Response of ponderosa pine primary needles to separate and simultaneous ozone and pan exposures

Young ponderosa pine seedlings in the primary needle stage were exposed to acute dosages of ozone peroxyacetylnitrate (PAN), or the combined pollutants. The seedlings were visibly injured by ozone but not by PAN. Exposure to the two pollutants simultaneously caused significantly less injury on 6 week old plants than did ozone alone, indicating an antagonistic response at this age. The same trend was observed for 10 week old plants, but the reduction in injury caused by the combined pollutants was not statistically significant. The magnitude of host reaction was affected by pollutant dosage.