Influence of the magnesium content in apple leaves photosynthetic capacity V. Blackhall 1,a , M. Curetti 2 , G.A. Orioli 3 and G.M. Colavita 1 1 CITACC, Universidad Nacional del Comahue - Consejo Nacional de Investigaciones Cientı́ficas y Técnicas, IBAC, Facultad de Ciencias Agrarias, Cinco Saltos, Rı́o Negro 8303, Argentina; 2 INTA E.E.A. Alto Valle, General Roca, Rı́o Negro, Argentina; 3 CERZOS, Consejo Nacional de Investigaciones Cientı́ficas y Técnicas, Departamento de Agronomı́a, Universidad Nacional del Sur, Bahı́a Blanca, Buenos Aires 8000, Argentina. Abstract Fruit trees photosynthesis is determined by abiotic factors affecting crop productivity. The aim of this study was to evaluate the influence of magnesium nutritional status and exposure to solar radiation in the photosynthetic capacity of leaves of Malus domestica ‘Cripp’s Pink’. A two-factor experiment was conducted: high (+Mg) and low (-Mg) foliar Mg content (0.26 and 0.19%, respectively) in leaves were exposed to high (HR) and low (LR) solar radiation (1600 and 600 μmol m -2 s -1 , respectively). Chlorophyll concentration, maximum quantum yield of photosystem-II (Fv/Fm) and net photosynthesis rate was determined in leaves. HR and LR -Mg leaves had 11.5 and 1.8% less chlorophyll concentration respectively than HR and LR +Mg leaves; no effects were observed in chlorophyll concentration by exposure. Fv/Fm was affected by sun exposition but not by Mg content. In HR leaves Fv/Fm was 5-6% lower than in LR leaves. Photosynthesis was affected by magnesium content when leaves where exposed to high solar radiation. Maximum net photosynthesis in HR +Mg leaves was 20.0 μmol m -2 s -1 , while in HR -Mg was 11.1 μmol m -2 s -1 . Net photosynthesis in LR leaves was not affected by Mg content. These results show that apple leaves photosynthetic capacity could be affected by lower Mg content when exposed to high solar radiation. Keywords: high light stress, chlorophyll a fluorescence transient, apple, magnesium INTRODUCTION Magnesium (Mg) is not usually a limiting element in Alto Valle of Rı́o Negro region soils, although its absorption may be strongly depressed in the presence of other cations (Hawkesford et al., 2012). Under this situation it is not uncommon to find foliar symptoms of Mg deficiency in apple crops and the foliar applications of Mg salts is the most effective tool to correct its deficiency (Sanchez, 1999). Mg deficiency has been studied in apple plants (Malus domestica Borkh.), but most of the results are limited to symptoms occurring in leaves with medium to severe deficiencies (Ford, 1964; Oland and Opland, 1956; Sanchez, 1999). Little information is reported on the effects of Mg deficiency on apple tree physiology. Recent studies advanced the knowledge of the physiological and biochemical effects of Mg deficiency in fruit plants such as mango (Balakrishnan et al., 2000), coffee (Meireles da Silva et al., 2014), mulberry (Kumar Tewari et al., 2006), and citrus species (Li et al., 2017; Yang et al., 2012; Zhang et al., 2015). Several studies in cereals and horticultural crops have reported that Mg deficiency induces decreases in chlorophyll content (Cakmak and Marschner, 1992; Esfandiari et al., 2010; Hermans et al., 2004), proteins (Anza and Riga, 2001) and carotenoid compounds in leaves (El-Abdin Abdel-Kader, 2004). Photosynthetic rate may also be affected by Mg deficiency (Sun and Payn, 1999; Laing et al., 2000; Ridolfi and Garrec, 2000; Hariadi and Shabala, 2004; Farhat et al., 2016). Because Mg plays a key role in photosynthate phloem loading, Mg deficiency results in an accumulation of carbohydrates in source leaves, altering carbon photosynthetic metabolism and restricting CO2 fixation (Cakmak and Kirkby, 2008; a E-mail: valeria.blackhall@faca.uncoma.edu.ar Acta Hortic. 1217. ISHS 2018. DOI 10.17660/ActaHortic.2018.1217.19 Proc. VIII International Symposium on Mineral Nutrition of Fruit Crops Eds.: T. Mimmo, Y. Pii and F. Scandellari 151