DOI: 10.1007/s11099-018-0839-z PHOTOSYNTHETICA 56 (4): 1313-1325, 2018 1313 Effects of foliar applications of nitric oxide and spermidine on chlorophyll fluorescence, photosynthesis and antioxidant enzyme activities of citrus seedlings under salinity stress D. KHOSHBAKHT *, + , M. R. ASGHARI * , and M. HAGHIGHI ** Department of Horticultural Science, College of Agriculture, Urmia University, West Azarbaijan, Iran * Department of Horticulture Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran ** Abstract The effects of exogenous sodium nitroprusside (SNP), as nitric oxide donor, and spermidine (Spd) on growth and photosynthetic characteristics of Bakraii seedlings (Citrus reticulata × Citrus limetta) were studied under NaCl stress. In citrus plants, SNP- and Spd-induced growth improvement was found to be associated with reduced electrolyte leakage, malondialdehyde, hydrogen peroxide content, and leaf Na + and Cl – concentration. However, we found increased leaf Ca 2+ , Mg 2+ , and K + concentrations, relative water content, chlorophyll fluorescence parameters, antioxidant enzyme activities, such as ascorbate peroxidase, catalase, superoxide dismutase and peroxidase, as well as higher photosynthetic rate, intercellular CO 2 concentration, stomatal conductance, and transpiration rate under saline regime. Foliar application of SNP and Spd alone mitigated the adverse effect of salinity, while the combined application proved to be even more effective. Additional key words: abiotic stress; biomass; gas exchange; oxidative stress; photosystem II efficiency. Introduction Salinity is one of the major environmental stresses for plants that can severely limit crop production throughout the world (Sudhir and Murthy 2004, Ravindran et al. 2007, Munns and Tester 2008). Salinity stress has received increasing attention in recent years because it greatly reduces agricultural productivity (Parihar et al. 2015). Salinity causes various injuries in plants, such as tissue burning, yield reduction, and finally plant death (Romero- Aranda et al. 2001), causes leaf senescence, reduction PSII activities (Khayyat et al. 2016), enhances membrane permeability (Dhindsa et al. 1981), nutritional imbalances, and toxicity (Khoshbakht et al. 2014). NaCl inhibits photosynthetic rate (PN ) as a consequence of osmotic stress, which leads to a decrease in water potential and stomatal conductance (g s ), sugar accumulation, which causes feedback inhibition, and ion toxicity (e.g., an excess of Na + and Cl – ) accompanied by a reduction of K + and Ca 2+ (Khoshbakht and Asgharei 2015a). Moreover, salinity can cause oxidative stress through excessive production of reactive oxygen species (ROS), such as superoxide radical (O 2 • – ), hydroxyl radicals ( • OH), and hydrogen peroxide (H 2 O 2 ) (Wu et al. 2011). The accelerated accumulation of ROS is able to induce oxidative damage to many cellular constituents, such as proteins, membrane lipids, and nucleic acids (Mittler 2002). One of the most destructive impacts of oxidative damage is peroxidation of membrane lipids, which leads to the accompanying generation of malondialdehyde (MDA) (Wu et al. 2011). Consequently, a high MDA content is a helpful biomarker of lipid peroxidation and is therefore often used to detect oxidative ——— Received 30 June 2017, accepted 3 January 2018, published as online-first 21 June 2018. + Corresponding author; phone: +98 311 3913447, fax: +98 311 3913356, e-mail: davod.khoshbakht@gmail.com Abbreviations: APX – ascorbate peroxidase; Ca – atmospheric CO2 concentration; Ci – intercellular CO2 concentration; CAT – catalase; Chl – chlorophyll; E – transpiration rate; EL – electrolyte leakage; F0 – minimal fluorescence yield of the dark-adapted state; Fm – maximal fluorescence yield of the dark-adapted state; Fv – variable fluorescence; Fv/Fm – maximum photochemical efficiency of PSII; gs – stomatal conductance; LN – number of leaves per plant; MDA – malondialdehyde; NPQ – nonphotochemical quenching; PAs – polyamines; Put – putrescine; POD – peroxidase; PN – net photosynthetic rate; qP – photochemical quenching; ROS – reactive oxygen species; RWC – relative water content; SL – length of shoot; SNP – sodium nitroprusside; SOD – superoxide dismutase; S – salinity stress; Spd – spermidine; Spm – spermine; TPDM – total plant dry mass; TPFM – total plant fresh mass. Acknowledgements: Here we would like to thank to the Department of Horticulture, College of Agriculture, University of Urmia for financial support of the research.