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Concentrations of soluble and bound phenolic compounds were measured in needles of 3-yr-old loblolly pine (Pinus taeda L.) trees exposed from May to November 1993 to a range of ozone (O3) concentrations in open-lop field chambers. The treatments were charcoal-filtered air (CF). non-filtered air (NF), and NF air with O3 added at 1.5 times (NF 1.5) and 2.0 times (NF 2.0) the ambient O3 concentration for 12 h daily. Average daily 0800-2000 hours) O3 concentrations in the CF. NF, NE 1.5 and NF 20 treatments were c. 29, 47,76 and 98 nl l-1, respectively, for the 140 d treatment period. At the end of the treatment period, total phenolic and proanthocyanidin concentrations in the previous year's needles were 25-29", higher in the NF 2.0 treatment compared with the lower O3 treatments. Catechin concentration increased in the previous year's needles by as much as 81% between the N F 2.0 treatment and the lower O3 treatments. Catechin is an effective antioxidant, and elevated levels might confer some protection against O3 injury. No significant differences in total phenolics and proanthocyanidins in the previous year's needles were detected among the remaining O3 treatments, or among any O3, treatment for the current year's needles. Lignin content in needles of both years was nor significantly affected O3 exposure. Changes in the phenolic content of older needles in response to elevated O3 could alter plant pathogen interactions and slow down microbial decomposition. which could contribute to a decline in site soil quality [ABSTRACT FROM AUTHOR]