| Ozone (O3) is currently the most important air pollutant. Rice is arguably the most important food crops on the planet, therefore it is very important to accurately assess an exact effect of increasing surface O3 concentration ([O3]) on growth and development of rice. While a limited number of enclosure-based studies have examined the genotypic differences among rice (Oryza sativa L.) cultivars in response to increasing ozone concentration, no ozone experiment has been conducted to date under fully open-air field conditions to address this issue.Compared with enclosure, Free Air ozone Concentration Enrichment (FACE) system represent our best simulation of the future paddy field under higher [O3]. In order to investigate the effects of elevated [O3] on yield, yield components, dry matter production and distribution and soluble carbohydrates of stem and sheath of different rice cultivars, we conducted an experiment for the first time in the world with rice using unique FACE system at Xiaoji town, Jiangdu County, Jiangsu Province, China (119°42′0″E,32°35′5″N). Five Chinese rice cultivars: Wuyunjing 21 (inbred japonica cultivar), Wujing 15 (inbred japonica cultivar), Yangdao 6 (inbred indica cultivar), Shanyou 63 (three-line hybrid rice cultivar), Liangyoupeijiu (two-line hybrid rice cultivar), were grown at ambient or elevated (target at 50% above ambient) [O3]. Results showed as follows:1. On average, it showed value 136 g m-2 lower (relative decrease 15.4%) for elevated [O3] vs. ambient grown plants (P < 0.01). There was a significant interaction between O3 and cultivar: the magnitude of reduction was much larger in hybrid SY 63 than in inbred indica (-5.5%), inbred japonica cultivar WYJ 21 and WJ 15 (-10.5% and -16.1%, respectively) and inter-subspecific hybrid LYPJ (-13.7%). Elevated [O3] significantly reduced the number of panicles, number of spikelets per panicle and number of spikelets per unit area by 5.7% (P < 0.01), 9.2% (P < 0.05) and 14.2% (P < 0.01), while no O3 effect was detected for the filled grain percentage, blighted grain percentage, empty grain percentage, filled grain weight, blighted grain weight and average grain weight.There was a significant interaction between O3 and cultivars for the number of spikelets per panicle, number of spikelets per unit area, filled grain percentage, filled grain weight and blighted grain percentage.2. The maximum tiller number was significantly decreased by 14.7% under elevated [O3] on average, while the productive tiller ratio was significantly increased by 9.5% of all cultivars. Tillering ability of rice was decreased, which led to the significant decrease of panicle number. Single-stem unit dry matter weight and the formation of the existing capacity of spikelets in elevated [O3] was reduced by 7.4 and 2.9 percent on average, respectively. The reduction of weight of single stem contributed the decline of spikelet number per panicle in elevated [O3].3. The dry matter accumulation of all cultivars under elevated [O3] was decreased by 8.7%, 6.0%, 11.1% and 12.6% on average at tillering, jointing, heading and maturity stages, respectively. The effect was significant at jointing, heading and maturity stages, except of tillering stage. The biomass accumulated from transplanting to tillering stage, tillering to jointing stage, jointing to heading stage and heading to maturity was reduced by 9.0%, 4.7%, 17.1% and 17.9% on average, respectively, with the effect being significant during the last two stages. The final total biomass at maturity was decreased because of the reduction of biomass in reproductive stage. O3 and cultivars affected biomass production almost independently and no significant interaction between them.4. Both O3 and CK, the average leaf area index (LAI) and net assimilation rate (NAR) reached the maximum at the stage of heading and jointing-heading, respectively. With the reproductive process, the reduction of average LAI showed a rising trend, the reduction of NAR showed a trend that decline first then rising. Further analysis showed that ozone significantly decreased average LAI and NAR, which contributed to the reduction of biomass accumulation during jointing to heading stage in ozone treatment. The excessive decay speed of LAI in grain-filling period is the main reason for the significant reduction of biomass accumulation during heading to maturity stage. O3 and cultivars affected average LAI and NAR independently and no significant interaction between them at different growth stages.5. Elevated [O3] significantly increased the ratio of yellow leaf dry matter to total biomass of rice plant at different growth stages, while no O3 effect was detected for the ratio of green leave, leave, stem and sheath, root, panicle dry matter to total biomass at different growth stages, as well as harvest index (HI) was not significantly changed. There was a significant interaction between O3 and cultivars for the biomass allocation at different growth stages.6. The concentration of soluble carbohydrates in stem and sheath under high ozone concentrations was decreased by 9.2% and 13.5% on average at heading stage and maturity with no significance with all cultivars. The accumulation of soluble carbohydrates in stem and sheath under elevated [O3] was significantly decreased by 22.3% and 23.8% on average at heading stage and maturity with all cultivars. There was no significant interaction between O3 and cultivars for the soluble carbohydrates content and accumulation in stem and sheath at heading stage and maturity.
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