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Effects of Elevated CO2 and Soil Nitrogen Stress on Sunflowers
Zerihun, A., Gutschick, V.P. and BassiriRad, H.  2000.  Compensatory roles of nitrogen uptake and photosynthetic N-use efficiency in determining plant growth response to elevated CO2: Evaluation using a functional balance model.  Annals of Botany 86: 723-730.

What was done
The authors grew sunflowers (Helianthus annuus L.) for one month in pots of three different soil nitrogen concentrations that were placed within open-top chambers maintained at atmospheric CO2 concentrations of 360 and 700 ppm in an attempt to validate predicted growth responses to atmospheric CO2 enrichment using a functional balance model they developed.

What was learned
Atmospheric CO2 enrichment reduced average rates of root nitrogen uptake by about 25%, which reduction would normally tend to reduce tissue nitrogen contents and relative growth rates of seedlings.  However, the elevated CO2 also increased photosynthetic nitrogen-use efficiency by an average of 50%, which increase normally tends to increase the relative growth rates of seedlings.  Of these two competing effects, the latter was much more significant, ultimately leading to an increase in whole plant biomass.  After just one month, for example, CO2-enriched plants exhibited whole plant biomass values that were 44, 13 and 115% greater than those of control plants growing in ambient air at low, medium and high levels of soil nitrogen, respectively.  Thus, low tissue nitrogen contents, as predicted by the authors' model and validated by their data, do not necessarily preclude a growth response to atmospheric CO2 enrichment, particularly if photosynthetic nitrogen-use efficiency is enhanced, which is typically the case.

What it means
As the air's CO2 concentration increases, it is likely that sunflower seedlings will exhibit physiological adjustments in root nitrogen uptake rates and photosynthetic nitrogen-use efficiency that will ultimately lead to greater amounts of net carbon uptake and biomass production, even on soils severely depleted in nitrogen.  Thus, commercial growers of sunflower crops will likely experience enhanced yields as the atmospheric CO2 content continues to rise.