Growth, radiation use efficiency, and canopy reflectance of wheat and corn grown under elevated ozone and carbon dioxide atmosphere

Estimates of increases in future agricultural production in response to increases in carbon dioxide (CO2) concentrations in the atmosphere are often based on the beneficial physiological effect of CO2 enrichment on plant growth, especially in C3 plants. However, these estimates fail to consider the negative impact of ozone (O3) air pollution on crop production. Increases in tropospheric concentrations of both gases, CO2 and O3, have been observed over the past century, and both are predicted to continue to increase at even higher rates in the near future to levels when they may have a significant impact on agricultural production. Field studies with wheat (Triticum aestivum L.) in 1991 and 1992, and corn (Zea mays L.) in 1991 were conducted using open-top chambers to mimic atmospheric concentrations of CO2 (approximately 500 μL-1 CO2) and O3 (approximately 40 nL L-1 O3 above ambient air (O31 during 7 h day-1, 5 days week-1) that are predicted to occur at the Earth's surface during the first half of the 21st century. Wheat and corn (C3 vs. C4) produced clearly different responses to CO2 enrichment, but similar responses to O3 exposure. In wheat, O3 exposure led to reduced grain yield, biomass, and radiation use efficiency (RUE, phytomass production per unit of energy received); in both years, but reduction in accumulated absorbed photosynthetically active radiation (AAPAR) was observed only in 1991. Conversely, CO2 enrichment produced greater grain yield, dry biomass, and RUE. With CO2 enrichment, the O3-induced stress to wheat plants was apparently ameliorated since responses were equivalent to the control group (low O3 and ambient CO2) for all variables. In contrast, corn demonstrated no benefit to CO2 enrichment for measured variables, and corn grain yield was the only parameter negatively influenced by O3 exposure that is attributed to O3-induced damage during the flowering process. Additionally, no treatment differences were observed for leaf area index (LAI) as determined nondestructively using the LICOR LAI-2000 Plant Canopy Analyzer. Also, treatment differences for normalized difference vegetation index (ND) were only observed for wheat plants from the high-O3 and ambient-CO2 treatment, at some growing stages. Otherwise, ND data were not helpful for identifying damage due to O3 fumigation or benefits due to CO2 enrichment. Significant interactive effects of CO2 vs. O3 were observed only for wheat grain yield in 1991 (p < 0.10), indicated that the detrimental effect of O3 air pollution was more than overcome under the CO2-enriched environment.