THRESHOLDS FOR INJURY, GROWTH, AND YIELD LOSS CAUSED BY OZONE ON FIELD CORN HYBRIDS

A commercial field corn (Zea mays L.) hybrid 'Coker 16' was exposed to four chronic doses of ozone (O3) in open-top field chambers from 25 days after planting until maturity. The different doses were obtained by adding different but constant concentrations of O3 to the naturally varying ambient concentrations for 7 hr/day (from 09.30 to 16.30 hours). The threshold O3 concentrations causing foliar injury (between 0.02 and 0.07 ppm) were lower than concentrations required to decrease kernel yield (between 0.11 and 0.15 ppm). These thresholds were the same whether plants were grown in pots or in the ground. In the greenhouse, the sensitivity of Coker 16 to growth effects caused by chronic O3 exposures was intermediate to that of two open-pedigree hybrids. In the field, both open-pedigree hybrids were more sensitive than Coker 16. Exposure to a mean O3 concentration of 0.15 ppm for 7 hr/day decreased kernel yield of the open-pedigree hybrids by 37-40%, but that of Coker 16 was decreased by only 12%.