Yield stimulation of a commonly grown cultivar of Phaseolus vulgaris L. at near-ambient ozone concentrations.

Seeds of Phaseolus vulgaris L. ev. 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 1 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 (O ) was added between 10.00 and 18.00 h GMT. The seasonal 7 h mean O concentrations ranged from 10 nl l in the CF treatment to 50 nl l in the NF30 treatment. The growth and yield of P. vulgaris ev. Nerina demonstrated two distinct responses to O depending upon concentration. At seasonal 7 h means of 20, 26 and 32 nl l (NF, NF6 and NF12 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 final harvest. In contrast, seasonal 7 h mean O concentrations of 38, 45 and 50 nl l (NF18, NF25 and NF30 treatments) caused 26-42 % reductions in the yield of P. vulgaris. The leaves of such plants developed extensive symptoms of O 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.