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Acclimation (Tree Species: Citrus) -- Summary
Trees grown for long periods of time in air enriched with CO2 sometimes exhibit photosynthetic acclimation or down regulation. This phenomenon is characterized by modestly reduced rates of photosynthesis (compared to what is observed during short-term exposure to CO2-enriched air), which result from a slow decline in the activity and/or amount of the primary plant carboxylating enzyme rubisco; and in this Summary we report what has been learned about this phenomenon in the few experiments that have studied it in citrus trees.

Jifon et al. (2002) grew seedlings of sour orange (Citrus aurantium L.) and sweet orange (Citrus sinensis L.) for nearly three months in glasshouses maintained at atmospheric CO2 concentrations of either 360 or 700 ppm, where they were either inoculated with arbuscular mycorrhizal fungi or left non-inoculated as control plants. Thus, they studied the effects of both elevated CO2 and fungal presence on photosynthesis and growth in these two citrus species of contrasting fungal acceptance: sour orange, which displays strong associations with mycorrhizal symbionts, and sweet orange, which exhibits relatively weaker relationships with such fungi.

This protocol revealed that elevated CO2 increased photosynthetic rates in non-mycorrhizal and mycorrhizal sour orange tree seedlings by 18 and 118%, respectively. Similarly, elevated CO2 enhanced photosynthetic rates in non-mycorrhizal and mycorrhizal sweet orange seedlings by 50 and 67%, respectively. In terms of biomass production, however, the mycorrhizal sour orange seedlings exposed to ambient CO2 displayed 18% less growth than the non-mycorrhizal control seedlings; but at elevated CO2, the mycorrhizal seedlings displayed 15% more growth than the non-mycorrhizal seedlings. Thus, atmospheric CO2 enrichment more than compensated for the carbon costs associated with maintaining the mycorrhizal fungal symbiosis in the sour orange seedlings, while sweet orange seedlings exposed to elevated CO2 exhibited the same increase in biomass with or without fungal inoculation, indicating that this species is less dependent upon fungal symbiosis in eliciting CO2-induced growth responses.

As the air's CO2 content continues to rise, therefore, both of these citrus species will likely respond by exhibiting enhanced rates of photosynthesis and biomass production, irrespective of their associations (or not) with symbiotic mycorrhizal fungi. However, in sour orange trees, photosynthetic and growth responses to elevated CO2 will likely be greater when seedlings are involved in symbiotic relationships with soil fungi. In fact, Jifon et al. found that the degree of CO2-induced photosynthetic acclimation or downregulation in sour orange tree seedlings was significantly reduced when mycorrhizal fungi were present, as they served as a carbon sink for excess carbohydrates produced by photosynthesis, thereby alleviating the notorious end-product inhibition of photosynthesis. Thus, it is likely that increasing atmospheric CO2 concentrations may increase growth in nearly all tree species throughout their normal life spans, for most are ubiquitously involved in symbiotic relationships with one or more types of mycorrhizal fungi.

Working exclusively with sour orange trees, Adam et al. (2004) measured numerous plant physiological processes and properties throughout the fourteenth year of a long-term study of the effects of a 75% increase (from 400 ppm to 700 ppm) in the air's CO2 concentration on the growth and development of sour orange trees that had been grown from the seedling stage to maturity under well-watered and fertilized conditions out-of-doors at Phoenix, Arizona (USA) in clear-plastic-wall open-top enclosures, after which they compared certain of their results with those of similar measurements made in earlier years of the study.

This work revealed that in the second year of the experiment, net photosynthesis rates were 2.84 times greater in the CO2-enriched enclosures than in the ambient-air enclosures. By the sixth year of the study, however, this enhancement ratio had declined to 1.75, while in the fourteenth year it had dropped to 1.45. Plotting similarly-declining above-ground woody biomass ratios against these net photosynthesis ratios, Adam et al. derived a linear relationship with an r2 value of 0.997 that yielded a CO2-induced woody biomass enhancement ratio of 1.78 at the 14-year point of the study. This value for the woody biomass ratio had previously been found by Idso and Kimball (2001) to have been essentially constant from year 10 to year 14, leading Adam et al. to conclude that the CO2-induced net photosynthesis ratio they derived (1.45) had likely also been essentially constant over this period, indicative of the final equilibrium level at which it had apparently stabilized.

Other evidence for the down-regulation of photosynthesis in the CO2-enriched sour orange tree leaves was provided by the observation that in year 14 of the study, the pooled mean of the concentration of the large subunit of Rubisco in the CO2-enriched leaves was only 78% of that observed in the ambient-air leaves, while the concentration of the small subunit of Rubisco was reduced by 34% in the CO2-enriched leaves compared to the ambient-air leaves. In addition, the full and initial activities of Rubisco under CO2 enrichment were reduced, as were leaf chlorophyll a and total nitrogen concentrations.

As for the implications of these findings, Adam et al. concluded that "long-term CO2 enrichment can result in photosynthetic down-regulation in leaves of trees, even under non-limiting nitrogen conditions." It must be pointed out, however, that at the final equilibrium level of acclimation experienced in the sour orange trees of this long-term study, the 75% enhancement of the air's CO2 concentration still produced an equivalent percentage increase (or possibly slightly more) in both wood and fruit production (78 and 80%, respectively).

Three years later, Kimball et al. (2007) described the final state of the Phoenix sour orange trees at the termination of that record-breaking 17-year-long CO2 enrichment experiment. In terms of total biomass production, which was the primary focus of their summary report, they state that the CO2-enriched to ambient-treatment ratio of annual wood plus fruit production peaked in years 2-4 of the experiment at a value of approximately 2.4, but that following that peak, "there was a decline through year 8." Thereafter, however, they found that the annually-produced-biomass ratios were, as they describe it, "more or less at a plateau that corresponded with the value of the ratio at final harvest of 1.69."

In terms of harvestable yield, i.e., fruit production, the results were even better; for the four researchers wrote that "the cumulative amount of biomass due to fruit production over the duration of the experiment was increased 85% due to elevated CO2," which increase "was entirely from an increase in fruit number." In addition, they report that "the vitamin C content of the fruit was increased 7% based on samples taken from the fourth through the 12th years of the experiment," citing the study of Idso et al. (2002). Consequently, not only were a whole lot more oranges produced by the trees in the CO2-enriched chambers, a whole lot more better-quality oranges were produced.

In their concluding discussion of one of the major implications of the study, Kimball et al. say that "rather than a continual acclimation" -- i.e., rather than a gradual long-term decline in the aerial fertilization effect of the extra 300 ppm of CO2 supplied to the CO2-enriched trees (which dwindling away of the effect's efficacy is periodically postulated by many climate alarmists) -- "instead there was a sustained enhancement of about 70% in annual fruit and incremental wood production over the last several years of the experiment." This observation thus led them to conclude that "the effects of elevated CO2 on trees can be large and sustained for many years," as they indeed demonstrated to be the case with the sour orange trees they studied, as there had been a 70% sustained increase in total biomass production, and an 85% increase in fruit production, over the entire last decade of the experiment in response to the 75% increase in the air's CO2 content employed throughout the study.

Adam, N.R., Wall, G.W., Kimball, B.A., Idso, S.B. and Webber, A.N. 2004. Photosynthetic down-regulation over long-term CO2 enrichment in leaves of sour orange (Citrus aurantium) trees. New Phytologist 163: 341-347.

Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D. 2002. The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice. Agriculture, Ecosystems and Environment 90: 1-7.

Jifon, J.L., Graham, J.H., Drouillard, D.L. and Syvertsen, J.P. 2002. Growth depression of mycorrhizal Citrus seedlings grown at high phosphorus supply is mitigated by elevated CO2. New Phytologist 153: 133-142.

Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.

Last updated 13 October 2010