Abstract: Barnyard grass is one of the vicious weeds in rice field, which affects seriously the growth and development of rice and reduces the yield of rice. In order to clarify the effect of barnyard grass on rice growth and development and yield formation under the condition of elevated atmospheric carbon dioxide (CO2) concentration, the experiment was conducted with the Free-Air CO2 Enrichment system (FACE system), taking rice Jijing 88 as material. The experiment included 4 treatments: atmospheric CO2 concentration (400µmol·mol−1) without barnyard grass, rice planting and barnyard grass together with atmospheric CO2 concentration, elevated CO2 concentration (550µmol·mol−1) without barnyard grass, rice planting and barnyard grass together with elevated CO2 concentration. Physiological indicators were measured and analyzed at the tillering stage, jointing stage, heading stage, and mature stage of rice. The results showed that elevated CO2 concentration increased significantly the number of rice ears per hole and the seed setting rate, and eventually increased the rice yield. Barnyard grass significantly reduced the rice seed setting rate, and reduced rice yield ultimately. Interaction of elevated CO2 concentration and barnyard grass increased significantly thousand-grain weight of rice, yet, the interaction did not affected rice yield significantly. Elevated CO2 concentration increased significantly the dry matter of rice, and barnyard grass reduced significantly the dry matter of rice. However, the interaction of elevated CO2 concentration and the barnyard grass had no significant difference. Elevated CO2 concentration increased significantly the net photosynthetic rate of the flag leaf of rice, the concentration of intercellular CO2 of the flag leaf, and the Soil and Plant Analyzer Developrnent (SPAD) value of rice flag leaf. Barnyard grass treatment reduced significantly the net photosynthetic rate of the flag leaf of rice, the concentration of intercellular CO2 in the flag leaf, the stomatal conductance, the transpiration rate, the instantaneous water use efficiency, and the SPAD value. The interaction of elevated CO2 concentration and barnyard grass reduced the net photosynthetic rate of rice flag leaves at early stage, and increased it at later stage. In conclusion, Elevated CO2 concentration after heading stage had a greater impact on rice photosynthesis than barnyard grass on rice photosynthesis.

Key words: Climate change, Carbon dioxide, Barnyard grass, Rice, Photosynthesis