Journal of Agricultural Science; Vol. 10, No. 10; 2018 ISSN 1916-9752 E-ISSN 1916-9760 Published by Canadian Center of Science and Education 388 Antioxidant Protection of Photosynthesis in Two Cashew Progenies Under Salt Stress Anselmo F. da Silva 1 , Valéria F. de O. Sousa 2 , Gisele L. dos Santos 2 , Eugênio S. Araújo Júnior 3 , Sérgio L. F. da Silva 3 , Cristiane E. C. de Macedo 4 , Alberto S. de Melo 5 & Josemir M. Maia 6 1 Academic Unit of Agronomy, Federal University of Paraiba, Areia, PB, Brazil 2 Academic Unit of Tropical Horticulture, Federal University of Campina Grande, Pombal, PB, Brazil 3 Academic Unit of Production Vegetable, Rural Federal University of Pernambuco, Serra Talhada, PE, Brazil 4 Academic Unit of Fitotecnia, Rural Federal University of the Semi-Arid, Mossoró, RN, Brazil 5 Academic Unit of Agrarian Sciences, State University of Paraiba, Campina Grande, PB, Brazil 6 State University of Paraíba, Center for Human and Agrarian Sciences, Catolé do Rocha, Paraíba, Brazil Correspondence: Josemir M. Maia, State University of Paraíba, Center for Human and Agrarian Sciences, Catolé do Rocha, Paraíba, Brazil. E-mail: jmouram@gmail.com Received: June 28, 2018 Accepted: August 7, 2018 Online Published: September 15, 2018 doi:10.5539/jas.v10n10p388 URL: https://doi.org/10.5539/jas.v10n10p388 Abstract The present work evaluated the indicators of photosynthetic efficiency and antioxidative protection in cashew tree seedlings subjected to salinity stress. The study was conducted with seedlings of two advanced dwarf cashew clones (CCP09 and CCP76) subjected to salt stress with increasing doses of NaCl (0, control; 25; 50; 75; 100 mM) in the nutrient solution for 30 days under greenhouse conditions. The variables of gas exchange, CO 2 assimilation (P N ), stomatal conductance (g S ), transpiration (E), intercellular CO 2 concentration (C I ), photochemical activity, potential quantum efficiency (Fv/Fm), effective quantum efficiency (ΔF/Fm’) of photosystem II (PSII), photochemical quenching (qP), non-photochemical quenching (NPQ) electron transport rate (ETR) as well as the indicators of damage and oxidative protection were measured. Under these conditions, there was an intense accumulation Na + associated with a reduction in the K + /Na + ratio in the leaves of both clones in response to salt, with higher values for this ratio in clone CCP09 than in CCP76 the highest concentration of NaCl (100 mM). Salinity reduced P N , g S and E in the two clones evaluated, with lower reductions in CCP09 than in CCP76 at the highest salt dose. Instantaneous carboxylation (P N /C I ) and water use (P N /E) efficiencies were strongly restricted by salinity but were less affected in CCP09 than in CCP76. Salinity stress also increased hydrogen peroxide (H 2 O 2 ) levels in CCP09, whereas lipid peroxidation decreased in both progenies. The clones presented specific antioxidant responses due to greater enzymatic and non-enzymatic activity in CCP76, in addition to the activity of phenol peroxidase (POX) in CCP09. Keywords: Anacardium occidentale, oxidative stress, photosynthesis, salinity 1. Introduction Excess of salt in the soil solution causes metabolic disturbances in plants due to the osmotic and ionic effects of salinity, leading to reduced crop growth and productivity (Khan & Panda, 2008; Lima, Nobre, Gheyi, Soares, & Silva, 2014). The osmotic effect is immediate due to the difference in osmotic potential between the external and internal environments of the cell, whereas the ionic effect occurs later when the concentration of Na + and/or Cl - reaches toxic levels in the cytosol (Shavrukov, 2013). At the time of exposure to salinity, these osmotic/ionic effects act simultaneously, affecting essential metabolic processes such as nutritional balance, water relations and photosynthesis (Shaheen, Naseer, Ashraf, & Akram, 2013; Chen, Hawighorst, Sun, & Polle, 2014). Ionic toxicity caused by salinity stress results from increased Na + /K + , Na + /Ca +2 , Na + /Mg +2 and Cl - /NO 3 - ratios in plant tissue, causing cellular disorders related to the physiological function of these essential nutrients (Abbaspour, Kaiser, & Tyeman, 2014; Bessa, Lacerda, Amorim, Bezerra, & Lima 2016). K + is a macronutrient that participates in several cellular functions, acting on osmotic potential (osmosolute function) and the functioning of metabolic pathways due to its role as an enzymatic cofactor (Wang & Wu, 2013). Thus, the