Effects Of Elevated CO₂Concentration And Transplanting Density On Spikelet Formation Of Hybrid Shanyou63-A FACE Study

Rising atmospheric carbon dioxide concentration ([CO2]) is a major driving factor for global warming, but it also directly increases crop yield. Rice (Oryza Sativa L.) is one of the most important crops in the world. Grain yield of rice is determined by the number of spikelets per unit ground area, which is in turn associated with the process of spikelet formation (including spikelet differentiation and degeneration). However, little is known about the effect of [CO2] on spikelet formation, especially for hybrid rice cultivar. In order to investigate the interactive effects of [CO2] and transplanting density on the number of surviving spikelets per panicle and relevant parameters, we conducted a unique Free Air CO2Enrichment (FACE) experiment in2011, using hybrid cultivar Shanyou63. The target [CO2] was200μmol mol-1above that in the ambient air. Two levels of transplanting densities were designed:normal-density (ND,24plants m-2) and high-density (HD,32plants m-2). The main results were as follows:1. Averaged across the two densities, elevated [CO2] increased the number of surviving spikelets per panicle by29(relative increase of15%, p<0.05). As for ND and HD crops, the number of surviving spikelets per panicle was increased by17%(p<0.01)and12%(p<0.1), respectively. On average,elevated [CO2] increased the number of surviving spikelets on primary branches (PB) and secondary branches (SB) per panicle by5%and20%, respectively, but no CO2effect was detected on the proportion of surviving PB or SB spikelets. The number of surviving PB and SB per panicle increased by5%(p<0.01) and15%(p<0.05) under elevated [CO2], respectively, but no change was found on the number of surviving spikelets on each PB and SB. No interaction between CO2and density was observed for the above parameters. 2. Averaged across the two densities, elevated [CO2] increased the number of differentiated spikelets per panicle by33(relative increase of12%, p<0.05). As for ND and HD crops, the number of differentiated spikelets per panicle was increased by18%(p<0.01) and6%(p>0.1) respectively. On average, elevated [CO2] increased the number of differentiated spikelets on PB and SB per panicle by5%and14%, respectively, but no CO2effect was detected on the proportion of differentiated PB or SB spikelets. The number of differentiated PB and SB per panicle increased by5%(p<0.01) and10%(p<0.05) under elevated [CO2], respectively. The number of differentiated spikelets on each SB was increased by elevated [CO2], while no effect was detected on the number of differentiated spikelets on each PB. No significant interaction between CO2and density was observed for all the above parameters except the number of differentiated PB.3. No effect of elevated [CO2] was detected on the number of degenerated spiekelets per panicle, the number of degenerated PB and SB, the number of degenerated spikelets on PB and SB, as well as the five categories of spikelet degeneration. Similar trends were found under the two densitie.4. The number of surviving spikelets per panicle can be divided into two factors:shoot dry weight (SDW) and the ratio of surviving spikelets to SDW. Averaged across the two densities, elevated [CO2] increased SDW by18%(p<0.05), but no change was found on the ratio of surviving spikelets to SDW. No interaction between CO2and density was observed for the two parameters.5. The number of surviving spikelets per panicle is also a function of nitrogen (N) absorption per stem at heading and the ratio of surviving spikelets to N. Averaged across the two densities, elevated [CO2] increased the ratio of surviving spikelets to N by16%(p<0.05), but no effect of CO2was recorded on N absorption per stem at heading. No interaction between CO2and density was observed for the above parameters.6. Elevated [CO2] increased the concentrations of soluble sugar, starch and soluble carbohydrates in stem at heading by67%(p<0.05),61%(p<0.05) and63%(p<0.01), respectively, while no interaction between CO2and density was observed for the two parameters.