainz de la Maza, H., Gonzalez-Moro, B., Lacuesta, M., Mena-Petite, A. and Munoz-Rueda, A. 2007. Elevated CO2 alleviates the impact of drought on barley improving water status by lowering stomatal conductance and delaying its effects on photosynthesis. Environmental and Experimental Botany 59: 252-263.

Reviewed 27 April 2011