Abstract:To investigate the effects of high ozone concentration on spikelet formation of hybrid rice cultivars, an experiment using a unique free air ozone concentration enrichment (ozone-FACE) system was conducted in 2007. Two hybrid rice cultivars, Shanyou 63 (SY63) and Liangyoupeijiu (LYPJ), were grown at ambient and elevated (target at 50% above ambient) ozone concentration. The results showed that compared with ambient ozone concentration, elevated ozone reduced the spikelet number per panicle by 28 and 34 (relative decrease of 15% and 13%) for SY63 and LYPJ, respectively. This reduction in spikelet number per panicle was mainly contributed to the significant decrease in spikelet number on secondary branches (SB), while only minor response was detected for the spikelet number on primary branches (PB), resulting in an increase in percentage of primary branch spikelets and a decrease in percentage of secondary branch spikelets under ozone exposure. In terms of spikelet formation, the large ozone-induced reduction in spikelet number per panicle of the two hybrid cultivars was entirely due to the suppression of spikelet differentiation per panicle (especially that on SB), while the degenerated spikelets per panicle decreased rather than increased under ozone exposure. It was suggested that corresponding measures should be adopted to mitigate the detrimental effects of ozone on the spikelet differentiation to minimize yield loss under increasing surface ozone concentration.