Ozone is the most important phytotoxic gaseous pollutant in many parts of the world. The study reported was conducted to elucidate the response of gas exchange characteristics of rape (Brassica napus L.) to different O_３ concentrations, and fumigation regimes under equal ozone dose at a site on the Yangtze River Delta, China. Rape seeds were germinated in seedbeds on 20 October, 2004. The seedlings were directly transplanted into twelve 2m×2m plots on 18 November 2004. After it became warm and the rape turned green, twelve open top chambers (OTCs) were erected on 21 March 2005(the chamber was octagon, 2.2 m high and 2 m in diameter), where the plants were exposed to O_３ from 23 March 2005. Over the course of the fumigation, three OTCs were ventilated continuously (8h d^－１) with passing air through activated charcoal filter (CF, O_３ range: 5～15 nl•L^－１), three received 50 nl•L^－１ O_３ (50, O_３ range: 45～55 nl•L^－１) and three received 100 nl•L－１ O_３ (100, O_３ range: 90～110 nl•L^－１), which were ventilated continuously (8h d－１) with constant O３ concentration, respectively. The other three were exposed to another O３ regime (9:00～11:00: 50 nl•L^－１, 11:00～13:00: 100 nl•L^－１, 13:00～15:00: 200 nl•L^－１, 15:00～17:00: 50 nl•L^－１), although the exposure dose was the same as the third treatment. The additional ozone was carried out from 9:00 to 17:00 per day, and suspended when it rained. Each treatment was randomly arranged in field. Ozone was generated using pure compressed air by electric discharge (ozone generator, QHG-1, Yuyao, China) and mixed with charcoal filtered ambient air by means of flow controllers linked to a desktop computer, programmed with individual exposure profiles. To guarantee controlled and reproducible exposure conditions, ozone concentrations were measured continuously within each chamber at plant height on a 5 min interval by an ozone analyst (Monitor Labs Inc. ML9810B). After 25 days’ exposure to O_３, at the stage of rape anthesis, leaf CO_２/H₂O exchange in situ was tracked at 9:30 ～10:30 AM on 15 April, 2005. Leaf gas exchange rates were measured by a portable infra-red gas analyzer (IRGA) (CIRAS-1, PP system, UK). Measurements on individual full-spread flags were repeated 2 times, and for each time 2 leaves were selected, and for each leaf 2 data were recorded. During the measurements of leaf gas exchange, the relative humidity of the air passing into the cuvette was maintained at 52.3％±2.1%, and environmental temperatures averaged （26.4 ±1.0）℃, and the PAR ranged between 350 and 470μmol•m^-2•s^-1. Some parameters such as photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), intercellular CO_２ concentration (Ci) and ambient CO_２ concentration (Ca) were recorded automatically. Water use efficiency (WUE) and stoma limit value (Ls) can be calculated by the formula of WUE=Pn/Tr and Ls=1-Ci/Ca, respectively. Pn was measured under the different PAR by controlling the light source on the top of cuvette to achieve the response curve of Pn to PAR. By this curve, some parameters can be calculated, such as apparent quantum yield (AQY), dark respiration rate (Rd), light compensation point (LCP), light saturated point (LSP) and photo-saturated photosynthetic rate (Pmax). In addition, Pn was also measured under the different CO_２ concentrations to achieve the response curve of Pn to CO_２. By this curve, some parameters can be calculated, such as carboxylation efficiency (CE), light respiration rate (Rp) and CO_２ compensation point (Г).
On 17 April, 2005, ratios of dark-adapted variable to maximum chlorophyll a fluorescence (Fv/Fm, i.e. the optimal photochemical efficiency of photosystem II) were determined in situ, with a portable fluorometer (PEA, Hansatech, UK) on 5 leaves from field-grown plants in three replicate OTCs per treatment. Measurements were made on the 5th and 8th leaves from the top of the canopy at 10:00～11:00. Samples were dark-adapted for 20 min before recording fluorescence induction kinetics (5 s) using an actinic excitation beam of 400μmol•m^-2•s^-1.
The results indicated that there were no significant difference in Pn, Gs, Ci and Ls between ozone concentrations, and higher ozone concentration (100 nl•L^－１) decreased Tr and increased WUE in comparison with CF in the constant concentration exposure way. However, dynamic ozone exposure regime significantly decreased Pn, Gs, Ls and WUE and increased Tr as well relative to CF. At the dynamic exposure regime, Pn,Rd， Ls and WUE were 17.0%, 16.7% and 36.6% lower than those of 100 nl•L^－１ treatment, respectively, and higher than 29.4% Tr was observed despite the same exposure dose. In the constant concentration exposure regimes, higher ozone concentration (100 nl•L^－１) markedly decreased the AQY, LSP and Pmax and increased Rp and Г, but there was no significant difference in LCP and CE. In the dynamic exposure regime, AQY, LSP, Pmax and CE were 11.9%, 48.7%, 21.3% and 10.6% lower than those of CF, respectively. Whereas Rd, LCP, Rp and Г were 7.9%, 22.6%, 99.7% and 78.7% higher than those of CF, respectively. There were significant differences in the parameters such as Rd, LCP, LSP, Pmax and CE between O_３ exposure regimes. The increase of O_３ concentration induced significant decreases in Fv/Fo and Fv/Fm of the 8th leaves from the top canopy, but it had no any effect on the 5th full-spread leaves, no matter what exposure regimes were imposed. It can be concluded that dynamic ozone exposure regime has greater detrimental effects on the photosynthesis of rape in spite of equal exposure dose, suggesting that traditional exposure regime (invariable concentration) could not really reflect the response process of plants to elevated O_３ concentration.