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The Influence of Plant Age on the Sensitivity of Virginia Pine to Ozone. Donald D. Davis, Former Special Fellow, currently Assistant Professor and Extension Air Pollution Specialist, Department of Plant Pathology and Center for Air Environment Studies, The Pennsylvania State University, University Park 16802; Francis A. Wood, Research Associate, Center for Air Environment Studies, and Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 63:381-388. Accepted for publication 8 September 1972. DOI: 10.1094/Phyto-63-381.

Virginia pine seedlings in the cotyledon, primary needle, and secondary needle stages were injured after exposure to 25 pphm O₃ for 2 hr. All three needle types reached maximum sensitivity to O₃ at approximately 3-5 weeks following initiation of needle growth, after which time the sensitivity declined. Cotyledons became resistant at 16 weeks; primary needles were still sensitive at the oldest age studied; secondary needles became resistant at 18 weeks. The basal portions of the cotyledons were more sensitive than the tip or mid portions. The youngest primary and secondary needles were injured most severely at the needle tip, whereas older needles were more severely injured at the base. The three needle types showed similar symptoms, mainly chlorotic mottle and tissue necrosis, as well as similar dosage-response curves. Dormant 3-year-old plants exposed to O₃ in December were highly resistant.