Plant Nutrient and Medicinal Properties
In this section, we survey the peer-reviewed scientific literature pertaining to the effects of higher atmospheric CO2 concentrations on plant nutrient content and on specific plant compounds of direct medicinal value, such as antioxidants that inhibit oxidation, some of which (vitamins E, C, and beta carotene) are found in the human body and are thought to protect its cells from the damaging effects of oxidation.
Reactive oxygen species (ROS) generated during cellular metabolism or peroxidation of lipids and proteins play a causative role in the pathogenesis of cancer and coronary heart disease (CHD), as demonstrated by Slaga et al. (1987), Frenkel (1992), Marnett (2000), and Zhao et al. (2000). However, Yu et al. (2004) have noted “antioxidant treatments may terminate ROS attacks and reduce the risks of CHD and cancer, as well as other ROS-related diseases such as Parkinson’s disease (Neff, 1997; Chung et al., 1999; Wong et al., 1999; Espin et al., 2000; Merken and Beecher, 2000),” and they therefore state, “developing functional foods rich in natural antioxidants may improve human nutrition and reduce the risks of ROS-associated health problems.”
Spurred on by these findings and thoughts, Levine et al. (2008) grew well-watered and -fertilized wheat plants (Triticum aestivum, cv Yocoro roho) from seed in custom-designed root modules—“consisting of a porous tube embedded in Turface (1-2 mm particle size) substrate containing 5 g Osmocote time release fertilizer per liter”—which were housed in Plexiglas chambers kept at atmospheric CO2 concentrations of 400, 1,500, or 10,000 ppm for periods of 14, 21, and 28 days, while measurements were made of a number of plant metabolic properties, including the leaf concentrations of several flavonoids capable of scavenging ROS.
According to the 13 researchers, their results indicated “elevated CO2 promoted the accumulation of secondary metabolites (flavonoids) progressively to a greater extent as plants became mature.” And as best as can be determined from the bar graphs of their results, the percentage increase in total wheat leaf flavonoid concentration in going from an atmospheric CO2 concentration of 400 to 1,500 ppm was 22 percent, 38 percent, and 27 percent (the one exception to this general rule) at 14, 21, and 28 days after planting, respectively, and in going from a CO2 concentration of 400 to 10,000 ppm, the percentage increase in total flavonoid concentration was 38 percent, 56 percent, and 86 percent, respectively, at 14, 21, and 28 days after planting. In addition, they found “both elevated CO2 levels resulted in an overall 25% increase in biomass over the control plants.”
In addition to the potential for the types of benefits described at the beginning of this section, the U.S., Japanese, and German scientists write, “the increased accumulation of secondary metabolites in plants grown under elevated CO2 may have implications regarding plant-herbivore interactions, decomposition rates for inedible biomass, and potential beneficial effects on plant tolerance to water stress (Idso, 1988) and cold stress (Solecka and Kacperska, 2003) due to their potentials for the scavenging of reactive oxygen species (ROS).”
In another study published in 2008, Stutte et al. (2008) wrote, as background for their plant CO2-enrichment experiment, that “many Scutellaria species are rich in physiologically active flavonoids that have a wide spectrum of pharmacological activity.” They note leaf extracts of Scutellaria barbata “have been used in traditional Chinese medicine to treat liver and digestive disorders and cancers (Molony and Molony, 1998),” and “recent research has shown extracts of S. barbata to be limiting to the growth of cell lines associated with lung, liver, prostate and brain tumors (Yin et al., 2004).”
Stutte et al. grew S. barbata and S. lateriflora plants from seed in large, walk-in, controlled environment chambers—which were maintained at atmospheric CO2 concentrations of either 400, 1,200, or 3,000 ppm—to the time of flowering (35 days after planting) and to the time of seed drop (49 days after planting). The plants were then harvested, their fresh and dry weights were determined, and the concentrations of a host of plant flavonoids within their tissues were measured.
The results of this project indicated that at 49 days after planting, the shoot dry weight of S. barbata increased by 54 percent at 1,200 ppm CO2 and by 57 percent at 3,000 ppm CO2, and that of S. lateriflora increased by 44 percent and 70 percent, respectively, under the same CO2 concentrations. In addition, the average concentration of the six flavonoids the researchers measured was increased by 48 percent at 1,200 ppm CO2 and by 81 percent at 3,000 ppm CO2 in the vegetative tissues of S. barbata, and it was increased by more than 2.4-fold at 1,200 and 4.9-fold at 3,000 ppm CO2 in S. lateriflora. Stutte et al. reported that in the case of S. lateriflora, “there was a 4.2-fold increase in total flavonoid content when enriching from 400 to 1200 ppm CO2, and a 13.7-fold increase at 3000 ppm.” They state “these results are generally consistent with those of B. Schmidt, W.D. Clark and S.B. Idso (unpublished data) who grew S. baicalensis at 700 ppm CO2” and found “total dry biomass was increased significantly” and “the overall antioxidant capacity, based on the ferric reducing antioxidant power assay, was increased.”
The three researchers concluded their results “clearly demonstrate the potential to use controlled environments to increase the production and quality of Scutellaria species ... because the practice has the potential to increase the value of the product by reducing the time to harvest, increasing yield per unit area, and increasing bioactivity per gram of dry matter.” Likewise, their extremely positive results hint at the likelihood that the active ingredients of many other medicinal plants may also be similarly enhanced by atmospheric CO2 enrichment and that the historical rise in the air’s CO2 content may have already done much the same thing for many of the plants that people include in their everyday diets.
La et al. (2009) took up the challenge to explore the subject further, writing by way of background, “epidemiological studies show that there is a negative relationship between vegetable intake and the risk of a number of cancers (Wattenberg, 1993; Kohlmeier and Su, 1997; Price et al., 1998),” and “it has been widely recognized that some of the cancer-chemoprotective activities in these vegetables are attributable to their contents of glucosinolates (GSs) (Zhao et al., 1992; Wattenberg, 1993; Tawfiq et al., 1995; Fahey et al., 1997; Rosa et al., 1997; Holst and Williamson, 2004).” They decided to see what effect the ongoing rise in the air’s CO2 content might have on the production of these important cancer-fighting agents.
The five scientists placed seedlings of Chinese broccoli (Brassica alboglabra L. var. Sijicutiao), in pairs in 1.8-L pots within growth chambers maintained at either 350 or 800 ppm CO2, where the plant’s roots were immersed in culture solutions treated with either low, medium, or high nitrogen and allowed to grow for 35 days, after which the plants were separated into their primary morphological parts and weighed, while their bolting stems were ground into powder for glucosinolate (GS) analyses.
“Regardless of N concentration,” state the researchers in describing their findings, the elevated CO2 treatment “significantly increased plant height [15.64 percent], stem thickness [11.79 percent], dry weights of the total aerial parts [11.91 percent], bolting stems [15.03 percent], and roots [16.34 percent].” In addition, they report the elevated CO2 increased the total GS concentrations of the bolting stems in the low and medium N treatments by 15.59 percent and 18.01 percent, respectively, compared with those at ambient CO2, although there was no such effect in the high N treatment. Thus, in terms of the total amount of GS production within the bolting stems of Chinese broccoli, these results suggest increases of 33 to 36 percent may be obtained for plants growing in low to medium N conditions in response to a 450 ppm increase in the air’s CO2 concentration. Such results bode well for people who eat broccoli—and, in all likelihood, other cruciferous vegetables as well—especially for those who will live in the CO2-enriched world of the future.
Jin et al. (2009) grew well-watered and fertilized spinach (Spinacia oleracea cv. Huangjia) plants from seed for approximately three weeks in controlled-environment chambers containing ambient air of 350 ppm CO2 or enriched air of 800 ppm CO2, after which they harvested the plants, weighed them, and measured the concentrations of several of the nutritive or health-promoting substances contained in their leaves. As best as can be determined from Jin et al.’s graphs of their results, the extra 450 ppm of CO2 increased the fresh weight of the spinach shoots by about 67 percent and their dry weight by approximately 57 percent. In addition, it boosted the soluble sugar concentrations of their leaves by approximately 29 percent and their soluble protein concentrations by about 52 percent. As an added bonus, the extra CO2 also increased spinach leaf concentrations of ascorbate, glutathione, and total flavonoids by 21 percent, 16 percent, and 3 percent, respectively, suggesting that as time progresses and the air’s CO2 content continues its upward climb, spinach should become more nutritious.
Turning our attention to fruit, Bindi et al. (2001), working near Rapolano, Siena (Italy), conducted a two-year free-air CO2 enrichment (FACE) study of 21-year-old grapevines (Vitis vinifera L., cv Sangiovese), where they enriched the air around the plants to 550 and 700 ppm CO2 while measuring numerous plant parameters, including—after the fermentation process was completed—“the principal chemical compounds that determine the basic red wine quality.”
Their results indicated “elevated atmospheric CO2 levels had a significant effect on biomass components (total and fruit dry weight) with increases that ranged from 40 to 45% in the 550 ppm treatment and from 45 to 50% in the 700 ppm treatment.” In addition, they report “acid and sugar contents were also stimulated by rising CO2 levels up to a maximum increase in the middle of the ripening season (8–14%),” but as the grapes reached the maturity stage, the CO2 effect on these parameters gradually disappeared. In terms of the primary pigments contained in the wine itself, however, it can be calculated from the bar graphs of their results that in response to the ~50 percent increase in atmospheric CO2 concentration experienced in going from ambient to 550 ppm CO2, the concentrations of total polyphenols, total flavonoids, total anthocyanins and non-anthocyanin flavonoids in the wine rose by approximately 19 percent, 33 percent, 31 percent, and 38 percent, respectively. Given these findings, Bindi et al. concluded, “the expected rise in CO2 concentrations may strongly stimulate grapevine production without causing negative repercussions on quality of grapes and wine.”
Similar results have been reported by Goncalves et al. (2009). Working with a native grape variety (Touriga Franca, Vitis vinifera L.) in the Demarcated Region of Douro, northern Portugal, the six Portuguese researchers investigated “the impact of elevated carbon dioxide concentration on the quality of berries, must, and red wine (with special reference to volatile composition, phenolic content, and antioxidant activity)” in an experiment in which grapevines were grown in open-top chambers maintained at either 365 or 550 ppm CO2. As they describe their findings, “in general, the increase of CO2 did not affect berry characteristics” and “did not significantly change the total antioxidant capacity of the red wines.” In fact, “thirty-five volatile compounds belonging to seven chemical groups were identified,” and “generally, the same volatile compounds were present in all of the wines.” Although some of these compounds were “slightly affected,” they state “the red wine quality remained almost unaffected.”
In considering these findings, Goncalves et al. state their study showed “the predicted rise in CO2 might strongly stimulate grapevine photosynthesis and yield (data not shown) without causing negative impacts on the quality of grapes and red wine.” Putting their personal stamp of approval on their findings, they add that “the informal sensorial analysis carried out by the researchers” also showed “wine quality remained almost unaffected.”
Vurro et al. (2009) examined the effect of atmospheric CO2 enrichment on thyme (Thymus vulgaris L.), noting thyme has “a considerable economic value in the nutraceutical and pharmaceutical industry (Vardar-Uenlue et al., 2003; Konyalioglu et al., 2006),” and “thyme essential oil possesses per se considerable antioxidant capacity (Economou et al., 1991), and may therefore contribute towards the control of antioxidant status in the leaves.”
Against this backdrop, Vurro et al. grew well-watered one-year-old thyme plants for three additional months out-of-doors within a mini-FACE system at Ravenna, Italy, where the air’s CO2 concentration was maintained at approximately 500 ppm (during daylight hours only), and where control plants were continuously exposed to air of approximately 370 ppm CO2, and they measured several plant parameters at the end of each of the three months of the study.
In analyzing their results, the four researchers report “none of the plants grown under high levels of CO2 for 90 days presented either significant differences in fresh weight and dry weight compared with controls, or macroscopic alteration of morphogenesis (number and length of nodes/internodes, branching, leaf area and chlorosis, etc.), at any of the sampling times.” However, they did find that “in plants grown under elevated CO2, a relative increase in oil yield of 32, 34 and 32 percent was, respectively, recorded in the first, second and third sampling-time (July, August and September),” and they observed a “general depression of the oxidative stress under elevated CO2” that led to a “down-regulation of leaf reactive oxygen species-scavenging enzymes under elevated CO2.” Such findings, in the words of the Italian scientists, point to “a ‘low cost’ life strategy for growth under elevated CO2, not requiring synthesis/activation of energy-intensive and expensive metabolic processes,” which thus allows the plants to invest more energy in the production of essential plant oils of nutritional and pharmaceutical value.
In another study conducted with pharmaceutical considerations in mind, Ziska et al. (2008) evaluated “the growth and production of opiates for a broad range of recent and projected atmospheric carbon dioxide concentrations using wild poppy (P. setigerum) as a surrogate for P. somniferum,” noting that “among medicinal plants, the therapeutic uses of opiate alkaloids from poppy (Papaver spp.) have long been recognized.”
Specifically, Ziska et al. grew well-watered and fertilized plants from seed within growth chambers maintained at four different atmospheric CO2 concentrations—300, 400, 500, and 600 ppm—for 90 to 100 days, while quantifying plant growth and the production of secondary compounds including the alkaloids morphine, codeine, papaverine, and noscapine, which were derived from latex obtained from capsules produced by the plants.
The three researchers’ data indicate that relative to the plants grown at 300 ppm CO2, those grown at 400, 500, and 600 ppm produced approximately 200, 275, and 390 percent more aboveground biomass, respectively, as best as can be determined from their bar graphs. In addition, they report, “reproductively, increasing CO2 from 300 to 600 ppm increased the number of capsules, capsule weight and latex production by 3.6, 3.0 and 3.7 times, respectively, on a per plant basis,” with the ultimate result that “all alkaloids increased significantly on a per plant basis.” Based on these findings, Ziska et al. conclude, “as atmospheric CO2 continues to increase, significant effects on the production of secondary plant compounds of pharmacological interest (i.e. opiates) could be expected.” These effects, in their words, “are commonly accepted as having both negative (e.g. heroin) and positive (e.g. codeine) interactions with respect to public health.”
In one final study, we report the work of Oliveira et al. (2010). Writing as background for their investigation, the five Brazilian researchers state, “presently, there is a growing interest in the use of inulin as a health food ingredient, as an alternative for low-calorie sweeteners, and as a dietary fiber and fat substitute (Ritsema and Smeekens, 2003).” In addition, they note “it is suggested” that a daily intake of low amounts of inulin or its derivatives generate certain bifidogenic effects that promote the growth of beneficial bacteria in the intestinal tract, as well as anti-tumor effects, citing the writings of Roberfroid (2005). They explain that their experimental subject, Vernonia herbacea (Vell.) Rusby, is an Asteraceae from the Brazilian Cerrado that accumulates inulin-type fructans in certain underground organs called rhizophores.
In conducting their experiment, Oliveira et al. grew well-watered and fertilized V. herbacea plants in open-top chambers within a glasshouse for 120 days at atmospheric CO2 concentrations of either 380 or 760 ppm, during which period they measured plant net photosynthetic rates, water use efficiencies, and fructan concentrations after 15, 30, 60, 90, and 120 days of treatment, as well as above- and below-ground biomass at the end of the experiment. Results indicated that “plants under elevated CO2 presented increases in height (40%), photosynthesis (63%) and biomass of aerial (32%) and underground (47%) organs when compared with control plants.” In addition, they state, “water use efficiency was significantly higher in treated plants, presenting a 177% increase at day 60.” Finally, they report that although fructan concentration remained unchanged, the significant CO2-induced increase in underground organ biomass caused “a 24% increase in total fructan yield.”
Because of the significant enhancement of inulin-type fructan production by V. herbacea under conditions of atmospheric CO2 enrichment, the positive health effects of those compounds, and the great increase in water-use efficiency displayed by the plants while producing them, a CO2-enriched future would appear to bode well for their commercial production throughout much of the central fifth—the Cerrado—of Brazil.
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