GROWTH, RADIATION USE EFFICIENCY, AND CANOPY REFLECTANCE OF WHEAT ANDCORN GROWN UNDER ELEVATED OZONE AND CARBON-DIOXIDE ATMOSPHERES

Estimates of increases in future agricultural production in response t o increases in carbon dioxide (CO2) concentrations in the atmosphere a re often based on the beneficial physiological effect of CO2 enrichmen t on plant growth, especially in C-3 plants. However, these estimates fail to consider the negative impact of ozone (O-3) air pollution on c rop production. Increases in tropospheric concentrations of both gases , CO2 and O-3, have been observed over the past century, and both are predicted to continue to increase at even higher rates in the near fut ure to levels when they may have a significant impact on agricultural production. Field studies with wheat (Triticum aestivum L.) in 1991 an d 1992, and corn (Zea mays L.) in 1991 were conducted using open-top c hambers to mimic atmospheric concentrations of CO2 (similar to 500 mu L(-1) CO2) and O-3 (similar to 40 nL L(-1) O-3 above ambient air [O-3] during 7h day(-1) 5 days week(-1)) that are predicted to occur at the Earth surface during the first half of the 21st century. Wheat and co rn (C-3 vs. C-4) produced clearly different responses to CO2 enrichmen t, but similar responses to O-3 exposure. In wheat, O-3 exposure led t o reduced grain yield, biomass, and radiation use efficiency (RUE, phy tomass production per unit of energy received); in both years; but red uction in accumulated absorbed photosynthetically active radiation (AA PAR) was observed only in 1991. Conversely, CO2 enrichment produced gr eater grain yield, dry biomass, and RUE. With CO2 enrichment, the O-3- induced stress to wheat plants was apparently ameliorated since respon ses were equivalent to the control group (low O-3 and ambient CO2) for all variables. In contrast, corn demonstrated no benefit to CO2 enric hment for measured variables, and corn grain yield was the only parame ter negatively influenced by O-3 exposure that is attributed to O-3-in duced damage during the flowering process. Additionally, no treatment differences were observed for leaf area index (LAI) as determined nond estructively using the LICOR LAI-2000 Plant Canopy Analyzer. Also, tre atment differences for normalized difference vegetation index (ND) wer e only observed for wheat plants from the high-O-3 and ambient-CO2 tre atment, at some growing stages. Otherwise, ND were not helpful for ide ntifying damage due to O-3 fumigation or benefits due to CO2 enrichmen t. Significant interactive effects of CO2 vs. O-3 were observed only f or wheat grain yield in 1991 (p < 0.10), indicated that the detrimenta l effect of O-3 air pollution was more than overcome under the CO2-enr iched environment.