Reasonable air pollution control policies depend upon a comparison of the costs of air pollution losses with the costs of controls. Present estimates of national economic losses to agricultural and ornamental vegetation are based primarily on field observations and partially on growth and yield data obtained from closed‐top field chambers and greenhouses. This research describes the design and evaluation of an open‐top field chamber that was developed to provide an environment more closely resembling ambient conditions than the environment found in closed‐top chambers.Temperature and relative humidity within open‐top chambers were similar to ambient conditions. Direct sunlight reached the plants for a portion of each day and rain entered, although not always uniformly over the chamber base. Chambers receiving charcoal‐filtered air protected sensitive ‘Bel W3’ tobacco plants (Nicotiana tabacum L.) from ambient ozone concentrations. Plants growing in chambers receiving unfiltered air or in ambient air plots were severely injured.

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Ozone in the troposphere can cause plant stress, whereas elevated CO. generally causes positive responses. Little is known of how these gases interact to affect plant response. Interactive effects on yield and seed quality of soybean [Glycine max (L.) Merr.] grown in 14-L pots were measured in open-top field chambers. Essex was tested in 1993, and Essex, Holladay, and NK 6955 were tested in 1994. Plants were exposed from emergence to maturity to four CO; levels (ambient and 1.3,1.6, and 2.0 times ambient) and three O, levels (0.4, 0.9, and 1.5 times ambient) in 12 combinations. Increasing O, suppressed growth and yield, whereas CO, enrichment stimulated growth and yield. Carbon dioxide-induced stimulation was greater for plants stressed by O, than for non stressed plants. For example, CO 2 at 2.0 times ambient increased 2-yr mean seed yield of Essex by 16, 24, and 81% at O, levels of 0.4, 0.9, and 1.5 times ambient, respectively. Effects of O, and CO, on seed oil content were variable with numerous cultivar differences. Seed protein content was never affected. Elevated O, suppressed oleic acid content in seeds, whereas CO, increased it; the nature of the O, x CO 2 interaction for oleic acid was similar to that observed for most yield measures. Carbon dioxide-induced stimulation of plants stressed by O, was apparently caused partly by amelioration of O 3 stress. Interactions between O 3 and CO 2 must be considered for proper interpretation of cause-effect relationships in CO, enrichment studies.

RESEARCH R ising concentrations of atmospheric CO 2 are predicted to increase biomass production and yield of many C 3 crops (Ainsworth and Long, 2005; Jablonski et al., 2002; Kimball et al., 2002). It is expected, however, that the magnitude of these potential gains from CO 2 enrichment will be infl uenced by possible changes in other environmental factors such as temperature and soil water availability (

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