Nutrition
From ClimateWiki
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Rising concentrations of CO2 in the atmosphere affect human health indirectly by enhancing plant productivity, a topic examined at length in Chapter 7. In this section we review the scientific literature on CO2-induced changes to the quantity and quality of food crops—in particular the protein and antioxidants present in grains and fruits—and on the medicinal properties of some plants. We find the overwhelming weight of evidence indicates a positive effect of global warming on human health.
Contents |
Food Quantity
The concentration of CO2 in the earth’s atmosphere has risen approximately 100 ppm since the inception of the Industrial Revolution. To measure the effect this increase had on wheat, Mayeux et al. (1997) grew two cultivars of commercial wheat in a 38-meter-long soil container topped with a transparent tunnel-like polyethylene cover within which a CO2 gradient was created that varied from approximately 350 ppm at one end of the tunnel to about 200 ppm at the other end. Both wheat cultivars were irrigated weekly over the first half of the 100-day growing season, to maintain soil water contents near optimum conditions. Over the last half of the season, this regimen was maintained on only half of the wheat of each cultivar, in order to create both water-stressed and well-watered treatments.
At the conclusion of the experiment, the scientists determined that the growth response of the wheat was a linear function of atmospheric CO2 concentration in both cultivars under both adequate and less-than-adequate soil water regimes. Based on the linear regression equations they developed for grain yield in these situations, we calculate that the 100-ppm increase in atmospheric CO2 concentration experienced over the past century-and-a-half probably increased the mean grain yield of the two wheat cultivars by about 72 percent under well-watered conditions and 48 percent under water-stressed conditions, for a mean yield increase on the order of 60 percent under the full range of moisture conditions likely to have existed in the real world. In other words, the historical rise in CO2 concentrations may have increased wheat yields by 60 percent, clearly a benefit to a growing population.
This CO2-induced yield enhancement to wheat production also has been documented by Alexandrov and Hoogenboom, 2000a; Brown and Rosenberg, 1999; Cuculeanu et al., 1999; Dijkstra et al., 1999; Eitzinger et al., 2001; Harrison and Butterfield, 1996; Masle, 2000; Southworth et al., 2002; and van Ittersum et al., 2003. Nor is wheat the only food crop that benefits from CO2-fertilization. Research reviewed in Chapter 7 showing increased production by other crops exposed to enhanced CO2, includes the following:
- Alfalfa (De Luis et al., 1999; Luscher et al., 2000; Morgan et al., 2001; Sgherri et al., 1998)
- Cotton (Booker, 2000; Booker et al., 2000; Leavitt et al., 1994; Reddy et al., 1999; Reddy et al., 1998. Tischler et al., 2000)
- Corn (maize) (Baczek-Kwinta and Koscielniak, 2003; Bootsma et al., 2005; Conway and Toenniessen, 2003; Leakey et al., 2004; Magrin et al., 2005; Maroco et al., 1999; Shen et al., 2005; Watling and Press, 1997; Watling and Press, 2000)
- Peanuts (Alexandrov and Hoogenboom, 2000b; Prasad et al., 2003; Stanciel et al., 2000; Vu, 2000
- Potatoes (Bunce, 2003; Chen and Setter, 2003; Fangmeier and Bender, 2002; Kauder et al., 2000; Lawson et al., 2001; Louche-Tessandier et al., 1999; Ludewig et al., 1998; Magliulo et al., 2003; Miglietta et al., 1998; Olivo et al., 2002; Pruski et al., 2002; Schapendonk, et al., 2000; Sicher and Bunce, 1999; Wolf and van Oijen, 2002; Wolf and van Oijen, 2003)
- Rice (Baker et al., 2000; De Costa et al., 2003a; De Costa et al., 2003b; Gesch et al., 2002; Kim et al., 2003; Kim et al., 2001; Kobayashi et al., 2001; Tako, et al., 2001; Watling and Press, 2000; Weerakoon et al., 2000; Widodo et al., 2003; Ziska et al., 1997)
- Sorgham (Ainsworth and Long, 2005; Ottman et al., 2001; Prior et al., 2005; Watling and Press, 1997)
- Soybeans (Alexandrov and Hoogenboom, 2000b; Allen et al., 1998; Bernacchi et al., 2005; Birt et al., 2001; Bunce, 2005; Caldwell et al., 2005; Ferris et al., 1999; Heagle et al., 1998; Messina, 1999; Nakamura et al., 1999; Rogers et al., 2004; Serraj et al., 1999; Thomas et al., 2003; Wittwer, 1995; Ziska, 1998; Ziska and Bunce, 2000; Ziska et al., 2001a; Ziska et al., 2001b)
- Strawberries (Bunce, 2001; Bushway and Pritts, 2002; Deng and Woodward, 1998)
Based on this voluminous data and much more, Idso and Idso (2000) calculated that the increase in atmospheric CO2 concentration during the past 150 years probably caused mean yield increases on the order of 70 percent for wheat and other C3 cereals, 28 percent for C4 cereals, 33 percent for fruits and melons, 62 percent for legumes, 67 percent for root and tuber crops, and 51 percent for vegetables.
Such major increases in production by important food plants due to the historical increase in the air’s CO2 content have undoubtedly benefitted human health. In fact, it is safe to say that some of the people reading these words would not be alive today were it not for the CO2 enrichment caused by human industry since the beginning of the Industrial Revolution.
What does the IPCC say about this extraordinary benefit to human health made possible by rising CO2 levels? Incredibly, it is not mentioned anywhere in the contribution of Working Group I to the Fourth Assessment Report of the IPCC (IPCC 2007-I) or in the chapter on the impact of global warming on human health in the contribution of Working Group II (IPCC 2007-II). It is treated dismissively in the chapter on agriculture in the contribution of Working Group III (IPCC 2007-III), even though the proposals justified in the first two volumes and advanced in the third would reduce CO2 emissions and therefore have a negative impact on crop yields. To call this a gross oversight is to be kind to the authors of these reports.
In view of these observations, it is indisputable that the ongoing rise in the air’s CO2 content has bestowed a huge benefit to human health by expanding the yields of food crops.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2 science.org/subject/a/agriculture.php.
Food Quality
The quantity of food is mankind’s primary concern when it comes to survival. But after survival is assured, the quality of food rises to the fore. What role does the ongoing rise in the air’s CO2 content play here? In this section we survey the literature on the effects of higher CO2 air concentration on plant protein and antioxidant content.
Protein Content
Idso and Idso (2001) and Idso et al. (2001) cited studies where elevated levels of atmospheric CO2 either increased, decreased, or had no effect on the protein concentrations of various agricultural crops. The relationship, as we will see, is complex, though in the end it appears that enhanced atmospheric CO2 has a positive effect on the protein content of most crops.
Pleijel et al. (1999) analyzed the results of 16 open-top chamber experiments that had been conducted on spring wheat in Denmark, Finland, Sweden, and Switzerland between 1986 and 1996. In addition to CO2 enrichment of the air, these experiments included increases and decreases in atmospheric ozone (O3). The scientists found that while increasing O3 pollution reduced wheat grain yield it simultaneously increased the protein concentration of the grain. Removing O3 from the air led to higher grain yield but lower protein concentration. The opposite relationship was found for atmospheric CO2 enrichment, which increased grain yield but lowered protein concentration. Water stress, which was also a variable in one of the experiments, reduced yield and increased grain protein concentrations.
In an earlier study of CO2 and O3 effects on wheat grain yield and quality, Rudorff et al. (1996) found that “flour protein contents were increased by enhanced O3 exposure and reduced by elevated CO2” but that “the combined effect of these gases was minor.” They conclude that “the concomitant increase of CO2 and O3 in the troposphere will have no significant impact on wheat grain quality.”
Earlier, Evans (1993) had found several other crops to be greatly affected by soil nitrogen availability. Rogers et al. (1996) observed CO2-induced reductions in the protein concentration of flour derived from wheat plants growing at low soil nitrogen concentrations, but no such reductions were evident when the soil nitrogen supply was increased. Pleijel et al. concluded that the oft-observed negative impact of atmospheric CO2 enrichment on grain protein concentration would probably be alleviated by higher applications of nitrogen fertilizers.
The study of Kimball et al. (2001) confirmed their hypothesis. Kimball et al. studied the effects of a 50 percent increase in atmospheric CO2 concentration on wheat grain nitrogen concentration and the baking properties of the flour derived from that grain throughout four years of free-air CO2 enrichment experiments. In the first two years of their study, soil water content was an additional variable; in the last two years, soil nitrogen content was a variable. The most influential factor in reducing grain nitrogen concentration was determined to be low soil nitrogen. Under this condition, atmospheric CO2 enrichment further reduced grain nitrogen and protein concentrations, although the change was much less than that caused by low soil nitrogen. When soil nitrogen was not limiting, however, increases in the air’s CO2 concentration did not affect grain nitrogen and protein concentrations; neither did they reduce the baking properties of the flour derived from the grain. Hence, it would appear that given sufficient water and nitrogen, atmospheric CO2 enrichment can significantly increase wheat grain yield without sacrificing grain protein concentration in the process.
There are some situations where atmospheric CO2 enrichment has been found to increase the protein concentration of wheat. Agrawal and Deepak (2003), for example, grew two cultivars of wheat (Triticum aestivum L. cv. Malviya 234 and HP1209) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm alone and in combination with 60 ppb SO2 to study the interactive effects of elevated CO2 and this major air pollutant on crop growth. They found that exposure to the elevated SO2 caused a 13 percent decrease in foliar protein concentrations in both cultivars; but when the plants were concomitantly exposed to an atmospheric CO2 concentration of 600 ppm, leaf protein levels decreased only by 3 percent in HP1209, while they actually increased by 4 percent in Malviya 234.
In the case of rice—which according to Wittwer (1995) is “the basic food for more than half the world’s population,” supplying “more dietary energy than any other single food”—Jablonski et al. (2002) conducted a wide-ranging review of the scientific literature, finding that it too appeared to suffer no reduction in grain nitrogen (protein) concentration in response to atmospheric CO2 enrichment. Likewise, they found no CO2-induced decrease in seed nitrogen concentration in the studies of legumes they reviewed. This finding is also encouraging, since according to Wittwer (1995) legumes “are a direct food resource providing 20 percent of the world’s protein for human consumption,” as well as “about two thirds of the world’s protein concentrate for livestock feeding.” What is more, the biomass of the CO2-enriched wheat, rice, and legumes was found by Jablonski et al. to be significantly increased above that of the same crops grown in normal air. Hence, there will likely be a large increase in the total amount of protein made available to humanity in a future CO2-enriched world, both directly via food crops and indirectly via livestock.
With respect to the leguminous soybean, Thomas et al. (2003) additionally note that “oil and protein comprise ~20 and 40 percent, respectively, of the dry weight of soybean seed,” which “unique chemical composition,” in their words, “has made it one of the most valuable agronomic crops worldwide.” In addition, they say “the intrinsic value of soybean seed is in its supply of essential fatty acids and amino acids in the oil and protein, respectively,” and they report that Heagle et al. (1998) “observed a positive significant effect of CO2 enrichment on soybean seed oil and oleic acid concentration.”
Legumes and their responses to atmospheric CO2 enrichment also figure prominently in a number of studies of mixed forage crops. In a study of nitrogen cycling in grazed pastures on the North Island of New Zealand, for example, Allard et al. (2003) report that under elevated CO2, leaves of the individual species exhibited lower nitrogen concentrations but higher water-soluble carbohydrate (WSC) concentrations. They also say “there was a significantly greater proportion of legume in the diet at elevated CO2,” and that this “shift in the botanical composition towards a higher proportion of legumes counterbalanced the nitrogen decrease observed at the single species scale, resulting in a nitrogen concentration of the overall diet that was unaffected by elevated CO2.” They further report that “changes at the species level and at the sward level appeared to combine additively in relation to WSC,” and “as there was a significant correlation between WSC and digestibility (as previously observed by Dent and Aldrich, 1963 and Humphreys, 1989), there was also an increase in digestibility of the high CO2 forage,” which result, in their words, “matches that found in a Mini-FACE experiment under cutting (Teyssonneyre, 2002; Picon-Cochard et al., 2004),” where “digestibility also increased in response to CO2 despite reduced crude protein concentration.” These data, plus the strong relationship between soluble sugars (rather than nitrogen) and digestibility, led them to suggest that “the widespread response to CO2 of increased soluble sugars might lead to an increase in forage digestibility.”
Luscher et al. (2004) found much the same thing in their review of the subject, which was based primarily on studies conducted at the Swiss FACE facility that hosts what has become the world’s longest continuous atmospheric CO2 enrichment study of a naturally occurring grassland. In response to an approximate two-thirds increase in the air’s CO2 concentration, leaf nitrogen (N) concentrations of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were reduced by 7 percent and 18 percent, respectively, when they were grown separately in pure stands. However, as Luscher et al. report, “the considerably lower concentration of N under elevated CO2, observed for L. perenne leaves in pure stands, was found to a much lesser extent for L. perenne leaves in the bi-species mixture with T. repens (Zanetti et al., 1997; Hartwig et al., 2000).” Furthermore, as they continue, “under elevated CO2 the proportion of N-rich T. repens (40 mg N g-1 dry matter) increased in the mixture at the expense of the N-poor L. perenne (24 mg N g-1 dry matter when grown in monoculture),” the end result being that “the concentration of N in the harvested biomass of the mixture showed no significant reduction.”
Campbell et al. (2000) analyzed research conducted between 1994 and 1999 by a worldwide network of 83 scientists associated with the Global Change and Terrestrial Ecosystems (GCTE) Pastures and Rangelands Core Research Project 1 (CRP1) that resulted in the publication of more than 165 peer-reviewed scientific journal articles. Campbell et al. determined from this massive collection of data that the legume content of grass-legume swards was typically increased by approximately 10 percent in response to a doubling of the air’s CO2 content.
Luscher et al. (2004) state that “the nutritive value of herbage from intensively managed grassland dominated by L. perenne and T. repens … is well above the minimum range of the concentration of crude protein necessary for efficient digestion by ruminants (Barney et al., 1981).” They conclude that “a small decrease in the concentration of crude protein in intensively managed forage production systems [which may never occur, as noted above] is not likely to have a negative effect on the nutritive value or on the intake of forage.”
One final forage study is Newman et al. (2003), who investigated the effects of two levels of nitrogen fertilization and an approximate doubling of the air’s CO2 content on the growth and chemical composition of tall fescue (Festuca arundinacea Schreber cv. KY-31), both when infected and uninfected with a mutualistic fungal endophyte (Neotyphodium coenophialum Morgan-Jones and Gams). They found that the elevated CO2 reduced the crude protein content of the forage by an average of 21 percent in three of the four situations studied: non-endophyte-infected plants in both the low and high nitrogen treatments, and endophyte-infected plants in the high nitrogen treatment. However, there was no protein reduction for endophyte-infected plants in the low nitrogen treatment.
As noted by Newman et al., “the endophyte is present in many native and naturalized populations and the most widely sown cultivars of F. arundinacea,” so the first two situations in which the CO2-induced protein reduction occurred (those involving non-endophyte-infected plants) are not typical of the real world. In addition, since the dry-weight biomass yield of the forage was increased by fully 53 percent under the low nitrogen regime, and since the 10-times-greater high nitrogen regime boosted yields only by an additional 8 percent, there would appear to be no need to apply any extra nitrogen to F. arundinacea in a CO2-enriched environment. Consequently, under best management practices in a doubled-CO2 world of the future, little to no nitrogen would be added to the soil and there would be little to no reduction in the crude protein content of F. arundinacea, but there would be more than 50 percent more of it produced on the same amount of land.
With respect to the final plant quality studied by Newman et al.—i.e., forage digestibility— increasing soil nitrogen lowered in vitro neutral detergent fiber digestibility in both ambient and CO2-enriched air; this phenomenon was most pronounced in the elevated CO2 treatment. Again, however, under low nitrogen conditions there was no decline in plant digestibility. Hence, there is a second good reason not to apply extra nitrogen to F. arundinacea in a high CO2 world of the future and, of course, little to no need to do so. Under best management practices in a future CO2-enriched atmosphere, therefore, the results of this study suggest much greater quantities of good-quality forage could be produced without the addition of any, or very little, extra nitrogen to the soil.
But what about the unmanaged world of nature? Increases in the air’s CO2 content often—but not always (Goverde et al., 1999)—lead to greater decreases in the concentrations of nitrogen and protein in the foliage of C3 as compared to C4 grasses (Wand et al., 1999); as a result, in the words of Barbehenn et al. (2004a), “it has been predicted that insect herbivores will increase their feeding damage on C3 plants to a greater extent than on C4 plants” (Lincoln et al., 1984, 1986; Lambers, 1993).
To test this hypothesis, Barbehenn et al. (2004a) grew Lolium multiflorum Lam. (Italian ryegrass, a common C3 pasture grass) and Bouteloua curtipendula (Michx.) Torr. (sideoats gramma, a native C4 rangeland grass) in chambers maintained at either the ambient atmospheric CO2 concentration of 370 ppm or the doubled CO2 concentration of 740 ppm for two months, after which newly molted sixth-instar larvae of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid) were allowed to feed upon the grasses. As expected, foliage protein concentration decreased by 20 percent in the C3 grass, but by only 1 percent in the C4 grass, when grown in the CO2-enriched air. However, and “contrary to our expectations,” according to Barbehenn et al., “neither caterpillar species significantly increased its consumption rate to compensate for the lower concentration of protein in [the] C3 grass,” noting that “this result does not support the hypothesis that C3 plants will be subject to greater rates of herbivory relative to C4 plants in future [high-CO2] atmospheric conditions (Lincoln et al., 1984).” In addition, and “despite significant changes in the nutritional quality of L. multiflorum under elevated CO2,” they report that “no effect on the relative growth rate of either caterpillar species on either grass species resulted” and there were “no significant differences in insect performance between CO2 levels.”
In a similar study with the same two plants, Barbehenn et al. (2004b) allowed grasshopper (Melanoplus sanguinipes) nymphs that had been reared to the fourth instar stage to feed upon the grasses; once again, “contrary to the hypothesis that insect herbivores will increase their feeding rates disproportionately in C3 plants under elevated atmospheric CO2,” they found that “M. sanguinipes did not significantly increase its consumption rate when feeding on the C3 grass grown under elevated CO2,” suggesting this observation implies that “post-ingestive mechanisms enable these grasshoppers to compensate for variable nutritional quality in their host plants,” and noting that some of these post-ingestive responses may include “changes in gut size, food residence time, digestive enzyme levels, and nutrient metabolism (Simpson and Simpson, 1990; Bernays and Simpson, 1990; Hinks et al., 1991; Zanotto et al., 1993; Yang and Joern, 1994a,b).” In fact, their data indicated that M. sanguinipes growth rates may have actually increased, perhaps by as much as 12 percent, when feeding upon the C3 foliage that had been produced in the CO2-enriched air.
In conclusion, the ongoing rise of the air’s CO2 concentration is not reducing the protein concentration in, or digestibility of, most important plant crops. In cases where protein concentration might by reduced, the addition of nitrogen fertilizer appears to offset the effect.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2 science.org/subject/p/ protein.php.
Antioxidant Content
Antioxidants are chemical compounds that inhibit oxidation. Some antioxidants found in the human diet, such as vitamin E, vitamin C, and beta carotene, are thought to protect body cells from the damaging effects of oxidation. Scurvy—a condition characterized by general weakness, anemia, gum disease (gingivitis), and skin hemorrhages—is induced by low intake of vitamin C. There is some evidence that the condition may be resurgent in industrial countries, especially among children (Dickinson et al., 1994; Ramar et al., 1993; Gomez-Carrasco et al., 1994). Hampl et al. (1999) found that 12-20 percent of 12- to 18-year-old school children in the United States “drastically under-consume” foods that supply vitamin C. Johnston et al. (1998) determined that 12-16 percent of U.S. college students have marginal plasma concentrations of vitamin C.
Since vitamin C intake correlates strongly with the consumption of citrus juice (Dennison et al., 1998), and since the only high-vitamin-C juice consumed in any quantity by children is orange juice (Hampl et al., 1999), even a modest role played by the ongoing rise in the air’s CO2 content in increasing the vitamin C concentration of orange juice could prove to be of considerable significance for public health in the United States and elsewhere. Thus, determining if rising CO2 concentrations increase or hinder the production of antioxidants in human food is relevant to the issue of what effect the historical rise in CO2 concentrations is having on human health.
Antioxidant concentrations in plants are generally observed to be high when environmental stresses are present, such as exposure to pollutants, drought, intense solar radiation, and high air or water temperatures. Stress generates highly reactive oxygenated compounds that damage plants, and ameliorating these stresses typically involves the production of antioxidant enzymes that scavenge and detoxify the highly reactive oxygenated compounds. In a study of two soybean genotypes, Pritchard et al. (2000) reported that three months’ exposure to twice-ambient CO2 concentrations reduced the activities of superoxide dismutase and catalase by an average of 23 and 39 percent, respectively. Likewise, Polle et al. (1997) showed that two years of atmospheric CO2 enrichment reduced the activities of several key antioxidative enzymes, including catalase and superoxide dismutase, in beech seedlings. Moreover, Schwanz and Polle (1998) demonstrated this phenomenon can persist indefinitely, as they discovered similar reductions in these same enzymes in mature oak trees that had been growing near natural CO2-emitting springs for 30 to 50 years.
The standard interpretation of these results is that the observed reductions in the activities of antioxidative enzymes under CO2-enriched conditions imply that plants exposed to higher-than-current atmospheric CO2 concentrations experience less oxidative stress and thus have a reduced need for antioxidant protection. This conclusion further suggests that “CO2-advantaged” plants will be able to funnel more of their limited resources into the production of other plant tissues or processes essential to their continued growth and development.
On the other hand, when oxidative stresses do occur under high CO2 conditions, the enhanced rates of photosynthesis and carbohydrate production resulting from atmospheric CO2 enrichment generally enable plants to better deal with such stresses by providing more of the raw materials needed for antioxidant enzyme synthesis. Thus, when CO2-enriched sugar maple seedlings were subjected to an additional 200 ppb of ozone, Niewiadomska et al. (1999) reported that ascorbate peroxidase, which is the first line of enzymatic defense against ozone, significantly increased. Likewise, Schwanz and Polle (2001) noted that poplar clones grown at 700 ppm CO2 exhibited a much greater increase in superoxide dismutase activity upon chilling induction than clones grown in ambient air. In addition, Lin and Wang (2002) observed that activities of superoxide dismutase and catalase were much higher in CO2-enriched wheat than in ambiently grown wheat following the induction of water stress.
In some cases, the additional carbon fixed during CO2-enrichment is invested in antioxidative compounds, rather than enzymes. One of the most prominent of these plant products is ascorbate or vitamin C. In the early studies of Barbale (1970) and Madsen (1971, 1975), a tripling of the atmospheric CO2 concentration produced a modest (7 percent) increase in this antioxidant in the fruit of tomato plants. Kimball and Mitchell (1981), however, could find no effect of a similar CO2 increase on the same species, although the extra CO2 of their study stimulated the production of vitamin A. In bean sprouts, on the other hand, a mere one-hour-per-day doubling of the atmospheric CO2 concentration actually doubled plant vitamin C contents over a seven-day period (Tajiri, 1985).
Probably the most comprehensive investigation of CO2 effects on vitamin C production in an agricultural plant—a tree crop (sour orange)—was conducted by Idso et al. (2002). In an atmospheric CO2 enrichment experiment begun in 1987 and still ongoing, a 75 percent increase in the air’s CO2 content was observed to increase sour orange juice vitamin C concentration by approximately 5 percent in run-of-the-mill years when total fruit production was typically enhanced by about 80 percent. In aberrant years when the CO2-induced increase in fruit production was much greater, however, the increase in fruit vitamin C concentration also was greater, rising to a CO2-induced enhancement of 15 percent when fruit production on the CO2-enriched trees was 3.6 times greater than it was on the ambient-treatment trees.
Wang et al. (2003) evaluated the effects of elevated CO2 on the antioxidant activity and flavonoid content of strawberry fruit in field plots at the U.S. Department of Agriculture’s Beltsville Agricultural Research Center in Beltsville, Maryland, where they grew strawberry plants (Fragaria x ananassa Duchesne cv. Honeoye) in six clear-acrylic open-top chambers, two of which were maintained at the ambient atmospheric CO2 concentration, two of which were maintained at ambient + 300 ppm CO2, and two of which were maintained at ambient + 600 ppm CO2 for a period of 28 months (from early spring of 1998 through June 2000). The scientists harvested the strawberry fruit, in their words, “at the commercially ripe stage” in both 1999 and 2000, after which they analyzed them for a number of different antioxidant properties and flavonol contents.
Before reporting what they found, Wang et al. provide some background by noting that “strawberries are good sources of natural antioxidants (Wang et al., 1996; Heinonen et al., 1998).” They further report that “in addition to the usual nutrients, such as vitamins and minerals, strawberries are also rich in anthocyanins, flavonoids, and phenolic acids,” and that “strawberries have shown a remarkably high scavenging activity toward chemically generated radicals, thus making them effective in inhibiting oxidation of human low-density lipoproteins (Heinonen et al., 1998).” In this regard, they note that previous studies (Wang and Jiao, 2000; Wang and Lin, 2000) “have shown that strawberries have high oxygen radical absorbance activity against peroxyl radicals, superoxide radicals, hydrogen peroxide, hydroxyl radicals, and singlet oxygen.”
They determined, first, that strawberries had higher concentrations of ascorbic acid (AsA) and glutathione (GSH) “when grown under enriched CO2 environments.” In going from ambient to ambient + 300 ppm CO2 and ambient + 600 ppm CO2, for example, AsA concentrations increased by 10 and 13 percent, respectively, while GSH concentrations increased by 3 and 171 percent, respectively. They also learned that “an enriched CO2 environment resulted in an increase in phenolic acid, flavonol, and anthocyanin contents of fruit.” For nine different flavonoids, for example, there was a mean concentration increase of 55 ± 23 percent in going from the ambient atmospheric CO2 concentration to ambient + 300 ppm CO2, and a mean concentration increase of 112 ± 35 percent in going from ambient to ambient + 600 ppm CO2. In addition, they report that the “high flavonol content was associated with high antioxidant activity.” As for the significance of these findings, Wang et al. note that “anthocyanins have been reported to help reduce damage caused by free radical activity, such as low-density lipoprotein oxidation, platelet aggregation, and endothelium-dependent vasodilation of arteries (Heinonen et al., 1998; Rice-Evans and Miller, 1996).”
In summarizing their findings, Wang et al. say “strawberry fruit contain flavonoids with potent antioxidant properties, and under CO2 enrichment conditions, increased the[ir] AsA, GSH, phenolic acid, flavonol, and anthocyanin concentrations,” further noting that “plants grown under CO2 enrichment conditions also had higher oxygen radical absorbance activity against [many types of oxygen] radicals in the fruit.”
Deng and Woodward (1998) reported that after growing strawberry plants in air containing an additional 170 ppm of CO2, total fresh fruit weights were 42 and 17 percent greater than weights displayed by control plants grown at high and low soil nitrogen contents, respectively. Bushway and Pritts (2002) reported that a two- to three-fold increase in the air’s CO2 content boosted strawberry fruit yield by an average of 62 percent. In addition, Campbell and Young (1986), Keutgen et al. (1997), and Bunce (2001) reported positive strawberry photosynthetic responses to an extra 300 ppm of CO2 ranging from 9 percent to 197 percent (mean of 76 percent ± 15 percent); and Desjardins et al. (1987) reported a 118 percent increase in photosynthesis in response to a 600 ppm increase in the air’s CO2 concentration.
Other researchers have found similar enhancements of antioxidative compounds under enriched levels of atmospheric CO2. Estiarte et al. (1999), for example, reported that a 180-ppm increase in the air’s CO2 content increased the foliar concentrations of flavonoids, which protect against UV-B radiation damage, in field-grown spring wheat by 11 to 14 percent. Caldwell et al. (2005) found that an ~75 percent increase in the air’s CO2 content increased the total isoflavone content of soybean seeds by 8 percent when the air temperature during seed fill was 18°C, by 104 percent when the air temperature during seed fill was 23°C, by 101 percent when the air temperature was 28°C, and by 186 percent and 38 percent, respectively, when a drought-stress treatment was added to the latter two temperature treatments.
Lastly, in an experiment conducted under very high atmospheric CO2 concentrations, Ali et al. (2005) found that CO2 levels of 10,000 ppm, 25,000 ppm, and 50,000 ppm increased total flavonoid concentrations of ginseng roots by 228 percent, 383 percent, and 232 percent, respectively, total phenolic concentrations by 58 percent, 153 percent, and 105 percent, cysteine contents by 27 percent, 65 percent, and 100 percent, and non-protein thiol contents by 12 percent, 43 percent, and 62 percent, all of which substances are potent antioxidants.
In summary, as the CO2 content of the air rises, plants typically experience less oxidative stress, and since they thus need fewer antioxidants for protection, antioxidant levels in their leaves decline, which enables them to use more of their valuable resources for other purposes. However, elevated CO2 also provides more of the raw materials needed for oxidant enzyme synthesis, leading to higher levels of antioxidative compounds—such as ascorbate, or vitamin C. Research shows this happens with enough frequency that higher CO2 levels will lead to higher concentrations of antioxidants, leading to better health.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2 science.org/subject/a/antioxidants.php.
Medicinal Constituents
Primitive medical records indicate that extracts from many species of plants have been used for treating a variety of human health problems for perhaps the past 3,500 years (Machlin, 1992; Pettit et al., 1993, 1995). In modern times the practice has continued, with numerous chemotherapeutic agents being isolated (Gabrielsen et al., 1992a). Until recently, however, no studies had investigated the effects of atmospheric CO2 enrichment on specific plant compounds of direct medicinal value.
Stuhlfauth et al. (1987) studied the individual and combined effects of atmospheric CO2 enrichment and water stress on the production of secondary metabolites in the woolly foxglove (Digitalis lanata EHRH), which produces the cardiac glycoside digoxin that is used in the treatment of cardiac insufficiency. Under controlled well-watered conditions in a phytotron, a near-tripling of the air’s CO2 content increased plant dry weight production in this medicinal plant by 63 percent, while under water-stressed conditions the CO2-induced dry weight increase was 83 percent. In addition, the concentration of digoxin within the plant dry mass was enhanced by 11 percent under well-watered conditions and by 14 percent under conditions of water stress.
In a subsequent whole-season field experiment, Stuhlfauth and Fock (1990) obtained similar results. A near-tripling of the air’s CO2 concentration led to a 75 percent increase in plant dry weight production per unit land area and a 15 percent increase in digoxin yield per unit dry weight of plant, which combined to produce a doubling of total digoxin yield per hectare of cultivated land.
Idso et al. (2000) evaluated the response of the tropical spider lily (Hymenocallis littoralis Jacq. Salisb.) to elevated levels of atmospheric CO2 over four growing seasons. This plant has been known since ancient times to possess anti-tumor activity; in modern times it has been shown to contain constituents that are effective against lymphocytic leukemia and ovary sarcoma (Pettit et al., 1986). These same plant constituents also have been proven to be effective against the U.S. National Cancer Institute’s panel of 60 human cancer cell lines, demonstrating greatest effectiveness against melanoma, brain, colon, lung, and renal cancers (Pettit et al., 1993). In addition, it exhibits strong anti-viral activity against Japanese encephalitis and yellow, dengue, Punta Tora, and Rift Valley fevers (Gabrielsen et al., 1992a,b).
Idso et al. determined that a 75 percent increase in the air’s CO2 concentration produced a 56 percent increase in the spider lily’s below-ground bulb biomass, where the disease-fighting substances are found. In addition, for these specific substances, they observed a 6 percent increase in the concentration of a two-constituent (1:1) mixture of 7-deoxynarciclasine and 7-deoxy-trans-dihydronarciclasine, an 8 percent increase in pancratistatin, an 8 percent increase in trans-dihydronarciclasine, and a 28 percent increase in narciclasine. Averaged together and combined with the 56 percent increase in bulb biomass, these percentage concentration increases resulted in a total mean active-ingredient increase of 75 percent for the plants grown in air containing 75 percent more CO2.
Other plant constituents that perform important functions in maintaining human health include sugars, lipids, oils, fatty acids, and macro- and micro-nutrients. Although concerns have been raised about the availability of certain of the latter elements in plants growing in a CO2-enriched world (Loladze, 2002), the jury is still out with respect to this subject as a consequence of the paucity of pertinent data.
Literally thousands of studies have assessed the impact of elevated levels of atmospheric CO2 on the quantity of biomass produced by agricultural crops, but only a tiny fraction of that number have looked at any aspect of food quality. From what has been learned about plant protein, antioxidants, and the few medicinal substances that have been investigated in this regard, there is no reason to believe these other plant constituents would be present in lower concentrations in a CO2-enriched world and ample evidence that they may be present in significantly higher concentrations and greater absolute amounts.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2 science.org/subject/h/co2healthpromoting.php.
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