The experiment was conducted at Anzhen city, Wuxi，Jiangsu, China, to study the effect of free-air CO2 enrichment [FACE, 200 μmol·mol-1 higher than ambient(AMB)] on spikelet differentiation and degeneration, using the japonica cultivar Wuxiangjing 14, in 2001–2003. Results of FACE treatment showed that the survived spikelets per panicle have decreased by 10.4 to 12.6, i.e. 6.88% to 8.49%. There were no significant effects on the differentiated spikelets per panicle and the degenerated spikelets of the primary branches per panicle, while an increase of the number and percentage of spikelets degenerated on the secondary branches per panicle compared with those under AMB; The plant N accumulation per differentiated spikelet at heading and N absorption per differentiated spikelet from stem elongating to heading were significantly lower, whereas the content of soluble carbohydrates in stem and sheath at heading and the ratio of that to nitrogen content in rice plant were significantly higher compared with those under AMB; the degenerated spikelets per panicle negatively and significantly correlated with the N accumulation per differentiated spikelet at heading (－0.848**) and with the N absorption per differentiated spikelet from elongating to heading (－0.961**), respectively; While the degenerated spikelets per panicle positively and significantly correlated with the content of soluble carbohydrates in stem and sheath (0.808*) and with the ratio of the soluble carbohydrates content in stem and sheath to nitrogen content in rice plant at heading stage (0.837*), respectively. It was speculated that the ability of N absorption weakened, carbon metabolism strengthened and nitrogen metabolism weakened due to FACE during the period of spikelets formation, were the primary causes of largely degenerating number of differentiated spikelets and decreasing number of spikelets per panicle under FACE condition.
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