DOI: 10.1007/s11099-011-0006-2 PHOTOSYNTHETICA 49 (1): 145-148, 2011 145 BRIEF COMMUNICATION Reduced nitrogen allocation to expanding leaf blades suppresses ribulose-1,5-bisphosphate carboxylase/oxygenase synthesis and leads to photosynthetic acclimation to elevated CO 2 in rice S. SENEWEERA Department of Agriculture and Food Systems, Melbourne School of Land and Environment, The University of Melbourne, Private Box 260, Horsham, VIC 3401, Australia Abstract Net photosynthetic rate (P N ) measured at elevated CO 2 concentration (C e ), ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco), and nitrogen (N) content in rice leaves decreased significantly after exposure to long term C e . The reduction in P N , Rubisco, and leaf N at C e was similar for the last fully expanded leaf blade (LFELB) and expanding leaf blade (ELB). Spatial leaf N content in the ELB was highest in the zone of cell division, sharply declined as cell expansion progressed and gradually increased with cell maturation. Maximum reduction in spatial leaf N and Rubisco content was found at C e only within cell expansion and maturation zones. The spatial leaf N content correlated well with the amount of Rubisco synthesized during leaf expansion, suggesting that N deposition into the expanding leaf blade may be the key for Rubisco synthesis and possibly photosynthetic acclimation to C e . Additional key words: elevated CO 2 ; photosynthesis; ribulose-1,5-bisphosphate carboxylase/oxygenase; rice; spatial nitrogen deposition. —— Net photosynthetic rate (P N ) acclimation to elevated CO 2 concentration (C e ) is characterized by lower leaf CO 2 assimilation rates associated with a reduction in the concentrations of Rubisco and leaf N (Nakano et al. 1997, Stitt and Krapp 1999). The underlying mechanism for the reduction in leaf Rubisco content by C e is still not resolved. Rubisco content is the rate-limiting factor for P N at the current atmospheric CO 2 concentration (C a ) (Farquhar et al. 1980) and accounts for 15–35 % of total leaf N in C 3 plants (Evans 1989, Makino and Osmond 1991). The concentration of Rubisco in leaf blades is determined by the balance between protein synthesis and degradation. Rubisco concentration rapidly increases during leaf expansion, reaching a maximum when the leaf is fully expanded (Imai et al. 2008, Suzuki et al. 2001). Suppression of Rubisco synthesis at C e is suggested to be mediated through the sugar sensing mechanism (Moore et al. 1999, Pandurangam et al. 2006). However, a negative relationship between accumulation of soluble sugar and Rubisco synthesis is not always reported (Ludewig et al. 1998, Nakano et al. 2000). We tested the hypothesis that N deposition in the growing leaf blade during leaf expansion is linked to Rubisco synthesis and then P N acclimation to C e . Rice is a good model system to test this hypothesis because leaf growth, protein and pigment synthesis, and nutrient deposition is unidirectional (Gastal and Nelson 1994, Schäufele and Schnyder 2001, Seneweera and Conroy 2005). Rice (Oryza sativa L. ) plants were grown hydropo- nically in environmentally controlled growth chambers at 1,000 μmol photons m –2 s –1 (in the 400–700 nm range), day/night cycle of 12/12h and day/night temperature of 25/20°C respectively. Vapour pressure deficit (VPD) was maintained at 1.5–1.9 kPa throughout the experiment. The seedlings were transferred to pots containing a nutri- ent solution as described by (Mae and Ohira 1981). N was supplied at a concentration of 2.0 mM (1.0 mM NH 4 NO 3 ). Plants were grown for 70 days at CO 2 concentrations of ——— Received 14 April 2010, accepted 1 January 2011. Tel.: +61 3 5362 2164, fax +61 3 5362 0782, e-mail: samans@unimelb.edu.au Abbreviations: C a – ambient CO 2 concentration; C e – elevated CO 2 concentration; DAP – days after planting; DM – dry mass; ELB – expanding leaf blade; FM – fresh mass; LER – leaf blade elongation rates; LFELB – last fully expanded leaf blade; N – nitrogen; P N – net photosynthetic rate; PPFD – photosynthetic photon flux density; Rubisco – ribulose 1,5-bisphosphate carboxylase/oxygenase. Acknowledgment: This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.