Intraspecific variation in leaf growth of wheat (Triticum aestivum) under Australian Grain Free Air CO 2 Enrichment (AGFACE): is it regulated through carbon and/or nitrogen supply? Chamindathee L. Thilakarathne A,B , Sabine Tausz-Posch A , Karen Cane C , Robert M. Norton D , Glenn J. Fitzgerald C , Michael Tausz A and Saman Seneweera A,E,F A Department of Agriculture and Food Systems, Melbourne School of Land and Environment, The University of Melbourne, Water Street, Creswick, Vic. 3363, Australia. B Central Research Station, Department of Export Agriculture, Matale, Sri Lanka. C Department of Environment and Primary Industries, Horsham, Vic. 3400, Australia. D International Plant Nutrition Institute, Horsham, Vic. 3400, Australia. E Centre for Systems Biology, University of Southern Queensland, Toowoomba, Qld 4350, Australia. F Corresponding author. Email: saman.seneweera@usq.edu.au Abstract. Underlying physiological mechanisms of intraspecific variation in growth response to elevated CO 2 concentration [CO 2 ] were investigated using two spring wheat (Triticum aestivum L.) cultivars: Yitpi and H45. Leaf blade elongation rate (LER), leaf carbon (C), nitrogen (N) in the expanding leaf blade (ELB, sink) and photosynthesis (A) and C and N status in the last fully expanded leaf blade (LFELB, source) were measured. Plants were grown at ambient [CO 2 ] (~384 mmol mol –1 ) and elevated [CO 2 ] (~550 mmol mol –1 ) in the Australian Grains Free Air CO 2 Enrichment facility. Elevated [CO 2 ] increased leaf area and total dry mass production, respectively, by 42 and 53% for Yitpi compared with 2 and 13% for H45. Elevated [CO 2 ] also stimulated the LER by 36% for Yitpi compared with 5% for H45. Yitpi showed a 99% increase in A at elevated [CO 2 ] but no A stimulation was found for H45. There was a strong correlation (r 2 = 0.807) between LER of the ELB and soluble carbohydrate concentration in LFELB. In ELB, the highest spatial N concentration was observed in the cell division zone, where N concentrations were 67.3 and 60.6 mg g –1 for Yitpi compared with 51.1 and 39.2 mg g –1 for H45 at ambient and elevated [CO 2 ]. In contrast, C concentration increased only in the cell division and cell expansion zone of the ELB of Yitpi. These findings suggest that C supply from the source (LFELB) is cultivar dependent and well correlated with LER, leaf area expansion and whole-plant growth response to elevated [CO 2 ]. Additional keywords: diurnal regulation of LER, elevated [CO 2 ], gas exchange, growth analysis, leaf blade elongation rate. Received 28 April 2014, accepted 13 October 2014, published online 4 December 2014 Introduction Atmospheric carbon dioxide concentration ([CO 2 ]), which was ~280 mmol mol –1 before the industrial revolution, is now ~400 mmol mol –1 and is predicted to reach ~550 mmol mol –1 by the middle of this century (IPCC 2007). As CO 2 is the primary substrate for photosynthesis (A), increasing [CO 2 ] will substantially increase A for most C 3 plants and possibly support greater plant growth and yield (Seneweera et al. 2005; Ainsworth et al. 2008). However, the magnitude of the response to elevated [CO 2 ] varies among plant functional groups and even within the same species (Kimball et al. 2002; Thilakarathne et al. 2013). There is a limited understanding of the underlying physiological mechanisms of how plant growth responds to elevated [CO 2 ]. Plant growth response to elevated [CO 2 ] is mainly characterised by accelerated leaf area production, resulting in higher relative growth rate (RGR) and then leaf area ratio (LAR) (Poorter 1993; Seneweera et al. 1996; Makino et al. 1997; Thilakarathne et al. 2013). This early advantage at elevated [CO 2 ] results from faster LER, which leads to greater leaf area, biomass production and grain yield (Conroy et al. 1994; Jitla et al. 1997). Photosynthate (carbon, C) supply to the expanding leaf blades (ELB) is largely determined by the photosynthetic capacity of the last fully expanded leaf blade (LFELB) as well as N supply, the latter of which is mainly determined by the availability of soil N (Seneweera 2011). Expanding leaf blade growth depends directly on C assimilation and on remobilisation of C intermediates from LFELB (Seneweera et al. 1995; Aoki CSIRO PUBLISHING Functional Plant Biology http://dx.doi.org/10.1071/FP14125 Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/fpb