Leaf carbon management in slow-growing plants exposed to elevated CO 2 IKER ARANJUELO *, ANTONI PARDO *w , CARMEN BIEL w , ROBERT SAVE ´ w , JOAQUIM AZCO ´ N-BIETO * and S A L V A D O R N O G U E ´ S * *Unitat de Fisologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain, wInsitut de Recerca i Tecnologı ´a Agroalimenta `ries (IRTA), Horticultura Ambiental Torre Marimon, Carretera C59 km 12.1, 08140 Caldes de Montbui, Barcelona, Spain Abstract Two slow-growing plant species (Chamaerops humilis, L. and Cycas revoluta Thunb.) were exposed to elevated CO 2 conditions over a 20-month period in order to study the CO 2 effect on growth, photosynthetic capacity and leaf carbon (C) management. The ambient isotopic 13 C/ 12 C composition (d 13 C) of the greenhouse module corresponding to elevated CO 2 (800 lmol mol 1 CO 2 ) conditions was changed from d 13 C ca. 12.8  0.3% to ca. 19.2  0.2%. Exposure of these plants to elevated CO 2 enhanced dry mass (DM) by 82% and 152% in Chamerops and Cycas, respectively, mainly as a consequence of increases in plant level photosynthetic rates. However, analyses of A–C i curve parameters revealed that elevated CO 2 diminished leaf photosynthetic rates of Chamaerops whereas in Cycas, no photosynthetic acclimation was detected. The fact that Chamaerops plants had a lower DM increase, together with a longer leaf C residence time and a diminished capacity to respire recently fixed C, suggests that this species was unable to increase C sink strength. Furthermore, the consequent C source/sink imbalance in Chamaerops might have induced the downregulation of Rubisco. Cycas plants were capable of avoiding photosynthetic downregulation due to a greater ability to increase C sink strength, as was confirmed by DM values, and 12 C-enriched CO 2 labeling data. Cycas developed the ability to respire a larger proportion of recently fixed C and to reallocate the recently fixed C away from leaves to other plant tissues. These findings suggest that leaf C management is a key factor in the responsiveness of slow-growing plants to future CO 2 scenarios. Keywords: carbon isotopic labeling, Chamaerops humilis, Cycas revoluta, global change, leaf carbon management, photosynthetic acclimation, slow-growing plants Received 8 October 2008 and accepted 4 November 2008 Introduction According to the IPCC, overall atmospheric concentra- tions of carbon dioxide have increased markedly as a result of human activities since 1750 and now far exceed preindustrial values (Alley et al., 2007). Furthermore, it is predicted that the by the end of this century, the CO 2 concentration in the Earth’s atmosphere is going to reach 750 mmol mol 1 . Plants can act as a sink (Suni et al., 2003) or a source (Lindroth et al., 1998) for atmospheric carbon dioxide. Because the net exchange of carbon dioxide is the balance between respiration and photosynthetic pro- cesses, small changes in either process can significantly impact the net exchange. An increase in CO 2 concentra- tion from 300–350 to 680 mmol mol 1 has been described as enhancing plant growth by 7% to 25% (Hadley et al., 1995; Daepp et al., 2000; Aranjuelo et al., 2006). Accord- ing to the literature (von Caemmerer & Farquhar, 1981; Long et al., 2004), this increase in growth is caused by a stimulation in photosynthetic rates of between 35% and 60%. Although the initial stimulation of net photosynth- esis associated with elevated CO 2 is sometimes retained during long-term exposure (Davey et al., 2006), it is often partially reversed in an acclimation process, often referred to as ‘down-regulation’ (Long et al., 2004; Aranjuelo et al., 2005a, b; Ainsworth & Rogers, 2007). Correspondence: Iker Aranjuelo, tel. 1 34 934021465, fax 1 34 934112842, e-mail: iker.aranjuelo@ub.edu Global Change Biology (2009) 15, 97–109, doi: 10.1111/j.1365-2486.2008.01829.x r 2008 The Authors Journal compilation r 2008 Blackwell Publishing Ltd 97