SUMMARYSeeds of Phaseolus vulgaris L, cv, Nerina were hand-sown inside open-top chambers (OTCs) in which the plants were exposed to charcoal-filtered air (CF treatment), ambient air (NF), or ambient air with added 6, 12, 18, 25 or 30 nl r^ of ozone (NF6, NF12, NF18, NF25 and NF30 treatments, respectively). Exposure commenced as the crop emerged and continued on a daily basis until the end of the season. Ozone (O3) was added between 10,00 and 18,00 h GMT, The seasonal 7 h mean O3 concentrations ranged from 10 nl I"^ in the CF treatment to 50 nl r ' in the NF30 treatment.The growth and yield of P. vulgaris cv, Nerina demonstrated two distinct responses to O3 depending upon concentration. At seasonal 7 h means of 20, 26 and 32 nl 1"' (NF, NF6 and NFI 2 treatments respectively), the yield increased relative to that of the CF" treatment. The increase in yield peaked at 48% in the NF6 treatment {P < 0-05), and was associated with a 55 % increase in the maximum leaf number, longer retention of the canopy, and a 45 % increase in the number of pods present at the Rnal harvest. In contrast, seasonal 7 h mean O3 concentrations of 38, 45 and 50 nl T' (NFI8, NF25 and NF30 treatments) caused 26^2 "o reductions in the yield of P, vulgaris. The leaves of such plants developed extensive symptoms of O3 injury and were prematurely abscised from the plants. The reductions in yield were due to a decrease in the numbers of seeds per pod and in the weights of individual seeds.
