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Carbon Based Secondary Compounds -- Summary
Atmospheric CO2 enrichment stimulates photosynthesis in nearly all plants.  A major expectation of this phenomenon is that plants exposed to elevated levels of atmospheric CO2 would produce more non-structural carbohydrates that could be used to create more carbon-based secondary compounds (CBSCs) or phenolics.  Why is this important?

For one thing, phenolic compounds tend to inhibit the biodegradation of organic materials.  Hence, if atmospheric CO2 enrichment results in the production of more of these decay-resistant substances, one would expect the ongoing rise in the air's CO2 content to lead to improved carbon sequestration in the world's soils, for plant-produced organic matter supplied to soils would be more resistant to decomposition; and the end result of this phenomenon would be a slowing of the rate-of-rise of the air's CO2 content, which negative feedback would tend to reduce the magnitude of any CO2-induced global warming that may be occurring.

A second significant characteristic of CBSCs is that they tend to strengthen plant defensive mechanisms for coping with herbivores and plant pathogens, which also allows more plant material to be produced and stored in the planet's soils.  In addition, it has been found that certain CBSCs called tannins, when eaten by ruminants, lead to reduced methane emissions from this large and diverse group of mammals (see our Editorial of 7 Aug 2002), which phenomenon tends to slow the rate-of-rise of the world's second most important greenhouse gas.

So the question is: How does atmospheric CO2 enrichment impact the production of carbon-based secondary compounds?  Does it increase them?  Decrease them?  Or is it of little consequence one way or the other?

For a long time the picture was rather muddled.  Many studies reported the expected increases in CBSC concentrations with experimentally-created increases in the air's CO2 content.  Others, however, could find no significant plant phenolic content changes; while a few even detected CO2-induced decreases in CBSC concentrations.  Although chaos thus reigned in this area for some time, order was finally brought to the issue when Penuelas et al. (1997) identified the key role played by soil nitrogen concentration.

In analyzing the results of a number of different studies, Penuelas et al. noticed that when soil nitrogen supply was less than adequate, some of the CBSC responses to a doubling of the air's CO2 content were negative, i.e., a portion of the studies indicated that plant CBSC concentrations declined as the air's CO2 content rose.  When soil nutrient supply was more than adequate, however, the responses were almost all positive, with plant CBSC concentrations rising in response to a doubling of the air's CO2 concentration.  In addition, when the CO2 content of the air was tripled, all CBSC responses - under both low and high soil nitrogen conditions - were positive.  The solution to the conundrum was thus fairly simple: with a tripling of the air's CO2 content, nearly all plants exhibited increases in their production of CBSCs; but with only a doubling of the air's CO2 content, adequate nitrogen nutrition was needed for a positive CBSC response.

Two studies we have reviewed on our website that have shown little to no impact of elevated CO2 on plant phenolic concentrations are those of Heyworth et al. (1998) and Penuelas et al. (2002).  The former researchers worked with pine tree seedlings growing in open-top chambers maintained at ambient and double-ambient CO2 concentrations for a period of three years, while the latter group worked with three species of shrubs growing in the vicinity of CO2-emitting springs that provided essentially lifetime exposure to approximately the same pair of atmospheric CO2 concentrations.  The majority of the studies we have reviewed, however, have shown modest to substantial positive responses.

Gebauer et al. (1998) grew loblolly pine seedlings in glasshouses maintained at atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, the seedlings were subjected to four different soil nitrogen treatments.  At the end of five months, there were no significant interactions between atmospheric CO2 and soil nitrogen; but when averaged across the four soil nitrogen concentrations employed, the elevated CO2 concentration was found to have increased the above- and below-ground concentrations of total phenolics in the seedlings by approximately 21 and 35%, respectively.

Booker and Maier (2001) also studied loblolly pine trees, but via branch bags placed on the limbs of more mature trees growing out-of-doors, while utilizing a much smaller CO2 concentration increase of only 200 ppm.  Nevertheless, they observed an approximate 11% increase in the concentration of total soluble phenolics in the needles contained within the CO2-enriched branch bags, and they noted that this response "could in turn affect plant-pathogen interactions, decomposition rates and mineral nutrient cycling."

Hoorens et al. (2002) grew two species common to dune grasslands of the Netherlands and two species common to Dutch peatlands in greenhouses fumigated with air containing either 390 or 700 ppm CO2 for up to five months (until leaf senescence occurred), after which they measured the phenolic concentrations of the leaf litter.  One species of each group registered no CBSC response to the elevated CO2, but the other species of each group did, with the grassland species registering a 20% increase in leaf litter phenolic concentration and the peatland species registering a 32% increase.

Castells et al. (2002) grew 14 genotypes of two Mediterranean perennial grasses (Dactylis glomerata and Bromus erectus) in glasshouses maintained at atmospheric CO2 concentrations of 350 and 700 ppm to determine if elevated CO2 impacts phenolic production in a genotypic-dependent manner.  Although there were no significant interactions of this type in either species, the CO2-enriched air increased total phenolic compound concentrations in D. glomerata and B. erectus by 15 and 87%, respectively.

Last of all, Cornelissen et al. (2003) grew two species of oak (Quercus myrtifolia and Quercus geminata) for a number of years in open-top chambers maintained at atmospheric CO2 concentrations of 370 and 700 ppm, finding that the extra CO2 of the CO2-enriched chambers led to increases in foliar tannin concentrations of approximately 35% and 43%, respectively, in these two species.  We note that this specific type of CBSC nearly always is found in greater concentrations in plant tissues produced in CO2-enriched air (see, for example, Tannins in our Subject Index).  This fact is particularly important, for in addition to the properties of many CBSCs that tend to temper the anthropogenic-induced increase in the air's CO2 content, tannins possess a unique characteristic that results in less methane being released to the atmosphere by ruminants that eat plant tissues that contain them in greater abundance than is typical of plants growing in ambient air, as reported in the introduction to this summary.

In conclusion, we note that the ongoing rise in the air's CO2 content will likely increase the production of many different CBSCs in many of earth's plants, leading to a number of benefits to the biosphere, not the least of which may be a slowing of the rate-of-rise of the greenhouse-gas-induced radiative forcing that is believed by many to drive global warming.

References
Booker, F.L. and Maier, C.A.  2001.  Atmospheric carbon dioxide, irrigation, and fertilization effects on phenolic and nitrogen concentrations in loblolly pine (Pinus taeda) needles.  Tree Physiology 21: 609-616.

Castells, E., Roumet, C., Penuelas, J. and Roy, J.  2002.   Intraspecific variability of phenolic concentrations and their responses to elevated CO2 in two Mediterranean perennial grasses.  Environmental and Experimental Botany 47: 205-216.

Cornelissen, T., Stiling, P. and Drake, B.  2003.  Elevated CO2 decreases leaf fluctuating asymmetry and herbivory by leaf miners on two oak species.  Global Change Biology 10: 27-36.

Gebauer, R.L.E., Strain, B.R. and Reynolds, J.F.  1998.  The effect of elevated CO2 and N availability on tissue concentrations and whole plant pools of carbon-based secondary compounds in loblolly pine (Pinus taeda).  Oecologia 113: 29-36.

Heyworth, C.J., Iason, G.R., Temperton, V., Jarvis, P.G. and Duncan, A.J.  1998.  The effect of elevated CO2 concentration and nutrient supply on carbon-based plant secondary metabolites in Pinus sylvestris L.  Oecologia 115: 344-350.

Hoorens, B., Aerts, R. and Stroetenga, M.  2002.  Litter quality and interactive effects in litter mixtures: more negative interactions under elevated CO2Journal of Ecology 90: 1009-1016.

Penuelas, J., Estiarte, M. and Llusia, J.  1997.  Carbon-based secondary compounds at elevated CO2Photosynthetica 33: 313-316.

Penuelas, J., Castells, E., Joffre, R. and Tognetti, R.  2002.  Carbon-based secondary and structural compounds in Mediterranean shrubs growing near a natural CO2 spring.  Global Change Biology 8: 281-288.