Air Pollutants: Interactions with Elevated Carbon Dioxide
INTRODUCTION
The concentrations of various compounds in the atmosphere have undergone significant changes during the last century, and they continue to change. Many of these compounds interact with the terrestrial biosphere. Depending on the concentration in the atmosphere, gases such as SO2 or NO2 may be beneficial to natural and agricultural ecosystems or they may act as air pollutants affecting these systems in a negative or adverse manner.
In contrast, carbon dioxide (CO2) is the basic plant nutrient and has positive and growth-stimulating effects on vegetation. Because of their co-occurrence, the assessments of potential effects of atmospheric changes on vegetation have to consider these contrasting impacts and the interactions between effects of air pollutants and elevated CO2.
ATMOSPHERIC CHANGE: CONCENTRATIONS AND TRENDS
Anthropogenic activities have changed the concentrations of a wide variety of gaseous and particulate compounds in the atmosphere, including carbon dioxide (CO2), nitrogen monoxide (NO) and nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), ammonia (NH3), heavy metals, and volatile organic compounds (VOC).
Prominent examples of global importance are CO2 and tropospheric O3. Since the beginning of the 19th century, the concentration of CO2 [CO2] has increased globally to current values of about 360 ppmv (parts-per-million by volume). It is expected that [CO2] will continue to increase even more rapidly and may reach about 550–650 ppmv between 2050 and 2100.
As CO2 is the substrate for plant photosynthesis, this increase in [CO2] will have far-reaching consequences for most types of vegetation. Parallel to the increase in [CO2], ground-level O3 concentrations ([O3]) in most industrialized countries have nearly doubled during the last 100 years. The current mean [O3] in nonurban areas is between 40–75 ppbv (parts-per-billion by volume) during the growing season and 20–35 ppbv as an annual mean.
Today, O3 pollution has also become a major environmental problem in many developing countries. Predictions for the future development of [O3] are uncertain; in the case that the emission of the precursor compounds nitrogen oxides and volatile organic compounds remain high or continue to increase, [O3] will follow these emission trends. [O3] varies considerably in time and space and shows annual and diurnal patterns. In contrast to CO2, elevated [O3] is phytotoxic and affects plants negatively. However, quantification of the effects of O3 is difficult due to the large variability of exposure concentrations.
EFFECTS OF CO2 AND O3 ALONE
As current atmospheric [CO2] limits photosynthesis in most C3 plants, any increase in [CO2] results in a stimulation of plant physiological and growth processes. The most frequently observed effects of elevated [CO2] include stimulation of photosynthesis, enhanced concentrations of soluble carbohydrates, an increase in growth rates and leaf area, and stimulated biomass production and yield. Transpiration rate (per unit leaf area) and stomatal conductance, as well as tissue element concentrations (particularly nitrogen), usually decline.
Yield enhancements of up to 25– 35% as compared to ambient [CO2] have been observed when plants were exposed to 550–750 ppmv CO2. The initial stimulation of photosynthesis often decreases under long-term exposure to elevated [CO2], leading to smaller growth and yield enhancements than expected from the short-term photosynthetic responses. Plant species differ widely in their response to high [CO2].
By contrast, O3 is currently regarded as the most important phytotoxic pollutant in the atmosphere. Primary O3 effects include subtle biochemical and ultrastructural changes, which may result in impaired photosynthesis, alterations of carbon allocation patterns, symptoms of visible injury, enhanced senescence, reduced growth and economic yield, altered resistance to other abiotic and biotic stresses, reduced flowering and seed production, loss of competitive abilities of plant species in communities, and shifts in biodiversity. Current ambient [O3] in many industrialized areas is high enough to suppress crop yields of sensitive species and retard the growth and development of trees and other plant species of the nonwoody (semi-)natural vegetation. As with CO2, there is large inter- and intraspecific variability in the O3 susceptibility of plants.
INTERACTIVE EFFECTS OF CO2 AND AIR POLLUTANTS
While single exposures to elevated [CO2] or air pollutants may have contrasting effects on plant performance, it is of particular interest to understand how the individual changes in atmospheric constituents may interfere with each other in order to predict the likelihood of combined effects of atmospheric changes on terrestrial ecosystems.
CO2 and O3
A great number of studies on the combined effects of the two gases have shown that high [CO2] either partially or totally compensate for adverse O3 effects. This has been demonstrated, for example, for some crop species including soybean, wheat, and corn. However, summarized over the total available database with different plant species and cultivars, the information is not entirely consistent, as several studies revealed that elevated [CO2] may not always protect plants from the adverse effects of O3.
The proposed mechanisms to explain the protective effect of elevated [CO2] against the phytotoxic effects of O3 include
1) reduced uptake or flux of O3 through the stomata due to a CO2-induced stomatal closure,
2) improved supply of carbon skeletons supporting the synthesis of antioxidants involved in the destruction of O3 and its toxic products,
3) protection of the Rubisco protein from O3-induced degradation and
4) CO2-induced changes in the cell surface/volume ratio.
However, it has been shown that in spite of decreased stomatal conductance under elevated [CO2], adverse effects of O3 may still occur. As CO2 effects on stomatal conductance may be species-specific, it is not yet possible to support a general concept of a CO2-induced reduction in the flux of O3 into the plant. Moreover, in a given plant species, protection by high [CO2] from a particular adverse effect is not necessarily associated with the protection against another adverse effect.
CO2 and Other Air Pollutants Hardly any studies have addressed the combined effects of elevated [CO2] and of other air pollutants. Studies of the combined effects of elevated [CO2] and nitrogen oxides (NO, NO2) are confined to commercial greenhouses under conditions of horticultural crop production and are not considered here. SO2 has been found to adversely affect agricultural crops and forest plants in a large number of studies.
Reduced photosynthesis, altered water relations, growth retardations, yield losses and altered susceptibilities to other stresses are common plant responses observed under SO2 stress. It was shown for a range of plant species under various exposure conditions that elevated [CO2] reduced the sensitivity of the plants to SO2 injury or protected them from negative effects of SO2 on growth and yield.
With the combined exposure of crop species to both gases, the yield increments were sometimes even larger when compared to the stimulation observed with exposure to elevated [CO2] alone, suggesting that the plants were able to use the airborne sulphur more effectively under the conditions of enhanced carbon availability. It must be kept in mind that low to moderate SO2 concentrations may confer a nutritional benefit to plants, particularly under conditions of low sulphur availability in the soil.
CONCLUSION
Rising atmospheric concentrations of CO2 and air pollutants can have interactive effects on agricultural and wild plant species. Existing evidence on potential interactions is almost exclusively restricted to CO2 and tropospheric O3, the concentrations of which are increasing globally. While high CO2 levels are beneficial to plants, current ambient [O3] is high enough to impair plants in many regions of the world. There is ambiguous information in the literature concerning the protective effect of elevated [CO2] from adverse effects of O3, but it has been demonstrated to occur in many experimental studies.
The mechanisms by which elevated [CO2] and O3 interact at the physiological and metabolic level remain uncertain. There is also some evidence that rising [CO2] may protect plants against phytotoxic SO2 concentrations. Overall the existing database on air pollutant/CO2 interactions lacks a general conceptual model of the potential modes of interactions. Additional long-term field experiments will be necessary, combined with a better understanding of how other plant and environmental variables, such as plant genotype, soil water deficit, nutrient availability, or temperature, may modify the interaction.