Effect of elevated CO 2 , temperature and drought on dry matter partitioning and photosynthesis before and after cutting of nodulated alfalfa Gorka Erice a , Juan J. Irigoyen a , Pilar Pe ´rez b , Rafael Martı ´nez-Carrasco b , Manuel Sa ´nchez-Dı ´az a, * a Departamento de Biologı ´a Vegetal, Facultades de Ciencias y Farmacia, Universidad de Navarra, Irunlarrea s/n, 31008, Pamplona, Navarra, Spain b Instituto de Recursos Naturales y Agrobiologı ´a de Salamanca, CSIC, Apartado 257, 3701 Salamanca, Spain Received 29 September 2005; received in revised form 23 December 2005; accepted 26 December 2005 Available online 3 February 2006 Abstract The rising atmospheric CO 2 concentration resulting from industrial development may enhance photosynthesis and plant growth. However, there is a lack of research concerning the effect of combined factors such as CO 2 , temperature and water availability on plant regrowth following cutting or grazing, which represent the usual methods of managing forage legumes like alfalfa. Elevated CO 2 , temperature and drought can interact with cutting factors (e.g. cutting frequency or height), and source-sink balance differences before and after defoliation can modify photosynthetic behaviour and dry matter accumulation, as well as dry matter partitioning between above- and belowground organs. The aim of our study was to determine the interactive effect of CO 2 (ambient, around 350 mmol mol À1 versus 700 mmol mol À1 ), temperature (ambient versus ambient + 4 8C) and water availability (well-irrigated versus partially irrigated) on dry matter partitioning and photosynthesis in nodulated alfalfa after vegetative normal growth and during regrowth. At the end of vegetative normal growth, CO 2 enhanced dry matter accumulation despite photosynthesis being down-regulated at the end of this period. Photosynthesis was stimulated by elevated CO 2 and resulted in greater dry matter accumulation during the regrowth period. Aboveground organs were affected more by drought than belowground organs during the entire experiment, particularly during vegetative normal growth. The higher drought tolerance (greater growth) observed during the regrowth period may be related to higher mass and greater reserves accumulated in the roots of plants. # 2006 Elsevier Ireland Ltd. All rights reserved. Keywords: Alfalfa; Regrowth; Climate change; Plant dry matter; Photosynthesis; Temperature gradient greenhouses 1. Introduction Human economic development since the industrial revolution has increased the atmospheric CO 2 concentration from 280 mmol mol À1 in 1750 to 372 mmol mol À1 [1]. This increasing concentration may reach 550 mmol mol À1 by the middle of this century and 700 mmol mol À1 by the end of the century [1]. According to the Intergovernmental Panel on Climate Change [2], the global temperature may increase by the middle of this century by 1.4–5.8 8C due to the greenhouse properties of atmospheric carbon dioxide. In C 3 plants such as alfalfa, photosynthesis is CO 2 -limited [3] and an increase in this greenhouse gas will enhance photosynthesis and plant growth [3–5]. The magnitude of this beneficial effect on growth and dry matter (DM) accumulation depends on other factors such as temperature, relative humidity, drought and mineral nutrition [6–8]. Elevated CO 2 and temperature interact in climate systems, so the combined study of both parameters is particularly relevant. The beneficial effect of elevated CO 2 on plant growth is reportedly greater at high temperatures [9]. Alfalfa (Medicago sativa L.) is a temperate crop that usually faces low water availability under growth conditions subjected to a Mediterranean climate. As reported by Chaves et al. [10], water deficit has been recognized as one of the most important environmental factors limiting photosynthesis, plant growth www.elsevier.com/locate/plantsci Plant Science 170 (2006) 1059–1067 * Corresponding author. Tel.: +34 948425600x6227; fax: +34 948425649. E-mail address: msanchez@unav.es (M. Sa ´nchez-Dı ´az). 0168-9452/$ – see front matter # 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2005.12.018