RESPONSES OF BEAN (PHASEOLUS-VULGARIS L. CV PROS) TO CHRONIC OZONE EXPOSURE AT 2 LEVELS OF ATMOSPHERIC AMMONIA

Plants of bean (Phaseolus vulgaris cv. Pros) were exposed to a range o f O-3 concentrations up to 70 nl litre(-1) for 9 h day(-1) in the pres ence (45 nl litre(-1)) and absence (21 nl litre(-1)) of enhanced NH3 i n 12 open-top chambers. Treatment effects on visible injury, growth an d yield were assessed after 49 (intermediate harvest) and 62 days of e xposure (final harvest). The proportion of leaves with visible injury at final harvest increased with increasing concentrations of O-3 Enhan ced NH3 did not cause any symptoms and did not affect injury by O-3. T he estimated seasonal mean concentration corresponding with 5% injury was circa 23 nl litre(-1) O-3. Biomass production and green pod yield decreased with increasing concentrations of O-3 and were generally sti mulated by enhanced NH3 at both harvests. Adverse effects of O-3 on bi omass and pod yield did not depend on the NH3 level. Relative yield re sponse to increasing 9-h daily mean O-3 concentrations was nonlinear a nd yield losses of 5 and 10% were calculated to occur at seasonal dayt ime mean concentrations of 27 and 33 nl litre(-1) O-3, respectively. L inear regression showed that the Accumulated exposures Over a Threshol d of 30 (AOT30) and 40 nl litre(-1) (AOT40) O-3 performed equally well . The estimated accumulated O-3 exposures corresponding with a yield l oss of 5% were 1600 nl litre(-1) h for AOT30 and 400 nl litre(-1) h fo r AOT40. The results are discussed in relation to the long-term critic al level that is used as a guideline to protect crops against adverse effects by O-3. (C) 1998 Elsevier Science Ltd. All rights reserved.