Growth of Continental-Scale Metro-Agro-Plexes, Regional Ozone Pollution, and World Food Production

Impacts of ozone on the growth and yield of field-grown winter wheat

Seed of winter wheat (Triticum aestivum L. cv Riband) was sown on 29 August 1992 in eight field plots. Four plots were exposed to elevated ozone (O(3)) concentrations on 16 days between 29 August and 2 October 1992, for 6 h day(-1), and on 27 days between 29 March and 24 August 1993, for 7 h day(-1). Mean daily O(3) concentrations were approximately 30 and 80 nmol mol(-1) in ambient and fumigated plots, respectively. Plants were sampled on 5 November (1992), 14 January, 16 February, 1 April, 25 May, 23 June and 24 August (1993). No visible symptoms of O(3) damage or premature senescence were observed at any time over the course of the experiment. Exposure to elevated O(3) decreased the above ground biomass by reducing plant density and individual plant relative growth rate. However, there was no significant influence of the pollutant on the growth of the root relative to the shoot. Assessment of yield characteristics at the final harvest revealed an O(3)-induced decrease in the number of grains per ear, as a result of fewer grains per spikelet and an increase in the number of infertile florets per spikelet. No significant effects of the pollutant on the number of ears per plant, spikelets per ear, or 1000 grain weight were found. As a result of the combined effects on the number of grains per ear and the decrease in plant density and growth rate, O(3) exposure reduced grain and straw yields (tonnes ha(-1)) by 13 and 8%, respectively. However, no significant change in the partitioning of dry matter between the grain and the straw was observed in fumigated plots. The findings are discussed within the context of United Nation Economic Commission for Europe critical level guidelines for the protection of crop yields, in relation to their application to winter-sown crops.

Photochemical oxidants: state of the science

Atmospheric photochemical processes resulting in the production of tropospheric ozone (O(3)) and other oxidants are described. The spatial and temporal variabilities in the occurrence of surface level oxidants and their relationships to air pollution meteorology are discussed. Models of photooxidant formation are reviewed in the context of control strategies and comparisons are provided of the air concentrations of O(3) at select geographic locations around the world. This overall oxidant (O(3)) climatology is coupled to human health and ecological effects. The discussion of the effects includes both acute and chronic responses, mechanisms of action, human epidemiological and plant population studies and briefly, efforts to establish cause-effect relationships through numerical modeling. A short synopsis is provided of the interactive effects of O(3) with other abiotic and biotic factors. The overall emphasis of the paper is on identifying the current uncertainties and gaps in our understanding of the state of the science and some suggestions as to how they may be addressed.

Ozone sensitivity in hybrid poplar is correlated with a lack of defense-gene activation

Ozone is a major gaseous pollutant thought to contribute to forest decline. Although the physiological and morphological responses of forest trees to ozone have been well characterized, little is known about the molecular basis for these responses. Our studies compared the response to ozone of ozone-sensitive and ozone-tolerant clones of hybrid poplar (Populus maximowizii x Populus trichocarpa) at the physiological and molecular levels. Gas-exchange analyses demonstrated clear differences between the ozone-sensitive clone 388 and the ozone-tolerant clone 245. Although ozone induced a decrease in photosynthetic rate and stomatal conductance in both clones, the magnitude of the decrease in stomatal conductance was significantly greater in the ozone-tolerant clone. RNA-blot analysis established that ozone-induced mRNA levels for phenylalanine ammonia-lyase, O-methyltransferase, a pathogenesis-related protein, and a wound-inducible gene were significantly higher in the ozone-tolerant than in the ozone-sensitive plants. Wound- and pathogen-induced levels of these mRNAs were also higher in the ozone-tolerant compared with the ozone-sensitive plants. The different physiological and molecular responses to ozone exposure exhibited by clones 245 and 388 suggest that ozone tolerance involves the activation of salicylic-acid- and jasmonic-acid-mediated signaling pathways, which may be important in triggering defense responses against oxidative stress.

Global tropospheric and total ozone monitoring with a double-etalon fabry-perot interferometer. I. Instrument concept

The global distribution of tropospheric ozone (O(3)) can be observed from a satellite-based instrument by spectrally isolating the pressure-broadened wings of strong O(3) lines. The Fabry-Perot interferometer (FPI) provides high spectral resolution and high-throughput capabilities that are essential for performing such a measurement. Through proper selection of channel spectral regions, the FPI optimized for tropospheric O(3) measurements can simultaneously observe a stratospheric component and thus the total O(3) column abundance. We present a conceptual instrument design that involves a double-etalon fixed-gap series configuration FPI along with an ultranarrow bandpass filter to achieve single-order operation with an overall spectral resolution of approximately 0.068 cm(-1), sampling the narrow 1054.2-1055.2 cm(-1) spectral region within the strong 9.6-mum ozone infrared band from a nadir-viewing satellite configuration.

Enhanced ozone-tolerance in wheat grown at an elevated CO2 concentration: ozone exclusion and detoxification

Elevated [CO₂ ] has been shown to protect photosynthesis and growth of wheat against moderately elevated [O₃ ]. To investigate the role of ozone exclusion and detoxification in this protection, spring wheat (Triticum aestivum L. ev. Wembley) was grown from seed, in controlled-environment chambers, under reciprocal combinations of [CO₂ ] at 350 or 700 μmol mol^-1 and [O₃ ] peaking at < 5 or 60 nmol mol^-1 , respectively. Cumulative ozone dose to the mesophyll and antioxidant status were determined throughout flag leaf development. Catalase activity correlated with rates of photorespiration and declined in response to elevated [CO₂ ] and/or [O₃ ]. Superoxide dismutase activity was not significantly affected by either condition. Neither ascorbate nor glutathione content was enhanced by elevated [CO₂ ]. In wheat, at moderately elevated [O₃ ], our results show that stomatal exclusion plays a major role in the protective effect of elevated [CO₂ ] against O₃ damage.

Ground-based differential absorption lidar system for day or night measurements of ozone throughout the free troposphere

The National Oceanic and Atmospheric Administration Aeronomy Laboratory's rapid tunable daylight differential absorption lidar system for monitoring ozone throughout the free troposphere is described. The system components are optimized to provide continuously and rapidly profiles of ozone, day or night, with a vertical resolution of 1 km and an absolute accuracy of +/-10% to the tropopause under clear sky conditions. Routine observations of ozone with frequent error assessments are made by scanning wavelengths between 286 and 292 nm.

Effects of elevated O3 and CO2 on chlorophyll fluorescence and gas exchange in Scots pine during the third growing season

Naturally regenerated, 30-year-old Scots pines (Pinus Sylvestris L.) were grown in open-top chambers and exposed in situ to doubled ambient O(3), doubled ambient CO(2) and a combination of elevated O(3) and CO(2) from 15 April to 15 September for three growing seasons (1994-1996). To examine the effects of O(3) and/or CO(2) on photosynthesis, chlorophyll a fluorescence and gas exchange were measured simultaneously. Doubled ambient O(3) significantly decreased the rates of photosynthesis at all levels of photon flux density. This was related mainly to a significant decrease in the photochemical efficiency of photosystem II (PS II) and the rate of whole electron transport, rather than to a decrease in stomatal conductance. When measurements were made at doubled ambient concentration of CO(2) (700 micromol mol(-1)), doubled ambient CO(2) treatment did not lead to a significant change in the intrinsic capacity of photosynthesis, as manifested by no changes in PS II, the rate of electron transport, the maximal rate of photosynthesis and the apparent quantum yield of CO(2) assimilation. However, elevated CO(2) increased the sensitivity of stomatal conductance to light and decreased maximal stomatal conductance. When O(3) and CO(2) were combined, the O(3)-induced decrease in photosynthesis rate was reduced significantly by a high concentration of CO(2). This may be partly related to the decrease in stomatal conductance induced by the high concentration of CO(2). The complete mechanism behind this interaction is, however, still unclear.

ENERGY AND RESOURCE CONSTRAINTS ON INTENSIVE AGRICULTURAL PRODUCTION

▪ Abstract  This review explores the potential energy, soil, and water constraints on highly productive agricultural systems. It focuses on the process of agricultural intensification during the past 50 years, and it shows that multiple constraints—as opposed to a single constraint, such as energy—are needed to assess the future sustainability of intensive agricultural production. Recent studies documenting changes in total factor productivity based on long-term experimental trials and field surveys are discussed in detail. The results of these studies are worrisome; they indicate that degradation in soil quality and in the overall natural resource base may threaten the long-run viability of several of the world's most intensive agricultural systems. Other studies are reviewed that support a more optimistic view of resource availability and the ability of improved technology and management to overcome these physical constraints. However, the combined evidence suggests that the increase in agricultural prices required to induce the necessary changes in technology could be devastating to low-income households. Most of the world's poor consume more agricultural output than they produce, and they spend up to 80% of their incomes on food.

ANTHROPOGENIC MOBILIZATION OF SULPHUR AND NITROGEN: Immediate and Delayed Consequences

▪ Abstract  Global mobilization and dispersal of sulphur (S) and nitrogen (N) have been significantly increased by human activities. They are projected to increase even more in the future owing to growth in population and per-capita consumption of food and energy in the developing world, primarily Asia. Increased mobilization and distribution result in changes in precipitation acidity, ecosystem alkalinity and nutrient status, tropospheric and stratospheric ozone concentrations, and energy balance of the troposphere. Although increases in S and N mobilization cause increased environmental impacts, a leveling or decrease in mobilization does not result in a lessening of environmental impacts because of the accumulation of reactive S and N in environmental reservoirs. As S and N accumulate, ecosystems become saturated and S and N dispersal increases. Environmental impacts will only begin to lessen if mobilization rates decrease and as accumulated reactive S and N are converted to nonreactive forms or stored in long-term reservoirs.

Ozone-induced responses in Arabidopsis thaliana: the role of salicylic acid in the accumulation of defense-related transcripts and induced resistance

Exposure of Arabidopsis thaliana to ozone results in the expression of a number of defense-related genes that are also induced during a hypersensitive response. A potential common link between the activation of defense gene expression during a hypersensitive response and by ozone treatment is the production of active oxygen species and the accumulation of hydrogen peroxide. Here we report that salicylic acid accumulation, which can be induced by hydrogen peroxide and is required for the expression of both a hypersensitive response and systemic acquired resistance, is also required for the induction of some, but not all, ozone-induced mRNAs examined. In addition, we show that ozone exposure triggers induced resistance of A. thaliana to infection with virulent phytopathogenic Pseudomonas syringae strains. Infection of transgenic plants expressing salicylate hydroxylase, which prevents the accumulation of salicylic acid, or npr1 mutant plants, which are defective in the expression of systemic acquired resistance at a step downstream of salicylic acid, demonstrated that the signaling pathway activated during ozone-induced resistance overlaps with the systemic acquired resistance activation pathway and is salicylic acid dependent. Interestingly, plants expressing salicylate hydroxylase exhibited increased sensitivity to ozone exposure. These results demonstrate that ozone activates at least two distinct signaling pathways, including a salicylic acid dependent pathway previously shown to be associated with the activation of pathogen defense reactions, and that this latter pathway also induces a protective response to ozone.

Ozone and plant health

Phytotoxic effects of ozone are described with emphasis on secondary plant metabolism. Numerous ozone-induced genes, enzymes and stress metabolites of antioxidative and phytopathological defense reactions have been discovered for herbaceous plants and forest tree species. Ozone induces reactions normally elicited by viral and microbial pathogens. The molecular basis (receptors, signal chains) for induction by ozone remains to be elucidated. The induced stress reactions seem to change plant predisposition to either enhanced tolerance or susceptibility for a second stressor. The following topics are discussed: ozone and biotic disease, the role of ozone on field sites and ozone limit values.

The interactive effects of elevated CO2 and O 3 concentration on photosynthesis in spring wheat

This study investigated the interacting effects of carbon dioxide and ozone on photosynthetic physiology in the flag leaves of spring wheat (Triticum aestivum L. cv. Wembley), at three stages of development. Plants were exposed throughout their development to reciprocal combinations of two carbon dioxide and two ozone treatments: [CO2] at 350 or 700 μmol mol(-1), [O3] at < 5 or 60 nmol mol(-1). Gas exchange analysis, coupled spectrophotometric assay for RuBisCO activity, and SDS-PAGE, were used to examine the relative importance of pollutant effects on i) stomatal conductance, ii) quantum yield, and iii) RuBisCO activity, activation, and concentration. Independently, both elevated [CO2] and elevated [O3] caused a loss of RuBisCO protein and Vcmax. In combination, elevated [CO2] partially protected against the deleterious effects of ozone. It did this partly by reducing stomatal conductance, and thereby reducing the effective ozone dose. Elevated [O3] caused stomatal closure largely via its effect on photoassimilation.

Restoring Value to the World's Degraded Lands

Roughly 43 percent of Earth's terrestrial vegetated surface has diminished capacity to supply benefits to humanity because of recent, direct impacts of land use. This represents an approximately 10 percent reduction in potential direct instrumental value (PDIV), defined as the potential to yield direct benefits such as agricultural, forestry, industrial, and medicinal products. If present trends continue, the global loss of PDIV could reach approximately 20 percent by 2020. From a biophysical perspective, recovery of approximately 5 percent of PDIV is feasible over the next 25 years. Capitalizing on natural recovery mechanisms is urgently needed to prevent further irreversible degradation and to retain the multiple values of productive land.

Climate change: potential effects of increased atmospheric carbon dioxide (CO2), ozone (O3), and ultraviolet-B (UV-B) radiation on plant diseases

Continued world population growth results in increased emission of gases from agriculture, combustion of fossil fuels, and industrial processes. This causes changes in the chemical composition of the atmosphere. Evidence is emerging that increased solar ultraviolet-B (UV-B) radiation is reaching the earth's atmosphere, due to stratospheric ozone depletion. Carbon dioxide (CO(2)), ozone (O(3)) and UV-B are individual climate change factors that have direct biological effects on plants. Such effects may directly or indirectly affect the incidence and severity of plant diseases, caused by biotic agents. Carbon dioxide may increase plant canopy size and density, resulting in a greater biomass of high nutritional quality, combined with a much higher microclimate relative humidity. This would be likely to promote plant diseases such as rusts, powdery mildews, leaf spots and blights. Inoculum potential from greater overwintering crop debris would also be increased. Ozone is likely to have adverse effects on plant growth. Necrotrophic pathogens may colonize plants weakened by O(3) at an accelerated rate, while obligate biotroph infections may be lessened. Ozone is unlikely to have direct adverse effects on fungal pathogens. Ozone effects on plant diseases are host plant mediated. The principal effects of increased UV-B on plant diseases would be via alterations in host plants. Increased flavonoids could lead to increased diseased resistance. Reduced net photosynthesis and premature ripening and senescence could result in a decrease in diseases caused by biotrophs and an increase in those caused by necrotrophs. Microbial plant pathogens are less likely to be adversely affected by CO(2), O(3) and UV-B than are their corresponding host plants. Changes in host plants may result in expectable alterations of disease incidence, depending on host plant growth stages and type of pathogen. Given the importance of plant diseases in world food and fiber production, it is essential to begin studying the effects of increased CO(2), O(3) and UV-B (and other climate change factors) on plant diseases. We know very little about the actual impacts of climate change factors on disease epidemiology. Epidemiologists should be encouraged to consider CO(2), O(3) and UV-B as factors in their field studies.