It is forecasted that atmospheric CO2 concentration till the middle of this century will be risen to about 550 µmol·mol-1 . Several experiments have shown that the yield of rice will be significantly increased by elevated CO2 as a result of an increase in number of grain per panicle or per plant. Less attention has been paid to the effect of elevated CO2 on the process of grain growth and development. This experiment was conducted to determine the grain growth and development during early stage of grain filling of rice (Oryza sativa L. sp. japonica. cv Wuxiangjing-14) under the conditions of ambient atmosphere CO2 concentration (CK) and FACE (Free Air CO2 Enrichment, 200 µmol·mol-1 higher than ambient) with 3 repetitions. The grain width, length and their product, contents of soluble carbohydrate (reducing sugar and sucrose) and starch, and activities of invertases including cell wall invertase (CWI), soluble acid invertase（SAI）and neutral invertase (NI) in grain were measured. Dry matter and filling rate on the basis of dry weight of grain with sampling every 3 days from anthesis till mature period, and grain weight and size at maturity were tested. The results showed that the process of growth and development of grain was accelerated and width and length of caryopsis were significantly increased by FACE. The maxima of grain width and size, and grain filling rate under FACE were about 3 days earlier than those under CK. The product of mature grain width and length under FACE was increased by 4.5% of CK. However, such obvious effect was not shown in the final grain weight (Fig.1, Fig.2). The contents of reducing sugar and sucrose, which were needed as materials and energy substance for the normal growth and development of grain, were increased during 1 week after anthesis (Fig.3), while the activities of CWI and NI enhanced during 5 days after anthesis by FACE (Fig.4). However, the content of starch and activitie of SAI in grain were not affected by elevated CO2. In conclusion, higher contents of soluble carbohydrate and higher activities of CWI and NI under FACE supply more materials for the growth of larger grain sink capacity. There is no significant effect on the final grain dry weight resulting from a slight increase in sink activities and the earlier decrease in grain filling rate under FACE. A new balance between sink and source needs to be established under elevated CO2 condition in order to improve grain yield by means of increasing the grain weight on the basis of higher panicle grain number in rice.
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