One way to maintain plant quality is to optimally fertilize soils with macro- and micro-nutrients to help reduce any deleterious imbalances that might otherwise occur under elevated atmospheric CO2 concentrations.  In the long-term experiment of Idso and Kimball (2001), for example, optimally-fertilized sour orange tree seedlings suddenly exposed to a 300-ppm increase in the air's CO2 concentration experienced significant reductions in leaf concentrations of a number of essential elements that were still substantial at the three-year point of the study (Gries et al., 1993).  By the eighth year of the experiment, however, most of the deficiencies had disappeared (Penuales et al., 1997), demonstrating the long-term power of the rapidly growing CO2-enriched trees to ultimately overcome this problem when supplied with adequate nutrients.

In many cases, however, proper fertilization may be difficult, if not impossible, to achieve.  What happens then?

As noted in the preceding Editorial, increased agricultural productivity made possible by elevated levels of atmospheric CO2 could partially or even fully compensate for decreased plant quality among the 800 million people of the world who are suffering from less than adequate calorie intake by providing them with more ample food supplies.  However, the diets of over half the world's population are deficient in one or more essential microelements, making 'hidden hunger' the world's top nutritional disorder (UN ACC/SCN in collaboration with IFPRI, 2000).  What is more, this problem could worsen, especially if synergetic decreases in multiple essential elements occur.  Hence, food-fortification programs may be particularly important in maintaining the health of humans in a high-CO2 world.  Also, an important ameliorative avenue for plant breeders to take is the promotion of morphological changes in plants that increase nutrient uptake, such as an increase in root:shoot ratio, and enhancements in root structure and activity, and root associations with mycorrhizae.

Elevated levels of atmospheric CO2 can also affect the concentrations of complex compounds in agricultural produce and medicinal plants; and these changes could have both health-promoting and health-diminishing effects.  With respect to the 2-15% increases in the orange juice vitamin C concentration of the CO2-enriched oranges studied by Idso et al. (2002), we note further that this vitamin enhancement was achieved without any selection of trees.  This suggests that plant breeders might possibly be able to enhance orange juice vitamin C concentrations in a high-CO2 world even more by careful selection for cultivars that direct the extra carbon provided by elevated CO2 into the making of more vitamin C.

In this regard, it is encouraging that not only vitamin C but also many complex compounds beneficial to human health are made exclusively of carbon, hydrogen, and oxygen, such as vitamins A, K and D.  For such compounds, increasing their plant tissue concentrations would not be stoichiometrically constrained in a high-CO2 world.  In the case of vitamin A, its subclinical deficiency affects an estimated 250 million pre-schoolers and is the second leading cause of blindness in the developing world (UN ACC/SCN in collaboration with IFPRI, 2000).  Plant breeders may be able to take advantage of elevated CO2 levels to increase the concentration of this important vitamin in edible plant tissues.

To summarize, there are several established facts on which we wholeheartedly agree:
1)   The air's CO2 content is rising rapidly on both evolutionary and ontogenetic time
      scales and will likely continue to do so over the next several decades.
2)   Increasing atmospheric CO2 concentrations can - and will - alter the chemical
      compositions of many plants, in terms of both individual elements and complex
      compounds; and these changes have the potential to significantly affect human
      nutrition and health.
3)   In sharp contrast with the plentiful literature on the effects of atmospheric CO2
      enrichment on plant productivity, there is a startling scarcity of data on the effects of
      elevated CO2 on plant tissue quality (aside from data on nitrogen concentration).
      Analogously, there are few data on the effects of other global change factors on
      plant stoichiometry, such as tropospheric ozone (O3), UV-radiation, and temperature.
4)   Further research on these subjects is urgent.  Scientists need to quantify the
      changes in plant quality induced by global change and rising atmospheric CO2
      concentrations in particular.  They need to determine the existence and extent of
      large-scale patterns in altered plant stoichiometry and understand how such patterns
      depend on plant type (i.e., C3, C4, nitrogen fixing, or mycorrhizal-infected plants) and
      growing conditions.  In addition, they need to learn how to use the extra plant
      carbohydrate production that occurs in CO2-enriched air, in order to improve plant
      quality via plant selection and breeding.

As we contemplate the future, these considerations must begin to play a more prominent role in the design of atmospheric CO2 enrichment experiments than they have in the past.  We know, as well as we know anything, that more CO2 in the air means more food on people's plates; but we know precious little about how the nutritive value of that food might change.  Some studies have provided tentative answers; but we need to expand both the breadth and the depth of this important work to gain a deeper understanding of the principles involved and the extent of their applicability.  Until we do, speculation will be the rule of the day, much as it is in the area of climate change.  Surely, we can do better in the realm of biology, where well-designed experiments can provide well-defined answers to important questions affecting each of us.

References
Gries, C., Kimball, B.A. and Idso, S.B.  1993.  Nutrient uptake during the course of a year by sour orange trees growing in ambient and elevated atmospheric carbon dioxide concentrations.  Journal of Plant Nutrition 16: 129-147.

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.

Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D.  2002.  The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice.  Agriculture Ecosystems & Environment 90: 1-7.

Jablonski, L.M., Wang, X. and Curtis, P.S.  2002.  Plant reproduction under elevated CO2 conditions: a meta-analysis of reports on 79 crop and wild species.  New Phytologist 156: 9-26.

Loladze, I.  2002.  Rising atmospheric CO2 and human nutrition: toward globally imbalanced plant stoichiometry?  Trends in Ecology & Evolution 17: 457-461.

Penuelas, J., Idso, S.B., Ribas, A. and Kimball, B.A.  1997.  Effects of long-term atmospheric CO2 enrichment on the mineral content of Citrus aurantium leaves.  New Phytologist 135: 439-444.

UN ACC/SCN in collaboration with IFPRI.  2000.  Fourth Report on the World Nutrition Situation, Geneva, Switzerland.