What was done
Well-watered and fertilized sour orange (Citrus aurantium L.) trees that are rooted in the ground have been grown from the seedling stage out-of-doors at Phoenix, AZ, USA in clear-plastic-wall open-top enclosures maintained at atmospheric CO2 concentrations of 400 and 700 ppm since November 1987 in the world's longest - and still ongoing - aerial CO2 enrichment study.  Idso and Kimball (2001) describe the effects of this 75% increase in the air's CO2 content on the fruit and woody biomass production of the trees (a long-term enhancement of 80% in both cases). In this paper, the authors describe the effects of the extra CO2 on the vitamin C concentrations of fully-ripened fruit harvested over the eight-year period 1992-1999.

What was learned
In years when the production of fruit was approximately doubled by the extra CO2, the fruit produced in the two CO2 treatments were of approximately the same size; and the vitamin C concentration of the juice of the CO2-enriched oranges was enhanced by approximately 7% above that of the juice of the ambient-treatment oranges.  In years when fruit numbers were more than doubled, however, the CO2-enriched fruit were slightly smaller than the fruit produced in normal air; and the vitamin C concentration of the juice of the CO2-enriched fruit rose even higher, to as much as 15% above that of the ambient-treatment fruit.  On the other hand, in years when fruit numbers were less than doubled, the CO2-enriched fruit were slightly larger than the ambient-treatment fruit; and the enhancement of the vitamin C concentration of the juice of the CO2-enriched fruit was somewhat less than the base value of 7% typical of equal-size fruit.

With respect to the likely long-term equilibrium response of the trees, the authors write that in five of the last six years of the study, "the 75% increase in atmospheric CO2 concentration has increased : (1) the number of fruit produced by the trees by 74 ± 9%, (2) the fresh weight of the fruit by 4 ± 2%, and (3) the vitamin C concentration of the juice of the fruit by 5 ± 1%."

What it means
On the basis of this study, in the words of the authors, "there is reason to believe that an atmospheric CO2 enrichment of the magnitude expected over the current century may induce a large and sustained increase in the number of fruit produced by orange trees, a small increase in the size of the fruit, and a modest increase in the vitamin C concentration of the juice of the fruit, all of which effects bode well for this key agricultural product that plays a vital role in maintaining good health in human populations around the globe."  Further support for this conclusion and its great significance is provided by Idso and Idso (2001), who write as follows.

"These findings take on great significance when it is realized that scurvy - which is brought on by low intake of vitamin C - may be resurgent in industrial countries, especially among children (Ramar et al., 1993; Gomez-Carrasco et al., 1994), and that subclinical scurvy symptoms are increasing among adults (Dickinson et al., 1994).  Furthermore, Hampl et al. (1999) have found that 12 to 20% of 12-18-year-old school children in the United States 'drastically under-consume' foods that supply vitamin C; while Johnston et al. (1998) have determined that 12 to 16% of U.S. college students have marginal plasma concentrations of vitamin C.  Hence, since vitamin C intake correlates strongly with the consumption of citrus juice (Dennison et al., 1998), and since the only high-vitamin-C juice consumed in any quantity by children is orange juice (Hampl et al., 1999), the modest role played by the ongoing rise in the air's CO2 content in increasing the vitamin C concentration of orange juice could ultimately prove to be of considerable significance for public health in the United States and elsewhere."

So chalk up another plus for the biosphere.  As the air's CO2 content continues to rise, the earth becomes an ever-better habitat for humanity.

References
Dennison, B.A., Rockwell, H.L. and Baker, S.L.  1998.  Fruit and vegetable intake in young children.  Journal of the American College of Nutrition 17: 371-378.

Dickinson, V.A., Block, G. and Russek-Cohen, E.  1994.  Supplement use, other dietary and demographic variables, and serum vitamin C in NHANES II.  Journal of the American College of Nutrition 13: 22-32.

Gomez-Carrasco, J.A., Cid, J.L.-H., de Frutos, C.B., Ripalda-Crespo, M.J. and de Frias, J.E.G.  1994.  Scurvy in adolescence.  Journal of Pediatric Gastroenterology and Nutrition 19: 118-120.

Hampl, J.S., Taylor, C.A. and Johnston, C.S.  1999.  Intakes of vitamin C, vegetables and fruits: which schoolchildren are at risk?  Journal of the American College of Nutrition 18: 582-590.

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.

Idso, S.B. and Kimball, B.A.  2001.  CO2 enrichment of sour orange trees: 13 years and counting.  Environmental and Experimental Botany 46: 147-153.

Ramar, S., Sivaramakrishman, V. and Manoharan, K.  1993.  Scurvy - a forgotten disease.  Archives of Physical Medicine and Rehabilitation 74: 92-95.