In a study of an agricultural plant, Harnos et al. (2002) grew winter wheat (Triticum aestivum L. cv. Emma) in open-top chambers maintained at atmospheric CO2 concentrations of 365 and 700 ppm. Among other things, the authors reported that elevated CO2 stimulated photosynthetic rates to a greater extent under light-limiting than under non-light-limiting conditions. In fact, twice-ambient CO2 concentrations increased net photosynthesis rates by approximately 100% in upper-canopy leaves and by about 770% further down in the canopy, where light intensity was 60% less than in the upper canopy, suggesting that as the atmospheric CO2 concentration increases in the years ahead, winter wheat plants will likely respond by exhibiting enhanced rates of photosynthesis, even in leaves deep within their canopies, where irradiance is severely reduced due to shading by upper-canopy leaves.

Louche-Tessandier et al. (1999) grew potato plantlets inoculated with an arbuscular mycorrhizal fungus at various light intensities and super CO2 enrichment of approximately 10,000 ppm, finding that the unusually high CO2 concentration produced an unusually high degree of root colonization by the beneficial mycorrhizal fungus, which typically helps supply water and nutrients to plants. And it did so irrespective of the degree of light intensity to which the potato plantlets were exposed.

With respect to trees, Leakey et al. (2002) grew seedlings of Shorea leprosula (an under-story rain forest tree) in controlled environments maintained at atmospheric CO2 concentrations of 376 and 711 ppm in combination with low irradiance treatments delivered in a uniform or intermittent (sunfleck) manner for about seven months, in order to study the effects of elevated CO2 and low light intensity on photosynthesis and growth in this species. Their results indicated that the initial steady-state rates of photosynthesis measured in the shade in CO2-enriched leaves were approximately 109% greater than those observed in ambient-grown leaves. In addition, seedlings in the sunfleck treatment that were grown in elevated CO2 displayed post-irradiance rates of photosynthesis that were 14% greater than those observed in control seedlings. Taken together, these increases in photosynthesis led to CO2-induced increases in carbon uptake that were 59 and 89% greater than those observed in control seedlings subjected to uniform and sunfleck light treatments, respectively. Ultimately, seedlings subjected to uniform irradiance produced more biomass than seedlings exposed to sunfleck irradiance; but the CO2-induced percentage increase in biomass was greater under the sunfleck irradiance regime (60%) than under the uniform irradiance regime (25%).

In another study, Pardos et al. (2006) grew seedlings of cork oak (Quercus suber L.) for five months at either high (83%) or low (32-34%) growing medium moisture under either high (600 µmol m^-2 s^-1) or low (60 µmol m^-2 s^-1) light intensity in growth chambers maintained at either ambient (360 ppm) or elevated (700 ppm) atmospheric CO2 concentration. Among other things, Pardos et al. report that seedling relative growth rate rose 2.2-fold when going from the lowest to the highest combined light-water treatment in the elevated CO2 environment, but only 1.6-fold when doing so in the ambient CO2 environment. As a result, the four Spanish researchers say that "elevated CO2 caused cork oak seedlings to improve their performance in dry and high light environments to a greater extent than under well-irrigated and low-light conditions, thus ameliorating the effects of soil water stress and high light loads on growth." Consequently, and because they believe these latter two stressful conditions are what "global change is likely to produce in the Mediterranean basin in the next decades," it can be appreciated that the ongoing rise in the air's CO2 concentration should help the cork oak species to better deal with those stresses, if they actually do occur.

Introducing their contribution to the subject, Rasineni et al. (2011) write that "excess light limits photosynthesis by photoinhibition, resulting in reduced carbon gain and also causing photo-damage (Oquist and Huner, 1993; Pastenes et al., 2003; Allakhverdiev and Murata, 2004; Nishiyama et al., 2006)," and they say that "plants grown in tropical climates usually experience significantly high irradiance leading to the strong midday depression of photosynthesis (Hymus et al., 2001)." Against this backdrop, Rasineni et al. utilized two open-top chambers in the Botanical Gardens of the University of Hyderabad, India -- each of which contained four six-month-old specimens of the fast-growing tropical Gmelina arborea tree, which they maintained at optimum moisture and nutrient levels -- to measure several plant physiological properties and processes related to leaf photosynthesis and photosystem II (PSII) photochemistry and photoinhibition at both ambient and elevated CO2 concentrations (360 and 460 ppm, respectively), working with "well-expanded and light-exposed leaves randomly chosen from the upper half of the plant canopy."

Based upon their analysis, the three Indian scientists determined that there were no significant differences in CO2 assimilation rates between the ambient and elevated CO2 grown plants during early morning hours; but they discovered that, thereafter, "photosynthesis typically maximized between 0900 hours and 1000 hours in both ambient and elevated CO2-grown plants," which experienced net photosynthetic rates of 20 and 32.5 µmol/m²/s, respectively, for a stunning CO2-induced enhancement of 62%, which for the more standard CO2 enrichment of 300 ppm would be roughly equivalent to an enhancement of 180%. Subsequently, during the following midday period of 1100-1300 hours, the net photosynthesis rate was still significantly enhanced by about 37% (roughly equivalent to a 300-ppm induced increase of more than 100%) in the elevated CO2 treatment, after which the difference between the net photosynthetic rates of the two CO2 treatments once again became insignificant. Noting that the "elevated CO2 treatment mitigated PSII-photoinhibition through enhanced electron transport rates and through efficient biochemical reactions in leaves of G. arborea," Rasineni et al. thus conclude that their data "demonstrate that future increases in atmospheric CO2 may have positive effects on photochemical efficiency in fast growing tropical tree species," allowing them to take great advantage of the high-light midday period of potential maximum growth in Earth's tropical regions.

Further proof that elevated atmospheric CO2 helps to ameliorate the stress of low light intensities in trees comes from the study of Kerstiens (1998), who analyzed the results of 15 previously published studies of trees having differing degrees of shade tolerance and found that elevated CO2 caused greater relative biomass increases in shade-tolerant species than in shade-intolerant or sun-loving species. In fact, in more than half of the studies analyzed, shade-tolerant species experienced CO2-induced relative growth increases that were two to three times greater than those of less shade-tolerant species.

In an extended follow-up review analyzing 74 observations from 24 studies, Kerstiens (2001) reported that twice-ambient CO2 concentrations increased the relative growth response of shade-tolerant and shade-intolerant woody species by an average of 51 and 18%, respectively. Moreover, similar results were reported by Poorter and Perez-Soba (2001), who performed a detailed meta-analysis of research results pertaining to this topic, and more recently by Kubiske et al. (2002), who measured photosynthetic acclimation in aspen and sugar maple trees. Low light intensity, therefore, is by no means a roadblock to the benefits that come to plants as a consequence of an increase in the air's CO2 content.

Of course, most general rules do have their exceptions. In one such study, a 200-ppm increase in the air's CO2 concentration enhanced the photosynthetic rates of sunlit and shaded leaves of sweetgum trees by 92 and 54%, respectively, at one time of year, and by 166 and 68% at another time (Herrick and Thomas, 1999). Likewise, Naumburg and Ellsworth (2000) reported that a 200-ppm increase in the air's CO2 content boosted steady-state photosynthetic rates in leaves of four hardwood understory species by an average of 60 and 40% under high and low light intensities, respectively. Thus, even though these photosynthetic responses were significantly less in shaded leaves, they were still substantial, with mean increases ranging from 40 to 68% for a 60% increase in atmospheric CO2 concentration. And that's anything but shabby!

Under extremely low light intensities, the benefits arising from atmospheric CO2 enrichment may be small, but oftentimes they are very important in terms of plant carbon budgeting. In the study of Hattenschwiler (2001), for example, seedlings of five temperate forest species favored with an additional 200-ppm CO2 under light intensities that were only 3.4 and 1.3% of full sunlight exhibited CO2-induced biomass increases that ranged from 17 to 74%. Similarly, in the study of Naumburg et al. (2001), a 200-ppm increase in the air's CO2 content enhanced photosynthetic carbon uptake in three of four hardwood understory species by more than two-fold in three of the four species under light irradiances that were as low as 3% of full sunlight.

In another important study, Sefcik et al. (2006) grew seedlings of two shade-tolerant northern hardwood tree species - sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia J.F. Ehrh.) - as well as seedlings of two shade-intolerant northern hardwood tree species - black cherry (Prunus serotina J.F. Ehrh.) and paper birch (Betula papyrifera Marsh.) - for two full growing seasons inside open-top chambers maintained at either ambient (383 ppm) or elevated (658 ppm) atmospheric CO2 concentrations within an overarching 90-year-old nitrogen-limited northern hardwood forest located in Michigan, USA, to determine their responses to atmospheric CO2 enrichment in two contrasting degrees of shade: moderate shade (14.2 µmol photons m^-2 s^-1 = 5.6% full sun) and deep shade (6.5 µmol photons m^-2 s^-1 = 2.2% full sun). In doing so, it was determined, according to Sefcik et al., that "the magnitude of enhancement from exposure to elevated CO2 was similar for both shade-tolerance groups," with the elevated CO2 treatment increasing the mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and by 67% in the shade-intolerant species. More importantly, however, they found that "seedlings grown in deep shade, regardless of shade-tolerance group, showed a greater long-term photosynthetic enhancement to elevated CO2 than those grown in moderate shade," with the mean long-term enhancement being 47% in moderate shade and a much larger 97% in deep shade.

Noting that the same type of photosynthetic response "has also been found in a number of other studies, suggesting that the impact of a CO2-enriched atmosphere increases as light becomes more limiting (Hattenschwiler, 2001; Granados and Korner, 2002; Leakey et al., 2002)," Sefcik et al. concluded that "if long-term enhancement of photosynthesis in elevated CO2 and deep shade translates into greater survival [and it seems only logical that it would], especially for shade-intolerant species [ditto], this could have profound successional implications for nitrogen-limited northern hardwood forest composition in a future higher CO2 atmosphere," with the likely end result of enhancing local species richness.

Last of all, elevated CO2 often reduces a plant's light compensation point, which is the light intensity at which the amount of carbon fixed by photosynthesis is equal to that lost by respiration. Above that particular light intensity, net photosynthesis is positive. Below it, net photosyntheis is negative; and if prolonged, the plant will ultimately die. Thus, this phenomenon is especially beneficial to vegetation growing in deep shade beneath forest canopies that block out much of the incoming sunlight (Kubiske and Pregitzer, 1996; Osborne et al., 1997); and it also helps aquatic plants extend their life zones to greater depths (Zimmerman et al., 1997).

So, whether light intensity is high or low, or leaves are shaded or sunlit, when the CO2 content of the air is increased, so too are the various biological processes that lead to plant robustness increased. Less than optimal light intensities, therefore, clearly do not negate the beneficial effects of atmospheric CO2 enrichment.

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