Net photosynthetic rate (P N ) of leaves grown under free-air CO 2 enriched condition (FACE, about 200 μmol mol -1 above ambient air) was significantly lower than P N of leaves grown at ambient CO 2 concentration (AC) when measured at CO 2 concentration of 580 μmol mol -1 . This difference was found in rice plants grown at normal nitrogen supply (25 g m -2 ; NN-plants) but not in plants grown at low nitrogen supply (15 g m -2 ; LN-plants). Namely, photosynthetic acclimation to FACE was observed in NN-plants but not in LN-plants. Different from the above results measured in a period of continuous sunny days, such photosynthetic acclimation occurred in NN-plants, however, it was also observed in LN-plants when P N was measured before noon of the first sunny day after rain. Hence strong competition for the assimilatory power between nitrogen (N) and carbon (C) assimilations induced by an excessive N supply may lead to the photosynthetic acclimation to FACE in NN-plants. The hypothesis is supported by the following facts: FACE induced significant decrease in both apparent photosynthetic quantum yield (Φ c ) and ribulose-1,5-bisphosphate (RuBP) content in NNplants but not in LN-plants.Additional key words: apparent quantum yield; carboxylation efficiency; net photosynthetic rate; nitrogen supply; Oryza; ribulose-1,5-bisphosphate regeneration.

Net photosynthetic rate (P N ) measured at the same CO 2 concentration, the maximum in vivo carboxylation rate, and contents of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBPCO) and RuBPCO activase were significantly decreased, but the maximum in vivo electron transport rate and RuBP content had no significant change in CO 2 -enriched [EC, about 200 μmol mol −1 above the ambient CO 2 concentration (AC)] wheat leaves compared with those in AC grown wheat leaves. Hence photosynthetic acclimation in wheat leaves to EC is largely due to RuBP carboxylation limitation.Additional key words: maximum in vivo electron transport rate; net photosynthetic rate; ribulose-1,5-bisphosphate carboxylase/oxygenase activase; RuBP regeneration limitation; sucrose-phosphate synthase; Triticum. --After long-term exposure to elevated CO 2 concentration (EC) leaf net photosynthetic rate (P N ) is often significantly lower in the plants grown at EC than that in plants grown in ambient air (AC) when measured at a same CO 2 concentration. This phenomenon is so called photosynthetic acclimation to EC (Xu et al. 1994). Usually, this acclimation has been almost entirely attributed to the decrease in active ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) (Jacob et al. 1995, Rogers and Humphries 2000, Pérez et al. 2007). Nevertheless, Chen et al. (2005 reported that the photosynthetic acclimation in rice leaves is related to both RuBP carboxylation limitation and RuBP regeneration limitation. The similar result was also observed in Norway spruce (Urban et al. 2003). Furthermore, Zhang et al. (2008) has demonstrated that RuBP regeneration limitation is predominant within the two limitations. However, it is not known whether such mechanism of photosynthetic acclimation operating in rice is speciesspecific or universal. In order to answer the question the photosynthetic acclimation was investigated further in wheat.The Chinese CO 2 enrichment facilities were located at Xiaoji village (119°42'0"E, 32°35'5"N), Yangzhou city in Jiangsu Province, East China. In the experimental field, there were six rings of 15 m diameter. Among them, three rings were sprayed with pure CO 2 as EC treatment, and the others were in the normal atmosphere as AC control. The distances between EC and AC rings were more than 90 m. Target CO 2 concentration in the centre of EC rings was about 200 μmol mol -1 above the AC. The EC treatment was made from emergence of wheat seedlings to harvesting. Seeds of wheat (Triticum aestivum L.cv. Ningmai 9) were sown in mid November. The cultivation of wheat was performed with typical agronomic management techniques for this region.---Abbreviations: C i -internal CO 2 concentration; Cyt -cytochrome; J max -maximum in vivo electron transport rate; P N − net photosynthetic rate; RuBP -ribulose-1,5-bisphosphate; RuBPCO -ribulose-1,5-bisphosphate carboxylase/oxygenase; SPS -sucrosephosphate synthase; V cmax -maximum in vivo carboxylation rate. Acknowledgements: The Chinese Rice/Wheat free-air CO 2 enrichment project ...

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