Figure 1.

(a) Open-top chambers (OTCs) were located at Montardon site (10 km north of Pau, France). Technical characteristics of OTCs, close to those described by Heagle et al. [32, 33], have been already reported [34]. Each circular OTC had a diameter of 3 m and an open-top diameter of 2 m and was 2.8 m tall. It consisted of a galvanised iron frame covered with a polyethylene foil (Deltatex T2E). Ozone-free air (filtered air) or O₃-enriched air was blown all around the chamber above the canopy level. The flow rate was controlled to achieve an air exchange rate of 3.14 times per min at the canopy level. When supplied, extra ozone was generated by electrical discharge of pure oxygen and injected into the air stream. Extra ozone was equally released only from 10 a.m. to 5 p.m. (GMT) in order to simulate the normal period of ozone exposure. The control chamber received filtered air (before ambient air was blown in the chamber, it passed through a charcoal filter that removed almost totally ozone), while, in O₃-enriched OTC, O₃ concentration reached 100 μg/m³. Ozone was sequentially monitored in the three OTCs with an UV ozone analyser (Environnement SA, O₃ 41 M) under the control of a computer recording system. (b) Inside a filtered air chamber: in the foreground mature Dactylis glomerata plants with inflorescences can be seen. The air suction device of the OTC allowing measurement of the ozone concentration is noted with a red arrow.