Fluctuating asymmetry (FA) is the terminology used to describe small variations from perfect symmetry in otherwise bilaterally-symmetrical characters in an organism (Moller and Swaddle, 1997). It is believed to arise in consequence of developmental instability experienced during ontogeny that is caused by various stresses, including both genetic and environmental factors (Martel et al., 1999; Cornelissen and Stiling, 2005); and it has been studied extensively in animals but less so in plants (Moller and Shykoff, 1999).
In the first study to address the effects of atmospheric CO2 enrichment on leaf asymmetry and how herbivores respond to these effects, Cornelissen et al. (2004) opened up a whole new window through which to view the world of the future in terms of the potential effects of the ongoing rise in the air's CO2 content on the plant and animal components of the biosphere. The Cornelissen et al. study was conducted on a native scrub-oak community at the Kennedy Space Center, Titusville, Florida, USA, which is dominated by myrtle oak (Quercus myrtifolia) and sand live oak (Quercus geminata) under atmospheric CO2 concentrations of approximately 370 and 700 ppm.
Based on measurements of (1) distances from the leaf midrib to the left and right edges of the leaf at its widest point and (2) leaf areas on the left and right sides of the leaf midrib, Cornelissen et al. determined that "asymmetric leaves were less frequent in elevated CO2, and, when encountered, they were less asymmetric than leaves growing under ambient CO2." In addition, they found that "Q. myrtifolia leaves under elevated CO2 were 15.0% larger than in ambient CO2 and Q. geminata leaves were 38.0% larger in elevated CO2 conditions." As a bonus, they also determined that "elevated CO2 significantly increased tannin concentration for both Q. myrtifolia and Q. geminata leaves" and that "asymmetric leaves contained significantly lower concentrations of tannins than symmetric leaves for both Q. geminata and Q. myrtifolia."
In commenting on their primary findings of reduced percentages of leaves experiencing asymmetry in the presence of elevated levels of atmospheric CO2 and the lesser degree of asymmetry exhibited by affected leaves in the elevated CO2 treatment, Cornelissen et al. say that "a possible explanation for this pattern is the fact that, in contrast to other environmental stresses, which can cause negative effects on plant growth, the predominant effect of elevated CO2 on plants is to promote growth with consequent reallocation of resources (Docherty et al., 1996)." Another possibility they discuss "is the fact that CO2 acts as a plant fertilizer," and, as a result, that "elevated CO2 ameliorates plant stress compared with ambient levels of CO2," which is one of the well-documented biological benefits of atmospheric CO2 enrichment (Idso and Idso, 1994).
With respect to the ancillary finding of CO2-induced increases in tannin concentrations in the leaves of both oaks (a mean increase of approximately 35% for Q. myrtifolia and 43% for Q. geminata), it should be noted that this phenomenon may provide both species with greater protection against herbivores, and that part of that protection may be associated with the observed CO2-induced reductions in the amount and degree of asymmetry in the leaves of the CO2-enriched trees. Consistent with this hypothesis, for example, Stiling et al. (1999, 2003) found higher abundances of leaf miners in the leaves of the trees in the ambient CO2 chambers, where asymmetric leaves were more abundant, while in the current study it was determined that leaf miners attacked asymmetric leaves more frequently than would be expected by chance alone in both CO2 treatments.
In further support of this CO2-induced benefit, Cornelissen and Stiling (2005) evaluated patterns of asymmetry in 40 leaves from each of 30 trees of each of two species of oak - sand live oak (Quercus geminata) and turkey oak (Q. laevis) - at the University of South Florida Botanical Garden in Tampa, Florida, USA, well before any herbivores had begun to attack the trees that growing season. Thereafter, patterns of leaf asymmetry, leaf quality and herbivory were examined for 30 individual trees of each of the two oak species from March to October of the same year.
The "before and after" measurements clearly indicated that differential herbivory patterns neither caused nor affected patterns of leaf FA. However, they revealed, in the words of the authors, that "herbivores may use asymmetry as a cue to plant quality and suitable oviposition sites," as plants with a higher percentage of asymmetric leaves were attacked more frequently by various leaf miners, as were leaves on the same plant that were more asymmetric. One of the reasons for these choices may have been, as Cornelissen and Stiling report, that "asymmetric leaves of both plant species exhibited better nutritional quality for herbivores than symmetric leaves," with asymmetric leaves possessing "significantly lower concentrations of tannins [-22% for Q. geminata and -36% for Q. laevis] and higher nitrogen content [+8% for both species]."
In one additional study, Kaligaric et al. (2008) measured the degree of FA in "undamaged (not grazed, not visibly attacked by herbivores or pathogens) fully developed leaves" of the Mediterranean shrub Myrtus communis L. growing along an atmospheric CO2 gradient (570, 530, 490, 450, 410 and 370 ppm) moving away from a natural CO2 spring "I Borboi" near Lajatico (Pisa, Tuscany, Italy) at distances of 2, 18, 34, 50, 66 and 82 m, respectively, from the CO2 source.
The four researchers report they found "a significant and negative correlation between CO2 concentration and leaf FA," such that "with increased CO2 concentration the leaf FA decreased," which result, in their words, "confirms what was obtained by Cornelissen et al. (2004) on Quercus myrtifolia and Quercus geminata (in a short-term experiment)." In addition, they note that "Myrtus communis, grown under elevated CO2 concentration at 'I Borboi,' showed a reduction in xylem embolism and an increase in hydraulic efficiency (Tognetti et al., 2001)," stating that "improved water relations could represent a good explanation for the observed reduction in leaf FA [as the air's CO2 content increased]."
In discussing their findings, Kaligaric et al. say that "adaptation and selection could explain the tendency towards decreased leaf FA in plants from the CO2 spring relative to ambient conditions," since "the more symmetrical leaves under long-term elevated CO2 concentration were more developmentally stable in these conditions."
In light of the results of the above studies, a reduction in leaf FA can be added to the ever-growing number of benefits plants will likely experience as the air's CO2 concentration continues to rise.
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Cornelissen, T., Stiling, P. and Drake, B. 2004. Elevated CO2 decreases leaf fluctuating asymmetry and herbivory by leaf miners on two oak species. Global Change Biology 10: 27-36.
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Kaligaric, M., Tognetti, R., Janzekovic, F. and Raschi, A. 2008. Leaf fluctuating asymmetry of Myrtus communis L., affected by increases in atmospheric CO2 concentration: Evidence from a natural CO2 spring. Polish Journal of Environmental Studies 17: 503-508.
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Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G. 2003. Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia 134: 82-87.
Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B. 1999. Decreased leaf-miner abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack. Ecological Applications 9: 240-244.
Tognetti, R., Longobucco, A., Raschi, A. and Jones, M.B. 2001. Stem hydraulic properties and xylem vulnerability to embolism in three co-occurring Mediterranean shrubs at a natural CO2 spring. Australian Journal of Plant Physiology 28: 257-268.Last updated 22 August 2012