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Growth Response to Very High CO2 Concentrations (Terrestrial Plants) -- Summary
Terrestrial plants grown in elevated atmospheric CO2 environments typically exhibit increased rates of photosynthesis and biomass production. Most of the studies that have established this fact have historically utilized CO2 concentration increases on the order of 300-400 ppm, which represents an approximate doubling of the atmosphere's current CO2 concentration. So what happens if the air's CO2 content is super-enriched, to a concentration an order of magnitude or more larger? Are the consequences of the extra CO2 still positive? Or are they detrimental? In what follows, we attempt to answer these questions by summarizing what we know about the subject via a brief review of the pertinent scientific literature.

Louche-Tessandier et al. (1999) grew potato plantlets inoculated with an arbuscular mycorrhizal fungus at ambient and super-elevated (10,000 ppm) CO2 for one month at a number of different light intensities. This protocol revealed that the high CO2 treatment stimulated root colonization by the fungus; but biomass production in the CO2-enriched inoculated plantlets only increased significantly when they were grown at high light intensity.

In a similar experiment, Gouk et al. (1999) grew orchid plantlets at ambient and super-elevated (10,000 ppm) CO2 for three months. In their study, the extra CO2 enhanced plant dry weight by more than 2-fold, stimulated the induction of new roots, increased the total chlorophyll contents of both roots and leaves, and boosted tissue starch contents by nearly 20-fold, all without any disruption or damage of chloroplasts.

Also working with orchids was Hew et al. (1995), who grew them at ambient and 10,000 ppm CO2 and found that the elevated CO2 boosted their dry weights by 28 to 37%. Likewise, Tisserat et al. (2002) fumigated mint and thyme with air containing 10,000 ppm CO2; and they determined that the super-CO2-enrichment increased the fresh weights of the two species by 3.1- and 5.8-fold, respectively.

In a study of an epiphytic fern, Ong et al. (1998) grew the seedless vascular species Pyrrosia piloselloides (which is less adapted to terrestrial habitats than its seed-producing relatives) from spores in small containers maintained at atmospheric CO2 concentrations of 350, 515 and 3360 ppm to study the effects of elevated CO2 on the fern's photosynthesis and growth. Forty days after germination, light-saturated rates of net photosynthesis were 22% and 114% greater at 515 and 3360 ppm, respectively, than they were at 350 ppm. Over time, however, the elevated CO2 induced photosynthetic acclimation in the plants, but in a concentration-dependent manner. After 100 days of exposure to elevated CO2, for example, the photosynthetic stimulation of plants grown at 515 ppm CO2 had dropped to 10%, which represents a 50% decline relative to their original stimulation; but the photosynthetic adjustment was much less at the super-enriched CO2 concentration of 3360 ppm, for plants in this treatment reduced their original photosynthetic enhancement by only 10%.

As part of their acclamatory response to elevated CO2, gametophytes exposed to 515 and 3360 ppm CO2 reallocated limiting resources away from their photosynthetic apparatus, as indicated by respective 11 and 28% reductions in their tissue chlorophyll contents. Despite these reductions, resulting from an optimization of resources at elevated CO2 concentrations, total gametophytic dry mass at 515 and 3360 ppm was still 43 and 214% greater, respectively, than it was at ambient CO2 at physiological maturity (100 and 80 days for plants grown at 515 and 3360 ppm CO2, respectively).

These findings suggest that earth's rising atmospheric CO2 content will likely promote the photosynthesis and growth of ferns, which are considered more primitive forms of terrestrial plant life than earth's more numerous seed-bearing plants. Thus, ferns should continue to maintain their presence in many ecosystems across the globe. In fact, Ong et al. concluded that the "sum responses of Pyrrosia piloselloides gametophytes to elevated CO2 concentration suggest greater success against competitors in the future environment, enabling this fern to continue to establish itself in a future world with high atmospheric CO2."

More recently, Teixeira Da Silva et al. (2006) grew ornamental Spathiphyllum cv. Merry plantlets for a period of five weeks in novel culture vessels on a sugar-free liquid medium at low light intensity in controlled-environment chambers maintained at atmospheric CO2 concentrations of either 375, 1000, 2000 or 3000 ppm. Relative to the growth experienced by the plantlets exposed to ambient air of 375 ppm CO2, the plantlets exposed to 1000, 2000 and 3000 ppm CO2 produced 39%, 81% and 129% more shoot dry weight, respectively, plus 316%, 639% and 813% more root dry weight, respectively, for corresponding total CO2-induced biomass enhancements of 61%, 127% and 185%.

One of the more exciting studies to be conducted in the field of super atmospheric CO2 enrichment was that of Ali et al. (2005), who worked with the ginseng plant (Panax ginseng) that is widely cultivated in China, South Korea and Japan, the roots of which have been used for medicinal purposes since Greek and Roman times and are well known for their anti-inflammatory, diuretic and sedative properties and are acknowledged to be effective healing agents (Gillis, 1997; Ali et al., 2005). Normally, however, four to six years are required for ginseng roots to accumulate the amounts of the various phenolic compounds that are needed to produce their health-promoting effects. Consequently, in an important step in the quest to develop an efficient culture system for the commercial production of ginseng roots, Ali et al. investigated the effects of growing them in suspension culture in bioreactors maintained in equilibrium with air enriched to CO2 concentrations of 10,000 ppm, 25,000 ppm and 50,000 ppm for periods of up to 45 days.

Of most immediate concern in such an experiment would be the effects of the ultra-high CO2 concentrations on root growth. Would they be toxic and lead to biomass reductions or even root death? The answer was a resounding no. After 45 days of growth at 10,000 ppm CO2, for example, root dry weight was increased by fully 37% relative to the dry weight of roots produced in bioreactors in equilibrium with normal ambient air, while root dry mass was increased by a lesser 27% after 45 days at 25,000 ppm CO2 and by a still smaller 9% after 45 days at 50,000 ppm CO2. Hence, although the optimum CO2 concentration for ginseng root growth likely resided at some value lower than 10,000 ppm in this study, the concentration at which root growth rate was reduced below that characteristic of ambient air was somewhere significantly above 50,000-ppm, for even at that extremely high CO2 concentration, ginseng root growth was still greater than it was in ambient air.

Almost everything else measured by Ali et al. was even more dramatically enhanced by the ultra-high CO2 concentrations they employed in their experiment. After 45 days of treatment, total root phenolic concentrations were 58% higher at 10,000 ppm CO2 than at ambient CO2, 153% higher at 25,000 ppm CO2 and 105% higher at 50,000 ppm CO2, as best we can determine from the bar graphs of their results. Likewise, total root flavonoid concentrations were enhanced by 228%, 383% and 232%, respectively, at the same ultra-high CO2 concentrations, while total protein contents rose by 14%, 22% and 30%, non-protein thiol contents by 12%, 43% and 62%, and cysteine contents by 27%, 65% and 100% under the identical respective set of conditions. What is more, there were equally large CO2-induced increases in the activities of a large number of phenol biosynthetic enzymes.

What are the implications of these results? Ali et al. write that "the consumption of foodstuffs containing antioxidant phytonutrients such as flavonoids, polyphenolics, ascorbate, cysteine and non-protein thiol is advantageous for human health," citing Cervato et al. (2000) and Noctor and Foyer (1998). Hence, they concluded that their technique for the culture of ginseng roots in CO2-enriched bioreactors could be used for the large-scale production of an important health-promoting product that could be provided to the public in much greater quantities than is currently possible.

We further note that as the air's CO2 content continues to climb, ginseng and many other medicinal plants will likely see the concentrations of their health-promoting carbon-based secondary compounds naturally increased, leading to better human health the world over. In fact, this phenomenon has likely played a role (the magnitude of which is yet to be determined) in the huge lengthening of human life span that has occurred since the inception of the Industrial Revolution, as the atmosphere's CO2 concentration has risen from something on the order of 280 ppm to a value that is currently close to 385 ppm.

Switching to the cultivation of trees, Tisserat (2005) notes that "vitrified shoots are characterized as being small, succulent (i.e., 'glassy' or 'wet' in appearance) and immature, but [are] capable of readily proliferating additional axillary shoots." However, he also notes that vitrified shoots of the type that are cultured in vitro "do not transfer readily into soil well." Consequently, because there is a need for literally millions of sweetgum seedlings to be planted annually (Lin et al., 1995), it would be advantageous if a technique could be developed to increase the success of transferring tissue-culture-produced vitrified shoots to ex vitro growth in soil.

In searching for a technique to accomplish this feat, Tisserat first produced sweetgum shoots in an automated plant culture system in which ten times more shoots developed than in prior plant culture systems, but where vitrification was observed in fully 80% of the shoots. Thus, he studied the effects of ultra-high atmospheric CO2 concentrations on the vitrified shoots when they were transferred to soil and grown in air enriched with CO2 to concentrations as high as 30,000 ppm.

After four weeks of growth at atmospheric CO2 concentrations of 350, 1500, 3000, 10000 and 30000 ppm, survival percentages of 1-cm-long explants were found to be 48.6, 56.5, 65.7, 93.1 and 67.1%, respectively, while corresponding survival percentages of 2-cm-long explants were 61.2, 64.1, 69.2, 93.9 and 64.3%. For these same CO2 concentrations, the numbers of leaves produced per shoot were 4.17, 5.38, 5.85, 6.14 and 4.83, while the numbers of roots produced per shoot were 5.35, 8.58, 9.19, 9.66 and 9.82. Also, leaf and shoot lengths were similarly enhanced by the suite of increased CO2 concentrations.

Within the context that formed the basis for this study, Tisserat concluded that the procedures he developed should "minimize the time and labor involved in sweetgum micropropagation," and that they "can be readily adapted to the micropropagation of other woody and non-woody plants." Within the contest of the ongoing rise in the air's CO2 concentration, Tisserat's results additionally suggest that man will never be able to pump enough CO2 into the air to negatively affect the growth and development of sweetgum trees and, by implication, many (if not most) of earth's other plants. Even in those cases where plant growth responses did decline between 10,000 and 30,000 ppm in Tisserat's study, for example, the responses at 30,000 ppm CO2 were still greater than those observed at 350 ppm.

In a companion study, Tisserat and Vaughn (2003) grew four-week-old loblolly pine seedlings for 30 days at the same suite of atmospheric CO2 concentrations within 17.6-liter transparent containers, where the seedlings were watered three times per week but not fertilized. Three repetitions of this procedure revealed that seedling fresh weight, needle number, root number, and shoot length increased 341%, 200%, 74%, and 75%, respectively, after 30 days of growth at 10,000 ppm CO2, but that there were no further increases - or decreases - when going to an atmospheric CO2 concentration of 30,000 ppm. They also report that associated with increased growth and morphogenesis was a corresponding increase in secondary metabolites (more than 99% of which were a- and -pinene) in the ultra-high CO2 environments; and they note that "high a- and -pinene levels may confer an additional positive survival advantage " on the seedlings, since these substances, in their words, "have fungicidal and insecticidal activity (Harbone, 1982; Klepzig et al., 1995)."

Finally, in an important field study, Fernandez et al. (1998) investigated the effects of even higher CO2 concentrations (some as great as 35,000 ppm) on an herb and a tree growing in the vicinity of natural CO2 springs in Venezuela. These high CO2 concentrations stimulated the photosynthetic rates of both plants in all seasons of the year. In the dry season, this effect was particularly important; for plants exposed to elevated CO2 continued to maintain positive net photosynthetic rates, while those exposed to ambient air a few tens of meters away exhibited negative rates that, if prolonged, would be expected to lead to their eventual demise. The researchers thus noted that their work provides "a positive answer to the question of whether increases in carbon assimilation will be sustained throughout the growing season and over multiple seasons." It also demonstrated that very high atmospheric CO2 concentrations, some as much as two orders of magnitude greater than the current global mean, were not detrimental to the plants investigated. In fact, they actually helped them.

In conclusion, the results of the several studies reviewed above clearly suggest that earth's plants are not harmed by super-elevated atmospheric CO2 concentrations an order of magnitude or more greater than that of the globe's current mean. Indeed, they all report positive growth responses, with some being particularly large, even huge. Hence, most plants should continue to display enhanced rates of photosynthesis and biomass production as the atmosphere's CO2 concentration continues to rise, no matter how high its ultimate level turns out to be.

References
Ali, M.B., Hahn, E.J. and Paek, K.-Y. 2005. CO2-induced total phenolics in suspension cultures of Panax ginseng C.A. Mayer roots: role of antioxidants and enzymes. Plant Physiology and Biochemistry 43: 449-457.

Cervato, G., Carabelli, M., Gervasio, S., Cittera, A., Cazzola, R.. and Cestaro, B. 2000. Antioxidant properties of oregano (Origanum vulgare) leaf extracts. Journal of Food Biochemistry 24: 453-465.

Fernandez, M.D., Pieters, A., Donoso, C., Tezara, W., Azuke, M., Herrera, C., Rengifo, E. and Herrera, A. 1998. Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinervia. New Phytologist 138: 689-697.

Gillis, C.N. 1997. Panax ginseng pharmacology: a nitric oxide link? Biochemical Pharmacology 54: 1-8.

Gouk, S.S., He, J. and Hew, C.S. 1999. Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid plantlet exposed to super-elevated CO2. Environmental and Experimental Botany 41: 219-230.

Harbone, J.B. 1982. Introduction to Ecological Biochemistry. Academic Press, New York, NY, USA.

Hew, C.S., Hin, S.E., Yong, J.W.H., Gouk, S.S. and Tanaka, M. 1995. In vitro CO2 enrichment of CAM orchid plantlets. Journal of Horticultural Science 70: 721-736.

Klepzig, K.D., Kruger, E.L., Smalley, E.B. and Raffa, K.F. 1995. Effects of biotic and abiotic stress on induced accumulation of terpenes and phenolics in red pines inoculated with bark beetle-vectored fungus. Journal of Chemical Ecology 21: 601-625.

Kubler, J.E., Johnston, A.M. and Raven, J.A. 1999. The effects of reduced and elevated CO2 and O2 on the seaweed Lomentaria articulata. Plant, Cell and Environment 22: 1303-1310.

Lin, X., Bergmann, B.A. and Stomp, A.-M. 1995. Effect of medium physical support, shoot length and genotype on in vitro rooting and plantlet morphology of sweetgum. Journal of Environmental Horticulture 13: 117-121.

Louche-Tessandier, D., Samson, G., Hernandez-Sebastia, C., Chagvardieff, P. and Desjardins, Y. 1999. Importance of light and CO2 on the effects of endomycorrhizal colonization on growth and photosynthesis of potato plantlets (Solanum tuberosum) in an in vitro tripartite system. New Phytologist 142: 539-550.

Noctor, G. and Foyer, C.H. 1998. Ascorbate and glutathione: keeping active oxygen under control. Annual Review of Plant Physiology and Plant Molecular Biology 49: 249-279.

Ong, B.-L., Koh, C.K-K. and Wee, Y.-C. 1998. Effects of CO2 on growth and photosynthesis of Pyrrosia piloselloides (L.) Price gametophytes. Photosynthetica 35: 21-27.

Teixeira Da Silva, J.A., Giang, D.D.T. and Tanaka, M. 2006. Photoautotrophic micropropagation of Spathiphyllum. Photosynthetica 44: 53-61.

Tisserat, B. 2005. Establishing tissue-cultured sweetgum plants in soil. HortTechnology 15: 308-312.

Tisserat, B. and Vaughn, S.F. 2003. Ultra-high CO2 levels enhance loblolly pine seedling growth, morphogenesis, and secondary metabolism. HortScience 38: 1083-1085.

Tisserat, B., Vaughn, S.F. and Silman, R. 2002. Influence of modified oxygen and carbon dioxide atmospheres on mint and thyme plant growth, morphogenesis and secondary metabolism in vitro. Plant Cell Reports 20: 912-916.

Last updated 4 October 2006