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Monoterpenes -- Summary
Monoterpenes constitute a major fraction of the biogenic volatile organic compounds or BVOCs given off by plants; and they help protect earth's terrestrial vegetation by acting as scavengers of reactive oxygen species that are produced within plants experiencing significant heat stress (Pe˝uelas and Llusia, 2003). They also function as deterrents of pathogens and herbivores, and are known to aid wound healing after herbivore damage (Pichersky and Gershenzon, 2002). In addition, monoterpenes may attract pollinators and herbivore predators (Penuelas et al., 1995; Shulaeve et al., 1997); and they have the ability to generate large quantities of organic aerosols that may alter the planet's climate by producing cloud condensation nuclei that can lead to a cooling of the earth's surface (during the day) via an enhanced reflection of incoming solar radiation. For more information on this latter phenomenon, see Clouds (Condensation Nuclei - Climatic Effects) and Aerosols (Biological - Terrestrial) in our Subject Index.

In light of these several observations, it is important to know how plant monoterpene production might be affected by the ongoing rise in the atmosphere's CO2 concentration, as well as by any warming that might yet occur as the earth continues to recover from the global chill of the Little Ice Age, during which time the planet experienced some of the coldest temperatures of the current interglacial. In what follows, therefore, we briefly review the findings of some studies that have investigated various aspects of this subject.

We begin with a study conducted by Vuorinen et al. (2004), who grew well-watered and fertilized white cabbage plants from seed for 25 days in growth chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm. One group of plants in each CO2 treatment experienced no larval insect feeding, another experienced 48 hours of feeding by larvae of a crucifer specialist (Plutella xylostella), while yet another group experienced 48 hours of feeding by larvae of a generalist herbivore (Spodoptera littoralis), after which several BVOCs released by each group of plants were collected from the air surrounding them and analyzed. This protocol revealed, in the words of the researchers who conducted the work, that "total monoterpene emission per shoot dry weight was approximately 27% reduced [our italics] from plants grown at elevated CO2; and they report there was no difference in larval-induced damage to the plants between the ambient-air and CO2-enriched treatments.

Interestingly, an earlier study of holly oak trees by Loreto et al. (2001) had also resulted in decreases in monoterpene emissions in response to atmospheric CO2 enrichment, while Constable et al. (1999) found no effect of elevated CO2 on monoterpene emissions from Ponderosa pine and Douglas fir trees. Does this thus mean that the effects of atmospheric CO2 enrichment on plant monoterpene emissions are nil or even negative?

The results of the study of Baraldi et al. (2004) might make such appear to be the case. The seven scientists exposed sections of a southern California chaparral ecosystem to atmospheric CO2 concentrations ranging from 250 to 750 ppm in 100-ppm increments for a period of four years within naturally-lit glass chambers, measuring net ecosystem CO2 exchange (NEE) and emission rates of BVOCs, which were mainly monoterpenes. In doing so, they found that NEE exhibited a marked linear increase in response to increasing atmospheric CO2 concentration, more than tripling its rate in going from 400 to 700 ppm at 0400, 1200 and 1600 hours in June, and rising from moderately negative to weak positive values in December. However, they report that "total trace gas emissions expressed on a ground area basis were low and did not respond to increasing CO2 concentrations" in the winter; and in summer, when "BVOC emissions were of an order of magnitude greater than during winter," they found that the different levels of CO2 still "did not affect the emission rates" of monoterpenes.

Concurrently, Rapparini et al. (2004) measured BVOC emissions from mature downy and holly oak trees growing close to a natural CO2 spring in central Italy, where atmospheric CO2 concentrations averaged about 1000 ppm, and at a nearby control site where the air's CO2 content was unaffected by the spring. They too found that long-term exposure to high levels of atmospheric CO2 did not significantly affect BVOC emissions from the trees. However, they say that "when leaves of plants grown in the control site were exposed for a short period to an elevated CO2 level by rapidly switching the CO2 concentration in the gas-exchange cuvette, monoterpene basal emissions "were clearly inhibited," where basal emissions are those that occur at standard measuring conditions of 30░C air temperature and 1000 Ámol m-2 s-1 light intensity.

At this point, it may indeed appear that atmospheric CO2 enrichment has little to no effect on plant monoterpene emissions, and that if there is any effect at all, it is negative. But the situation is not nearly that straightforward, as demonstrated by the results of the study of Staudt et al. (2001). These four researchers also grew holly oak, albeit holly oak seedlings, within two compartments of a controlled-environment greenhouse -- one of which was maintained at an atmospheric CO2 concentration of 350 ppm and one of which was maintained at 700 ppm -- where air temperature and vapor pressure deficit were set to track outside ambient conditions and the plants were exposed to natural sunlight and watered every other week, while various growth parameters and physiological responses of the seedlings were measured over a four-month period that began when the trees had been exposed to the two CO2 treatments for a total of ten months. This work revealed that the elevated CO2 treatment increased the leaf area of the young oaks by 40%, their leaf biomass by 50%, and their trunk and branch biomass by 90%. Most impressive of all, they say the plants in the elevated CO2 treatment "released 2.8-fold more monoterpenes per plant than plants grown in ambient CO2," on top of which Pe˝uelas and Llusia (2003) state that warming "increases the emission rates of most BVOCs exponentially."

Most recently, and focusing on rising temperatures and atmospheric CO2 concentrations, Raisanen et al. (2008) designed an experiment to see to what extent a doubling of the air's CO2 content and 2-6░C increases in air temperature (+2░C in summer, +4░C in spring and autumn, +6░C in winter), applied singly or together, might impact emissions of monoterpenes from 20-year-old Scots pine seedlings. To this end, they constructed closed-top chambers over parts of a naturally-seeded stand of the trees in eastern Finland, which they exposed to these treatments for a period of five years. Over the five-month growing season of May-September, the three researchers found that total monoterpene emissions in the elevated-CO2-only treatment were 5% greater than those in the ambient-CO2-ambient-temperature treatment, and that emissions in the elevated-temperature-only treatment were 9% less than those in ambient air. In the presence of both elevated CO2 and elevated temperature, however, there was an increase of fully 126% in the total amount of monoterpenes emitted over the growing season.

Clearly, the diverse results of these several studies do not paint a clear picture of what we should expect in the way of plant monoterpene emissions in a CO2-enriched and possibly warmer world of the future; and until a better vision is obtained, it would be unwise to speculate further about the situation.

References
Baraldi, R., Rapparini, F., Oechel, W.C., Hastings, S.J., Bryant, P., Cheng, Y. and Miglietta, F. 2004. Monoterpene emission responses to elevated CO2 in a Mediterranean-type ecosystem. New Phytologist 161: 17-21.

Constable, J.V.H., Litvak, M.E., Greenberg, J.P. and Monson, R.K. 1999. Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming. Global Change Biology 5: 255-267.

Loreto, F., Fischbach, R.J., Schnitzler, J.P., Ciccioli, P., Brancaleoni, E., Calfapietra, C. and Seufert, G. 2001. Monoterpene emission and monoterpene synthase activities in the Mediterranean evergreen oak Quercus ilex L. grown at elevated CO2 concentrations. Global Change Biology 7: 709-717.

Pe˝uelas, J. and Llusia, J. 2003. BVOCs: plant defense against climate warming? Trends in Plant Science 8: 105-109.

Pe˝uelas, J., Llusia, J. and Estiarte, M. 1995. Terpenoids: a plant language. Trends in Ecology and Evolution 10: 289.

Pichersky, E. and Gershenzon, J. 2002. The formation and function of plant volatiles: perfumes for pollinator attraction and defense. Current Opinion in Plant Biology 5: 237-243.

Raisanen, T., Ryyppo, A. and Kellomaki, S. 2008. Effects of elevated CO2 and temperature on monoterpene emission of Scots pine (Pinus sylvestris L.). Atmospheric Environment 42: 4160-4171.

Rapparini, F., Baraldi, R., Miglietta, F. and Loreto, F. 2004. Isoprenoid emission in trees of Quercus pubescens and Quercus ilex with lifetime exposure to naturally high CO2 environment. Plant, Cell and Environment 27: 381-391.

Shulaev, V., Silverman, P. and Raskin, I. 1997. Airborne signaling by methyl salicylate in plant pathogen resistance. Nature 385: 718-721.

Staudt, M., Joffre, R., Rambal, S. and Kesselmeier, J. 2001. Effect of elevated CO2 on monoterpene emission of young Quercus ilex trees and its relation to structural and ecophysiological parameters. Tree Physiology 21: 437-445.

Vuorinen, T., Reddy, G.V.P., Nerg, A.-M. and Holopainen, J.K. 2004. Monoterpene and herbivore-induced emissions from cabbage plants grown at elevated atmospheric CO2 concentration. Atmospheric Environment 38: 675-682.

Last updated 27 August 2008