As the CO2 content of the air progressively declined millions of years ago, certain plants evolved specialized biochemical pathways and anatomical adaptations that enabled them to increase their intracellular CO2 concentration at the site of its fixation, which allowed the primary carboxylating enzyme rubisco to function more efficiently. The CO2 concentrating mechanism possessed by these CAM plants operates by sequentially reducing CO2 into carbohydrates at two different times of day. The initial reduction of CO2 into a four-carbon sugar is done at night - when CAM plant stomata are open - by the enzyme PEP-carboxylase. Then, during the day when CAM plant stomata are closed, the four-carbon sugar is decarboxylated, increasing the plant's intercellular CO2 concentration, and the resulting CO2 is subsequently reduced back into a carbohydrate, but this time by rubisco.
Because this CO2 concentrating mechanism saturates rubisco, some researchers have suggested that CAM plants will not respond to rising levels of atmospheric CO2. However, it has recently been shown that despite the apparent saturation of rubisco, atmospheric CO2 enrichment often elicits robust photosynthetic enhancements in CAM plants.
Lootens and Heursel (1998) reported that two Phalaenopsis hybrids grown at an atmospheric CO2 concentration of 950 ppm exhibited net photosynthetic rates that were over 80% greater than those displayed by control plants grown at 350 ppm CO2. Similarly, pineapple plants grown at 700 ppm CO2 exhibited photosynthetic rates that were more than 80% greater than those of control plants grown in ambient air (Zhu et al., 1999); and in the study of Fernandez et al. (1999), a 200-ppm increase in the air's CO2 content boosted rates of net photosynthesis in an inducible CAM herb (Talinum triangulare Jacq.) by 250 and 350% at the onset of drought and four weeks into drought, respectively.
In addition to increasing photosynthetic rates, elevated CO2 can also enhance biomass production in CAM plants. Zhu et al. (1997), for example, reported that a 400-ppm increase in the air's CO2 content increased rates of net photosynthesis and dry mass production in pineapple by 36 and 23%, respectively. In addition, Graham and Nobel (1996) observed that twice-ambient CO2 concentrations enhanced daily net carbon uptake in Agave deserti plants by 50%, which contributed to an 88% increase in their biomass. Likewise, when exposed to twice-ambient concentrations of atmospheric CO2, the epiphytic orchid Mokara Yellow attained shoot and root dry mass values that were 31 and 98% greater, respectively, than those reached by control plants grown in ambient air (Li et al., 2002).
Positive results have also been obtained in studies with super-elevated atmospheric CO2 concentrations. Hew et al. (1995), for example, reported that Mokara White orchids exposed to a CO2 concentration of 10,000 ppm produced total dry weights that were approximately 33% greater than those displayed by ambiently-grown plants, while the fumigation of other epiphytic orchids with air containing 10,000 ppm CO2 resulted in total dry weight values that were twice as great as those exhibited by orchids grown at ambient CO2 (Gouk and Hew, 1999).
Clearly, therefore, CAM plants can, and do, respond positively to increases in the air's CO2 concentration, as do nearly all of earth's C3 and C4 plants.
References
Fernandez, M.D., Pieters, A., Azuke, M., Rengifo, E., Tezara, W., Woodward, F.I. and Herrera, A. 1999. Photosynthesis in plants of four tropical species growing under elevated CO2. Photosynthetica 37: 587-599.
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.
Graham, E.A. and Nobel, P.S. 1996. Long-term effects of a doubled atmospheric CO2 concentration on the CAM species Agave deserti. Journal of Experimental Botany 47: 61-69.
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.
Li, C.R., Gan, L.J., Xia, K., Zhou, X. and Hew, C.S. 2002. Responses of carboxylating enzymes, sucrose metabolizing enzymes and plant hormones in a tropical epiphytic CAM orchid to CO2 enrichment. Plant, Cell and Environment 25: 369-377.
Lootens, P. and Heursel, J. 1998. Irradiance, temperature, and carbon dioxide enrichment affect photosynthesis in Phalaenopsis hybrids. HortScience 33: 1183-1185.
Zhu, J., Bartholomew, D.P. and Goldstein, G. 1997. Effect of elevated carbon dioxide on the growth and physiological responses of pineapple, a species with crassulacean acid metabolism. Journal of the American Society of Horticultural Science 122: 233-237.
Zhu, J., Goldstein, G. and Bartholomew, D.P. 1999. Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature. Plant, Cell and Environment 22: 999-1007.