Impact of elevated ozone concentration on yield of four Chinese rice cultivars under fully open-air field conditions

Ozone is currently the most important air pollutant that negatively affects growth and yield of agricultural crops in most parts of the world, and rice is arguably the most important food crops on the planet. 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. In 2007, we conducted an experiment for the first time in the world with rice using free-air concentration enrichment (FACE) system at Xiaoji town, Jiangdu County, Jiangsu Province, China (119° 42′0″E, 32° 35′5″N). Four Chinese rice cultivars: Wujing 15 (WJ15, inbred japonica cultivar), Yangdao 6 (YD6, inbred indica cultivar), Shanyou 63 (SY63, three-line hybrid rice cultivar), Liangyoupeijiu (LYPJ, two-line hybrid rice cultivar), were grown at ambient or elevated (target at 50% above ambient) ozone concentration under nitrogen application rate of 15 g N m−2. The ozone enhancement strongly accelerated phenologycal development of WJ15 and SY63, with maturity being reached by 4 and 8 days earlier, respectively, but only 1 day earlier for YD6 and LYPJ. Elevated ozone concentration reduced the number of mainstem leaves (ca. by half a leaf) and plant height at maturity (ca. by 3–5 cm) of SY63 and LYPJ with no ozone effects detected in YD6 or WJ15. Among the cultivars tested, SY63 and LYPJ exhibited significant yield loss by exposure to ozone (−17.5%, −15%, respectively), while WJ15 and YD6 showed no responses. For all cultivars, no ozone effect was observed on panicle number per unit area as a result of no changes in both maximum tiller number or productive tiller ratio. However, the number of spikelets per panicle of SY63 and LYPJ showed a significant reduction due to ozone exposure, while those of WJ15 and YD6 remained unaffected. Meanwhile, ozone exposure also caused minor reductions in both filled spikelet percentage and individual grain mass. The results of this experiment indicated that yield loss due to ozone exposure differs among rice cultivars with hybrid cultivars (i.e., SY63 and LYPJ) exhibiting greater yield loss than inbred cultivars (i.e., WJ15 and YD6), which could be attributed to the suppression of spikelet formation in the hybrid cultivars under ozone stress.