| Current Intergovernmental Panel on Climate Change (IPCC) projections indicate that the atmospheric carbon dioxide concentration ([CO2]) will increase from current 381μmol mol-1 to at least 550μmol mol-1 by 2050. Rice (Oryza sative L.) is one of the most important crops in the world and the first food in China, providing a significant proportion of the people's dietary needs. The process of spikelet formation (including spikelet differentiation and degeneration) determines the final number of spikelets per panicle, which plays a critical role in securing a sufficient number of spikelets per unit area for a higher yield. Therefore, it is very important to assess the impact of elevated atmospheric CO2 concentration on spikelet formation of rice. In order to investigate the effects of elevated [CO2] on the number of surviving, differentiated or degenerated spilelets per panicle, dry weight per stem, nitrogen (N) uptake, soluble sugar and starch concentration at heading stage, we conducted a unique free-air CO2 enrichment (FACE) experiment in a rotation system of rice and wheat at Jiangdu, Jiangsu, China (32°35.5'N, 119°42'E) in 2008–2009. Four conventional Indica cultivars, i.e., Zhenxian 96 (ZX96), Yangdao 6 (YD6), Yangfuxian 6 (YFX6) and Yangdao 8 (YD8) were grown at ambient (AMB) or elevated (FACE, ca. 200μmol mol-1 above ambient) [CO2]. The main results were as follows:1. Averaged across the four cultivars, rice plants grown under FACE produced 8.4 fewer surviving spiekelts per panicle (relative increase of 5%, p < 0.01), compared with the plants grown in the ambient plots. With respect to different cultivars that were investigated, FACE increased the number of surviving spikelets per panicle by 5.6%, 3.4% and 13.3% at YD6, YFX6, and YD8, respectively, while only 1.3% decrease was observed for ZX96. As for different years, the average number of surviving spikelets per panicle was increased by 3.3% and 7.2% under FACE in 2008 and 2009, respectively.On average, the number of surviving spikelets on primary branches (PB) and secondary branches (SB) per panicle showed values 2.0 and 6.8 higher in FACE than AMB, respectively. FACE had no significant effect on the proportion of surviving PB or SB spikelets per panicle. The number of surviving PB and SB per panicle showed small but significant increase (p < 0.01 and p < 0.05). The number of surviving spikelets on each SB was significantly increased by FACE, while no CO2 effect was detected on the number of surviving spikelets on each PB. Statistical analysis of variance (ANOVA) indicated a significant CO2 by cultivar interaction for the number of surviving spikelets per panicle, surviving PB spikelets per panicle, surviving PB per panicle and surviving SB spikelets per SB, and significant CO2 and year interaction with respect to the number of surviving PB spikelets and surviving PB per panicle.2. Averaged across the four cultivars, rice plants grown under FACE produced 14.5 fewer differentiated spiekelts per panicle (relative increase of 6.1%, p < 0.01), compared with the plants grown in the ambient plots. With respect to different cultivars that were investigated, FACE increased the number of differentiated spikelets per panicle by 1.0%, 8.0%, 4.3% and 11.6% at ZX96, YD6, YFX6, and YD8. As for different years, the average number of differentiated spikelets per panicle was increased by 3.0% and 9.6% under FACE in 2008 and 2009, respectively. On average, the number of differentiated spikelets on PB and SB per panicle showed values 2.2 and 12.3 higher in FACE than AMB, respectively. FACE decreased the number of the proportion of differentiated PB spikelets per panicle by 0.8% (p < 0.05) and increased the number of the proportion of differentiated SB spikelets per panicle by 0.8% (p < 0.01). The number of differentiated PB and SB per panicle showed small but significant increase (p < 0.01). The number of differentiated spikelets on each SB was significantly increased by FACE, while no CO2 effect was detected on the number of differentiated spikelets on each PB. ANOVA indicated a significant CO2 by cultivar interaction for the number of differentiated PB spikelets per panicle and differentiated PB per panicle, and significant CO2 and year interaction with respect to the number of differentiated spikelets per panicle,differentiated SB spikelets and differentiated SB per panicle.3. Averaged across the four cultivars, rice plants grown under FACE produced 6.0 fewer degenerated spiekelts per panicle (relative increase of 8.9%, p < 0.01), compared with the plants grown in the ambient plots. With respect to different cultivars that were investigated, FACE increased the number of degenerated spikelets per panicle by 6.9%, 13.6%, 6.8% and 8.0% at ZX96, YD6, YFX6, and YD8. As for different years, the average number of degenerated spikelets per panicle was increased by 2.3% and 14.5% under FACE in 2008 and 2009, respectively. The number of degenerated SB spikelets and degenerated SB per panicle significant increase (p < 0.01), while no CO2 effect was detected on the number of degenerated PB spikelets and degenerated PB per panicle. FACE had no significant effect on the proportion of degenerated PB or SB spikelets per panicle. FACE had no significant effect on the number of degenerated spikelets on each PB or SB. ANOVA indicated no significant CO2 by cultivar interaction for these parameters, and significant CO2 and year interaction with respect to the number of degenerated spikelets per panicle, degenerated SB spikelets and degenerated SB per panicle.4. The degenerated spikelets can be divided into the following 5 categories: degenerated PB spikelets on degenerated PB (1), degenerated PB spikelets on surviving PB (2), degenerated SB spikelets on degenerated PB (3), degenerated SB spikelets on surviving SB (4), degenerated SB spikelets on surviving PB (5). The average number of degenerated SB spikelets on surviving PB was increased by 9.1% (p < 0.01) under FACE, but no effect on 4 types of front. ANOVA indicated a significant CO2 by cultivar interaction for the number of degenerated PB and SB spikelets on degenerated PB, and significant CO2 and year interaction with respect to the number of degenerated SB spikelets on surviving SB and PB.5. The surviving spikelets per panicle can be divided into two factors: dry weight per stem at heading stage and the ratio of surviving spikelets per panicle to dry weight per stem. Averaged across the cultivars, dry weight per stem at heading stage was increased by 11.2% due to FACE (p < 0.01), while the ratio of surviving spikelets per panicle to dry weight per stem was decreased by 4.2% (p < 0.01). There was a significant interaction between CO2 and cultivars for the ratio of surviving spikelets per panicle to dry weight per stem, but no significant interactions between CO2 and year was found for both parameters.6. The surviving spikelets per panicle can be divided into two factors: N uptake at heading stage and the ratio of surviving spikelets per panicle to N uptake. Averaged across the cultivars, N uptake at heading stage was decreased by 2.6% due to FACE (p > 0.05), while the ratio of surviving spikelets per panicle to N uptake was increased by 9.7% (p < 0.01). There was a significant interaction between CO2 and cultivars for the ratio of surviving spikelets per panicle to N uptake, but no significant interactions between CO2 and year was found for both parameters.7. FACE increased by 10.2%(p < 0.01), 5.4%(p < 0.01) and 7.6%(p < 0.01) at soluble sugar, starch and water-soluble carbohydrate content at heading stage. There was no significant interaction between CO2 and cultivars for these parameters, neither between CO2 and year.
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