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Agriculture (Species -- Wheat: CO2 vs. Stress of Soil Infertility) -- Summary
Atmospheric CO2 enrichment typically enhances photosynthesis and biomass production in wheat (Triticum aestivum L.) under normal growing conditions (see, for example, our Subject Index Summaries pertaining to Agriculture (Species - Wheat: Photosynthesis and Biomass).  But what happens when environmental conditions are less than ideal?  In this summarization of the results of studies for which we have produced Journal Reviews, we report on what has been learned when soil infertility, especially low soil nitrogen content, negatively impacts the growth and development of wheat.

In a two-month study conducted in environmentally-controlled growth chambers, Vilhena-Cardoso and Barnes (2001) grew wheat at atmospheric CO2 concentrations of 350 and 700 ppm at low, medium and high levels of soil nitrogen, finding that the doubling of the air's CO2 content increased total plant dry weight by 12, 29 and 44%, respectively, in the three nitrogen regimes.  Under more realistic field conditions and timeframes - a FACE study conducted at Maricopa, Arizona, USA - spring wheat was grown for two seasons at atmospheric CO2 concentrations of ambient and ambient + 200 ppm, with half of each plot receiving high amounts of nitrogen and half receiving low amounts.  Over the course of the two growing seasons, Brooks et al. (2000) found that the plants grown in the CO2-enriched plots accumulated 8 and 16% more carbon than the plants exposed to ambient air under the low and high soil nitrogen regimes, respectively.

Throughout the same experiment, Hunsaker et al. (2000) determined that the extra 200 ppm of CO2 reduced seasonal evapotranspiration by about 1 and 4% under conditions of low and high soil nitrogen, respectively.  As a result, the extra CO2 increased the water use efficiency of the wheat plants by approximately 10 and 20% in the low and high nitrogen treatments, respectively.  In an earlier study by Kimball et al. (1999), however, it was reported that the CO2-enriched plots experienced daily water losses that were approximately 20 and 7% less than those observed in the ambient plots in the low and high soil nitrogen regimes, respectively, which would have resulted in a slightly smaller difference between the CO2-induced increases in the water use efficiencies of the plants in the two nitrogen treatments.

Farage et al. (1998) conducted two five-week experiments to investigate the role played by nitrogen supply in hastening or hindering the photosynthetic acclimation of wheat to elevated CO2.  In one experiment, plants were grown in pots within growth chambers maintained at either 350 or 650 ppm CO2 while being irrigated with either low or high nitrogen solutions on a regular basis.  In the other experiment, plants exposed to the same CO2 concentrations were grown hydroponically in nutrient solutions containing low or high nitrogen concentrations to eliminate any pot-induced root restrictions on growth, which can also lead to photosynthetic acclimation to elevated CO2.

In the pot experiment, the plants exposed to elevated CO2 experienced significant photosynthetic down regulation, while leaf nitrogen contents and amounts of active rubisco were dramatically reduced.  In the hydroponically-grown plants, on the other hand, the extra CO2 caused no photosynthetic down regulation, regardless of nitrogen supply, as it increased the light-saturated rates of photosynthesis in both nitrogen treatments by 56%.

These data suggest that low soil fertility need not necessarily lead to an induction of photosynthetic acclimation in wheat in CO2-enriched air, as has sometimes been observed in field experiments.  Such a development can be kept at bay if nutrient acquisition by roots keeps pace with the CO2-enhanced growth rate of the plants, as occurred in Farage et al.'s hydroponic experiment (see also Nutrient Acquisition in our Subject Index).  In addition, even when acclimation does occur, there is usually still a positive response to the increase in the air's CO2 concentration; it may just not be as large it is when essential nutrients are non-limiting, as demonstrated in the review of Idso and Idso (1994).

References
Brooks, T.J., Wall, G.W., Pinter Jr., P.J., Kimball, B.A., LaMorte, R.L., Leavitt, S.W., Matthias, A.D., Adamsen, F.J., Hunsaker, D.J. and Webber, A.N.  2000.  Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes.  3.  Canopy architecture and gas exchange.  Photosynthesis Research 66: 97-108.

Farage, P.K., McKee, I.F. and Long, S.P.  1998.  Does a low nitrogen supply necessarily lead to acclimation of photosynthesis to elevated CO2Plant Physiology 118: 573-580.

Hunsaker, D.J., Kimball. B.A., Pinter, P.J., Jr., Wall, G.W., LaMorte, R.L., Adamsen, F.J., Leavitt, S.W., Thompson, T.L., Matthias, A.D. and Brooks, T.J.  2000.  CO2 enrichment and soil nitrogen effects on wheat evapotranspiration and water use efficiency.  Agricultural and Forest Meteorology 104: 85-105.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Kimball, B.A., LaMorte, R.L., Pinter Jr., P.J., Wall, G.W., Hunsaker, D.J., Adamsen, F.J., Leavitt, S.W., Thompson, T.L., Matthias, A.D. and Brooks, T.J.  1999.  Free-air CO2 enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat.  Water Resources Research 35: 1179-1190.

Vilhena-Cardoso, J. and Barnes, J.  2001.  Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3Journal of Experimental Botany 52: 1901-1911.