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Effects of Atmospheric CO2 on Apple
Reference
Pan, Q., Wang, Z. and Quebedeaux, B.  1998.  Responses of the apple plant to CO2 enrichment: changes in photosynthesis, sorbitol, other soluble sugars, and starch.  Australian Journal of Plant Physiology 25: 293-297.

What was done
The authors exposed one-year-old apple plants to CO2 concentrations of 200, 360, 700, 1000 and 1600 ppm for a total of eight days to determine the impact of short-term CO2 enrichment on photosynthesis and sugar production in leaves.

What was learned
Midday whole-plant photosynthesis rates measured one day into the experiment for trees receiving 200 ppm atmospheric CO2 (sub-ambient treatment) were approximately 50% lower than those observed at ambient CO2 concentration.  At elevated CO2 levels of 700, 1000 and 1600 ppm, however, rates were nearly 2.5, 4.0 and 8.0 times greater than those measured at ambient CO2 concentration, respectively.  However, by the eighth day of CO2 exposure, down regulation of photosynthesis had occurred, as indicated by photosynthetic rates that now were approximately 1.9, 2.8 and 4.5 times greater than those recorded at ambient CO2 for concentrations of 700, 1000 and 1600 ppm CO2, respectively.  Leaf concentrations of sucrose were unaffected by elevated CO2Sorbitol concentrations, however, were reported to be approximately 38% greater at 1600 ppm CO2 than they were at ambient CO2, while starch levels at 1600 ppm CO2 were determined to be 17-fold greater than those observed at ambient CO2.  It is possible that this tremendous starch increase induced the photosynthetic down regulation that was observed in these trees.

What it means
Although the phenomenon of down regulation was observed in these young apple trees, it was not complete; and elevated CO2 still caused significant photosynthetic enhancement after eight days of CO2 exposure.  The greater concentrations of sugars thereby produced with CO2 enrichment should thus provide the necessary resources to accommodate increases in tree biomass and fruit production that commonly result from higher CO2 levels.  Hence, apple trees should exhibit increasing growth rates as the CO2 content of the air continues to rise.

Reviewed 1 October 1998