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Down Regulation of White Birch Photosynthesis vs. Soil Nitrogen Content
Cao, B., Dang, Q.-L. and Zhang, S. 2007. Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings. Tree Physiology 27: 891-899.

The authors note that "nitrogen (N) is a key factor regulating photosynthetic responses to elevated CO2 because it is a major component of Rubisco and other photosynthetic enzymes and structures," and they say "it is believed that, when N supply is low, photosynthetic down-regulation in response to elevated CO2 occurs because an inadequate N supply restricts the development of new sinks and exacerbates the source-sink imbalance that tends to develop in response to elevated CO2."

What was done
Cao et al. grew seven-week-old well watered and fertilized (with the partial exception of N) seedlings within greenhouses whose atmospheres were maintained at either 360 or 720 ppm CO2 for either 60 or 80 additional days (at which points they measured several plant physiological processes and properties) in pots filled with a 1:1 mix of peat moss and vermiculite that was augmented with either 10, 80, 150, 220 or 290 mg N per liter of peat-vermiculite mix, where they describe the highest of the five N concentrations as being "comparable to the highest value found in birch trees growing under natural environmental conditions," and where they describe the lowest of the five concentrations as being "lower than that found for seedlings in the low-N treatments in nearly all other published studies."

What was learned
The three researchers found that "photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated CO2, and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration." When measured at the CO2 concentration at which the plants were grown, however, they report that "elevated CO2 increased the overall rate of photosynthesis and instantaneous water-use efficiency by up to 69% and 236%, respectively."

What it means
Because "even in the lowest N-supply regime, photosynthetic down-regulation was less than 60% and the photosynthetic rate of seedlings was higher in elevated CO2 than in ambient CO2 in all the N treatments," Cao et al. conclude that "white birch will likely benefit from increases in atmospheric CO2 across all sites, and the benefit will increase with increasing soil fertility."

Reviewed 28 November 2007