We begin our exploration of this important subject with some introductory comments of Chakraborty and Datta (2003), who note there are a number of CO2-induced changes in plant physiology, anatomy and morphology that have been implicated in increased plant resistance to disease and that "can potentially enhance host resistance at elevated CO2," among which phenomena they list "increased net photosynthesis allowing mobilization of resources into host resistance (Hibberd et al., 1996a.); reduced stomatal density and conductance (Hibberd et al., 1996b); greater accumulation of carbohydrates in leaves; more waxes, extra layers of epidermal cells and increased fibre content (Owensby, 1994); production of papillae and accumulation of silicon at penetration sites (Hibberd et al., 1996a); greater number of mesophyll cells (Bowes, 1993); and increased biosynthesis of phenolics (Hartley et al., 2000), among others." In what follows, we summarize the findings of some additional studies we have reviewed on our website that have dealt with these and other related phenomena.

Malmstrom and Field (1997) grew individual oat plants for two months in pots placed within phytocells maintained at atmospheric CO2 concentrations of 350 and 700 ppm, while they infected one-third of the plants with the barley yellow dwarf virus (BYDV), which plagues more than 150 plant species worldwide, including all major cereal crops. Over the course of their study, they found that elevated CO2 stimulated rates of net photosynthesis in all plants, regardless of pathogen infection. However, the greatest percentage increase occurred in diseased individuals (48% vs. 34%). Moreover, atmospheric CO2 enrichment decreased stomatal conductance by 50% in infected plants but by only 34% in healthy ones, which led to a CO2-induced doubling of the instantaneous water-use efficiency of the healthy plants, but an increase of fully 2.7-fold in the diseased plants. Last of all, after 60 days of growth under these conditions, they determined that the extra CO2 increased total plant biomass by 36% in infected plants, but by only 12% in healthy plants. In addition, while elevated CO2 had little effect on root growth in the healthy plants, it increased root biomass in the infected plants by up to 60%. Consequently, it can be appreciated that as the CO2 content of the air continues to rise, its many positive effects will likely offset some, if not most, of the negative effects of the destructive BYDV. Quoting Malmstrom and Field with respect to two specific examples, they say in their concluding remarks that CO2 enrichment "may reduce losses of infected plants to drought" and "may enable diseased plants to compete better with healthy neighbors."

Tiedemann and Firsching (2000) grew spring wheat plants from germination to maturity in controlled-environment chambers maintained at ambient (377 ppm) and elevated (612 ppm) concentrations of atmospheric CO2 and at ambient (20 ppb) and elevated (61 ppb) concentrations of ozone (and combinations thereof), the latter of which gases is typically toxic to most plants. In addition, half of the plants in each treatment were inoculated with a leaf rust-causing fungus. Under these conditions, the elevated CO2 increased the photosynthetic rates of the diseased plants by 20 and 42% at the ambient and elevated ozone concentrations, respectively. It also enhanced the yield of the infected plants, increasing it by 57%, even in the presence of high ozone concentrations.

Jwa and Walling (2001) grew tomato plants hydroponically for eight weeks in controlled-environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm. In addition, at week five of the study, half of all plants growing in each CO2 concentration were infected with a fungal pathogen that attacks plant roots and induces a water stress that decreases growth and yield. At the end of the study, they found that the pathogenic infection had reduced total plant biomass by nearly 30% at both atmospheric CO2 concentrations. However, the elevated CO2 had increased the total biomass of the healthy and diseased plants by the same amount (+30%), with the result that the infected tomato plants grown at 700 ppm CO2 had biomass values that were essentially identical to those of the healthy tomato plants grown at 350 ppm CO2. Thus, the extra CO2 completely counterbalanced the negative effect of the pathogenic infection on overall plant productivity.

Chakraborty and Datta (2003) studied the aggressiveness of the fungal anthracnose pathogen Colletotrichum gloeosporioides by inoculating two isolates of the pathogen onto two cultivars of the tropical pasture legume Stylosanthes scabra (Fitzroy, which is susceptible to the fungal pathogen, and Seca, which is more resistant) over 25 sequential infection cycles in controlled-environment chambers filled with air of either 350 or 700 ppm CO2. By these means they determined that the aggressiveness of the pathogen was reduced at the twice-ambient level of atmospheric CO2, where aggressiveness is defined as "a property of the pathogen reflecting the relative amount of damage caused to the host without regard to resistance genes (Shaner et al., 1992)." As they describe it, "at twice-ambient CO2 the overall level of aggressiveness of the two [pathogen] isolates was significantly reduced on both cultivars."

Simultaneously, however, pathogen fecundity was found to increase at twice-ambient CO2. Of this finding, Chakraborty and Datta report that their results "concur with the handful of studies that have demonstrated increased pathogen fecundity at elevated CO2 (Hibberd et al., 1996a; Klironomos et al., 1997; Chakraborty et al., 2000)." How this happened in the situation they investigated, according to Chakraborty and Datta, is that the overall increase in fecundity at high CO2 "is a reflection of the altered canopy environment," wherein "the 30% larger S. scabra plants at high CO2 (Chakraborty et al., 2000) makes the canopy microclimate more conducive to anthracnose development."

In view of these opposing changes in pathogen behavior at elevated levels of atmospheric CO2, it is difficult to know the ultimate outcome of atmospheric CO2 enrichment for this specific pathogen-host relationship. More research, especially under realistic field conditions, will be needed to clarify the situation; and, of course, different results are likely to be observed for different pathogen-host associations. What is more, results could also differ under different climatic conditions. Nevertheless, the large number of ways in which elevated CO2 has been demonstrated to increase plant resistance to pathogen attack gives us reason to believe that plants will gain the advantage as the air's CO2 content continues to climb in the years ahead.

Another study that fuels this optimism was conducted by Parsons et al. (2003), who grew two-year-old saplings of paper birch and three-year-old saplings of sugar maple in well-watered and fertilized pots from early May until late August in glasshouse rooms maintained at either 400 or 700 ppm CO2. In these circumstances, the whole-plant biomass of paper birch was increased by 55% in the CO2-enriched portions of the glasshouse, while that of sugar maple was increased by 30%. Also, concentrations of condensed tannins were increased by 27% in the paper birch (but not the sugar maple) saplings grown in the CO2-enriched air; and in light of this finding, Parsons et al. conclude that "the higher condensed tannin concentrations that were present in the birch fine roots may offer these tissues greater protection against soil-borne pathogens and herbivores."

Within this context, it is interesting to note that Parsons et al. report that CO2-induced increases in fine root concentrations of total phenolics and condensed tannins have also been observed in warm temperate conifers by King et al. (1997), Entry et al. (1998), Gebauer et al. (1998) and Runion et al. (1999), as well as in cotton by Booker (2000). See also, in this regard, Phenolics and Tannins in our Subject Index.

In another intriguing study, Gamper et al. (2004) begin by noting that arbuscular mycorrhizal fungi (AMF) are expected to modulate plant responses to elevated CO2 by "increasing resistance/tolerance of plants against an array of environmental stressors (Smith and Read, 1997)." In investigating this subject in a set of experiments conducted over a seven-year period of free-air CO2-enrichment on two of the world's most extensively grown cool-season forage crops (Lolium perenne and Trifolium repens) at the Swiss FACE facility near Zurich, they determined that "at elevated CO2 and under [two] N treatments, AMF root colonization of both host plant species was increased," and that "colonization levels of all three measured intraradical AMF structures (hyphae, arbuscules and vesicles) tended to be higher." Hence, they concluded that these CO2-induced benefits may lead to "increased protection against pathogens and/or herbivores."

Pangga et al. (2004) grew well-watered and fertilized seedlings of a cultivar (Fitzroy) of the pencilflower (Stylosanthes scabra) -- an important legume crop that is susceptible to anthracnose disease caused by Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. -- within a controlled environment facility maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, where they inoculated six-, nine- and twelve-week-old plants with conidia of C. gloeosporioides. Then, ten days after inoculation, they counted the anthracnose lesions on the plants and classified them as either resistant or susceptible.

Adherence to this protocol revealed, in their words, that "the mean number of susceptible, resistant, and total lesions per leaf averaged over the three plant ages was significantly (P<0.05) greater at 350 ppm than at 700 ppm CO2, reflecting the development of a level of resistance in susceptible cv. Fitzroy at high CO2." In fact, with respect to the plants inoculated at twelve weeks of age, they say that those grown "at 350 ppm had 60 and 75% more susceptible and resistant lesions per leaf, respectively, than those [grown] at 700 ppm CO2."

In terms of infection efficiency (IE), the Australian scientists say their work "clearly shows that at 350 ppm overall susceptibility of the canopy increases with increasing age because more young leaves are produced on secondary and tertiary branches of the more advanced plants." However, they report that "at 700 ppm CO2, IE did not increase with increasing plant age despite the presence of many more young leaves in the enlarged canopy," which finding, in their words, "points to reduced pathogen efficiency or an induced partial resistance to anthracnose in Fitzroy at 700 ppm CO2." Consequently, as the air's CO2 content continues to rise, it would appear that the Fitzroy cultivar of the pasture legume Stylosanthes scabra will indeed acquire a greater intrinsic resistance to the devastating anthracnose disease.

McElrone et al. (2005) "assessed how elevated CO2 affects a foliar fungal pathogen, Phyllosticta minima, of Acer rubrum [red maple] growing in the understory at the Duke Forest free-air CO2 enrichment experiment in Durham, North Carolina, USA ... in the 6th, 7th, and 8th years of the CO2 exposure." Surveys conducted in those years, in their words, "revealed that elevated CO2 [to 200 ppm above ambient] significantly reduced disease incidence, with 22%, 27% and 8% fewer saplings and 14%, 4%, and 5% fewer leaves infected per plant in the three consecutive years, respectively." In addition, they report that the elevated CO2 "also significantly reduced disease severity in infected plants in all years (e.g. mean lesion area reduced 35%, 50%, and 10% in 2002, 2003, and 2004, respectively)."

What underlying mechanism or mechanisms produced these beneficent consequences? Thinking it could have been a direct deleterious effect of elevated CO2 on the fungal pathogen, McElrone et al. performed some side experiments in controlled environment chambers. However, they found that the elevated CO2 benefited the fungal pathogen as well as the red maple saplings, observing that "exponential growth rates of P. minima were 17% greater under elevated CO2." And they obtained similar results when they repeated the in vitro growth analysis two additional times in different growth chambers.

Taking another tack when "scanning electron micrographs verified that conidia germ tubes of P. minima infect A. rubrum leaves by entering through the stomata," the researchers turned their attention to the pathogen's mode of entry into the saplings' foliage. In this investigation they found that both stomatal size and density were unaffected by atmospheric CO2 enrichment, but that "stomatal conductance was reduced by 21-36% under elevated CO2, providing smaller openings for infecting germ tubes." In addition, they concluded that reduced disease severity under elevated CO2 was also likely due to altered leaf chemistry, as elevated CO2 increased total leaf phenolic concentrations by 15% and tannin concentrations by 14%.

Because the phenomena they found to be important in reducing the amount and severity of fungal pathogen infection (leaf spot disease) of red maple have been demonstrated to be operative in most other plants as well, McElrone et al. say these CO2-enhanced leaf defensive mechanisms "may be prevalent in many plant pathosystems where the pathogen targets the stomata." Indeed, they state that their results "provide concrete evidence for a potentially generalizable mechanism to predict disease outcomes in other pathosystems under future climatic conditions." And because of their insightful work, that future is looking particularly bright in terms of the never-ending struggle between earth's plants and the pathogens that prey upon them. Although elevated CO2 helps both sides of the conflict, it helps plants more and in more ways.

Matros et al. (2006) grew tobacco plants (Nicotiana tabacum L.) in 16-cm-diameter pots filled with quartz sand in controlled-climate chambers maintained at either 350 or 1000 ppm CO2 for a period of eight weeks, where they were irrigated daily with a complete nutrient solution containing either 5 or 8 mM NH4NO3. In addition, some of the plants in each treatment were mechanically infected with the potato virus Y (PVY) when they were six weeks old. Then, at the end of the study, the plants were harvested and a number of their chemical constitutes identified and quantified.

This work revealed, in the researchers words, that "plants grown at elevated CO2 and 5 mM NH4NO3 showed a marked and significant decrease in content of nicotine in leaves as well as in roots," while at 8 mM NH4NO3 the same was found to be true of upper leaves but not of lower leaves and roots. With respect to the PVY part of the study, they further report that the "plants grown at high CO2 showed a markedly decreased spread of virus." Both of these findings would likely be considered beneficial by most people, as potato virus Y is an economically important virus that infects many crops and ornamental plants throughout the world, while nicotine is nearly universally acknowledged to have significant negative impacts on human health (Topliss et al., 2002).

Braga et al. (2006) conducted three independent experiments where they grew well-watered soybean (Glycine max (L.) Merr) plants of two cultivars (IAC-14, susceptible to stem canker disease, and IAC-18, resistant to stem canker disease) from seed through the cotyledon stage in 5-liter pots placed within open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm in a glasshouse, while they measured various plant properties and processes, concentrating on the production of glyceollins (the major phytoalexins, or anti-microbial compounds, produced in soybeans) in response to the application of ß-glucan elicitor (derived from mycelial walls of Phythophthora sojae) to carefully created and replicated wounds in the surfaces of several soybean cotyledons. In doing so, they found that the IAC-14 cultivar did not exhibit a CO2-induced change in glyceollin production in response to elicitation -- as Braga et al. had hypothesized would be the case, since this cultivar is susceptible to stem canker disease -- but they found that the IAC-18 cultivar (which has the potential to resist the disease to varying degrees) experienced a 100% CO2-induced increase in the amount of glyceollins produced after elicitation, a response the researchers described as remarkable. As for its significance, Braga et al. say the CO2-induced response they observed "may increase the potential of the soybean defense since infection at early stages of plant development, followed by a long incubation period before symptoms appear, is characteristic of the stem canker disease cycle caused by Dpm [Diaporthe phaseolorum (Cooke & Ellis) Sacc. f. sp. meridionalis Morgan-Jones]." Hence, they say the response they observed "indicates that raised CO2 levels forecasted for next decades may have a real impact on the defensive chemistry of the cultivars."

Last of all, in a study conducted within the BioCON (Biodiversity, Carbon dioxide, and Nitrogen effects on ecosystem functioning) FACE facility located at the Cedar Creek Natural History Area in east-central Minnesota, USA, Strengbom and Reich (2006) evaluated the effects of an approximate 190-ppm increase in the air's daytime CO2 concentration on leaf photosynthetic rates of stiff goldenrod (Solidago rigida) growing in monoculture for two full seasons, together with its concomitant effects on the incidence and severity of leaf spot disease. Although they found that elevated CO2 had no significant effect on plant photosynthetic rate in their study, they report that "both disease incidence and severity were lower on plants grown under elevated CO2." More specifically, they found that "disease incidence was on average more than twice as high [our italics] under ambient as under elevated CO2," and that "disease severity (proportion of leaf area with lesions) was on average 67% lower under elevated CO2 compared to ambient conditions."

In discussing their results, Strengbom and Reich say the "indirect effects from elevated CO2, i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher CO2," which as noted above was negligible in their study. Hence, they conclude "it may be necessary to consider potential changes in susceptibility to foliar diseases to correctly estimate the effects on plant photosynthetic rates of elevated CO2." In addition, they note that the plants grown in CO2-enriched air had lower leaf nitrogen concentrations than the plants grown in ambient air, as is often observed in studies of this type; and they say that their results "are, thus, also in accordance with other studies that have found reduced pathogen performance following reduced nitrogen concentration in plants grown under elevated CO2 (Thompson and Drake, 1994)." What is more, they conclude that their results are "also in accordance with studies that have found increased [disease] susceptibility following increased nitrogen concentration of host plants (Huber and Watson, 1974; Nordin et al., 1998; Strengbom et al., 2002)." It is possible, therefore, that the ongoing rise in the air's CO2 content may help many plants of the future reduce the deleterious impacts of various pathogenic fungal diseases that currently beset them, thereby enabling them to increase their productivities above and beyond what is typically provided by the more direct growth stimulation resulting from the aerial fertilization effect of elevated atmospheric CO2 concentrations.

In summation, the vast bulk of the available data clearly suggests that atmospheric CO2 enrichment asserts its greatest positive influence on infected as opposed to healthy plants. Moreover, it would appear that elevated CO2 has the ability to significantly ameliorate the deleterious effects of various stresses imposed upon plants by numerous pathogenic invaders. Consequently, as the atmosphere's CO2 concentration continues its upward climb, earth's vegetation should be increasingly better equipped to successfully deal with pathogenic organisms and the damage they have traditionally done to mankind's crops, as well as to the plants that sustain the rest of the planet's animal life.

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