HEAT STRESS The Alleviating Effect of Elevated CO 2 on Heat Stress Susceptibility of Two Wheat (Triticum aestivum L.) Cultivars S. Shanmugam 1 , K. H. Kjaer 2 , C.-O. Ottosen 2 , E. Rosenqvist 3 , D. Kumari Sharma 3 & B. Wollenweber 4 1 Department of Bioenergy, Tamilnadu Agricultural University Coimbatore, India 2 Department of Food Science, Aarhus University  Arslev, Denmark 3 Department of Plant and Environmental Sciences, University of Copenhagen Taastrup, Denmark 4 Department of Agroecology, Aarhus University Slagelse, Denmark Keywords carbohydrate metabolism; chlorophyll fluorescence; heat stress; photosynthesis; pigments; stomatal conductance Correspondence K. H. Kjaer Department of Food Science Aarhus University Kirstinebjergvej 10 5792  Arslev Denmark Tel.: +45 8715 8337 Fax: +45 8715 4891 Email: Katrine.kjaer@agrsci.dk Accepted February 8, 2013 doi:10.1111/jac.12023 Abstract This study analysed the alleviating effect of elevated CO 2 on stress-induced decreases in photosynthesis and changes in carbohydrate metabolism in two wheat cultivars (Triticum aestivum L.) of different origin. The plants were grown in ambient (400 lll À1 ) and elevated (800 lll À1 ) CO 2 with a day/night tempera- ture of 15/10 °C. At the growth stages of tillering, booting and anthesis, the plants were subjected to heat stress of 40 °C for three continuous days. Photosyn- thetic parameters, maximum quantum efficiency of photosystem II (PSII) photo- chemistry (F v /F m ) and contents of pigments and carbohydrates in leaves were analysed before and during the stress treatments as well as after 1 day of recovery. Heat stress reduced P N and F v /F m in both wheat cultivars, but plants grown in elevated CO 2 maintained higher P N and F v /F m in comparison with plants grown in ambient CO 2 . Heat stress reduced leaf chlorophyll contents and increased leaf sucrose contents in both cultivars grown at ambient and elevated CO 2 . The con- tent of hexoses in the leaves increased mainly in the tolerant cultivar in response to the combination of elevated CO 2 and heat stress. The results show that heat stress tolerance in wheat is related to cultivar origin, the phenological stage of the plants and can be alleviated by elevated CO 2 . This confirms the complex interre- lation between environmental factors and genotypic traits that influence crop performance under various climatic stresses. Introduction The increasing variability of climate poses a major risk to food security. Increased occurrences of severe drought and heat events (IPCC 2007) have become a major problem for cereal production increasing the risk of significant yield losses and reductions in grain quality. Thus, record crop yield losses were reported in 2003, where Europe experi- enced a heat wave with July temperatures up to 6 °C above average and annual precipitation 50 % below average, resulting in substantial reductions in the primary produc- tivity (Fink et al. 2004). Heat stress is defined as a rise in temperature above criti- cal threshold levels for a period of time sufficient to cause irreversible damage to plant growth and development. In general, a transient rise in temperature, usually 10–15 °C above ambient, is considered as heat shock (Wahid et al. 2007). Photosynthesis is one of the most heat sensitive pro- cesses in plants, and thermo-sensitive components are found in both light-dependent (PSII) (Heckathorn et al. 2002) and dark (Rubisco activase) (Crafts-Brandner and Salvucci 2002) reactions. Furthermore, heat stress also reduces the amount of photosynthetic pigments (Todorov et al. 2003) and increases the accumulation of soluble car- bohydrates in the leaves (Wardlaw et al. 2002, Wahid et al. 2007, Suwa et al. 2010) thereby enhancing stress tolerance via osmotic adjustment (Hare et al. 1998, Wahid et al. 2007). Wheat has an optimum temperature range of 17–23 °C over the course of the growing period, with temperatures © 2013 Blackwell Verlag GmbH, 199 (2013) 340–350 340 J Agro Crop Sci (2013) ISSN 0931-2250