Barley Growth and Its Underlying Components are Affected by Elevated CO 2 and Salt Concentration Usue Pe ´rez-Lo ´pez • Jon Miranda-Apodaca • Amaia Mena-Petite • Alberto Mun ˜ oz-Rueda Received: 23 November 2012 / Accepted: 20 March 2013 / Published online: 1 June 2013 Ó Springer Science+Business Media New York 2013 Abstract The future environment will exhibit increases in soil salt concentrations and atmospheric CO 2 . In general, plant growth is inhibited by salt stress and stimulated by elevated CO 2 . This study investigated whether elevated CO 2 could improve plant growth under salt stress and the mechanisms involved. We measured functional and mor- phological components of growth in barley (cv. Iranis) subjected to 0, 80, 160, or 240 mM NaCl and grown at either 350 (ambient) or 700 (elevated) lmol mol -1 CO 2 . Under nonsaline conditions, elevated CO 2 stimulated growth by increasing the relative growth rate (RGR). Maximum CO 2 stimulation was observed within the first 10 days of development, before the start of the salt treat- ment. Afterwards, salt stress caused reductions in biomass production and RGR by decreasing the photosynthetic rate and increasing the respiration rate; this resulted in a reduced net assimilation rate (functional component). In addition, salt stress caused nutritional imbalances, which reduced the leaf expansion capacity, and changed the root- to-shoot ratio. This resulted in reductions in the specific leaf area and leaf weight ratio (morphological compo- nents). However, the functional component became more relevant with increasing salt stress. Under elevated CO 2 conditions, salt stress inhibited growth less than that observed at ambient CO 2 . This occurred because (1) more dry biomass was synthesized for a given leaf area due to higher photosynthetic rates, and (2) greater leaf area and root biomass were maintained for photosynthesis and water and mineral uptake, respectively. Keywords Climate change  Elevated CO 2  Growth  Hordeum vulgare  Salt stress Introduction It has been predicted that by the end of this century atmospheric CO 2 could rise to 700 lmol mol -1 (IPCC 2007). Another consequence of the global changes pre- dicted might be an increase in salinized areas in the Mediterranean region, where water is an important factor that limits plant growth (Ruiz-Sa ´nchez and others 2007). Changes in CO 2 and water availability can affect various physiological processes in plants, but the understanding of the interaction between these factors remains incomplete. Barley has always been one of the world’s most exten- sively cultivated crops. It has been predicted that barley yield will increase by 0.35 % for every 1 lmol mol -1 increase in atmospheric CO 2 (Manderscheid and Weigel 1995). Growth is the balance between the carbon gain and loss from the plant. From a physiological point of view, growth expresses the quantitative changes that occur dur- ing plant development as a result, ultimately, of the mas- sive and rapid expansion of young cells produced by meristematic divisions. The processes of cell expansion depend, in part, on an inwardly directed water potential gradient, which generates turgor pressure and cell water uptake (that is, volume increase; Neumann 1997). How- ever, cell expansion and division processes also depend on interactions between other processes, including photosyn- thesis, respiration, and nutritional and hormonal status (Lazof and Bernstein 1999). The relative growth rate U. Pe ´rez-Lo ´pez (&)  J. Miranda-Apodaca  A. Mena-Petite  A. Mun ˜oz-Rueda Departamento de Biologı ´a Vegetal y Ecologı ´a, Facultad de Ciencia y Tecnologı ´a, Universidad del Paı ´s Vasco (UPV/EHU), Apdo. 644, 48080 Bilbao, Spain e-mail: usue.perez@ehu.es 123 J Plant Growth Regul (2013) 32:732–744 DOI 10.1007/s00344-013-9340-x