Journal of Plant Physiology 213 (2017) 98–107 Contents lists available at ScienceDirect Journal of Plant Physiology journa l h om epage: www.elsevier.com/locate/jplph Research paper Drought stress obliterates the preference for ammonium as an N source in the C 4 plant Spartina alterniflora Kamel Hessini a,b,∗ , Herbert J. Kronzucker c,d , Chedly Abdelly a , Cristina Cruz e a Laboratory of Extremophiles Plants, Center of Biotechnology of Borj Cedria, University of Tunis El Manar, Tunisia b Biology Department, Faculty of Science, Taif University, Taif 888, Saudi Arabia c Department of Biological Sciences & Canadian Centre for World Hunger Research, University of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada d School of BioSciences, The University of Melbourne, Parkville, Victoria 3010, Australia e Departamento de BiologiaVegetal, Faculdade de Ciencias de Lisboa, Centro de Biologia Ambiental-CBA, Campo Grande, Bloco C-2, Piso 4, 1749-016 Lisboa, Portugal a r t i c l e i n f o Article history: Received 20 December 2016 Received in revised form 6 March 2017 Accepted 7 March 2017 Available online 9 March 2017 Keywords: Ammonium Antioxidant enzymes Spartina alterniflora Drought stress a b s t r a c t The C 4 grass Spartina alterniflora is known for its unique salt tolerance and strong preference for ammo- nium (NH 4 + ) as a nitrogen (N) source. We here examined whether Spartina’s unique preference for NH 4 + results in improved performance under drought stress. Manipulative greenhouse experiments were car- ried out to measure the effects of variable water availability and inorganic N sources on plant performance (growth, photosynthesis, antioxidant, and N metabolism). Drought strongly reduced leaf number and area, plant fresh and dry weight, and photosynthetic activity on all N sources, but the reduction was most pronounced on NH 4 + . Indeed, the growth advantage seen on NH 4 + in the absence of drought, producing nearly double the biomass compared to growth on NO 3 − , was entirely obliterated under both interme- diate and severe drought conditions (50 and 25% field capacity, respectively). Both fresh and dry weight became indistinguishable among N sources under drought. Major markers of the antioxidant capacity of the plant, the activities of the enzymes superoxide dismutase, catalase, ascorbate peroxidase, and glu- tathione reductase, showed higher constitutive levels on NH 4 + . Catalase and glutathione reductase were specifically upregulated in NH 4 + -fed plants with increasing drought stress. This upregulation, however, failed to protect the plants from drought stress. Nitrogen metabolism was characterized by lower consti- tutive levels of glutamine synthetase in NH 4 + -fed plants, and a rise in glutamate dehydrogenase (GDH) activity under drought, accompanied by elevated proline levels in leaves. Our results support postulates on the important role of GDH induction, and its involvement in the synthesis of compatible solutes, under abiotic stress. We show that, despite this metabolic shift, S. alterniflora’s sensitivity to drought does not benefit from growth on NH 4 + and that the imposition of drought stress equalizes all N-source-related growth differences observed under non-drought conditions. © 2017 Elsevier GmbH. All rights reserved. 1. Introduction Under natural conditions of growth and development, plants are inevitably exposed to multiple stresses, such as drought, salin- ity, flooding, mineral deficiencies, and toxicity (Ben Hamed et al., 2013). Of these, drought is considered one of the most formidable Abbreviations: APX, ascorbate peroxidase; CAT, catalase; EL, electrolyte leakage; FC, field capacity; GR, glutathione reductase; GDH, glutamate dehydrogenase; GS, glutamine synthetase; GPX, guaiacol peroxidase; H2O2, hydrogen peroxide; SOD, superoxide dismutase. ∗ Corresponding author at: Biology Department, Faculty of Science, Taif University, Taif, 888, Saudi Arabia. E-mail address: kamelhessini@gmail.com (K. Hessini). challenges to agricultural productivity (Mahajan and Tuteja, 2005; Hessini et al., 2008, 2009b), and the greatest losses in productivity occur in arid and semiarid regions, where, in addition to scarcity, the quality of irrigation water is often low (Fernández-Cirelli et al., 2009). Drought inhibits plant growth by disturbing the uptake of ions and water, impeding N-metabolism, and causing oxidative stress (Gonzalez et al., 1998; Bhargava and Sawant, 2013). The extent of damage depends on plant genotype, the severity of the stress, and the type, quantity, and regime of fertilization (Hessini et al., 2009a; Waraich et al., 2011, 2012). The use of fertilizer to enhance crop productivity has increased five-fold since the 1960s, and about 65% of it is used on cereals. However, inadequate or inefficient fertil- ization perturbs plant growth and contributes to soil degradation http://dx.doi.org/10.1016/j.jplph.2017.03.003 0176-1617/© 2017 Elsevier GmbH. All rights reserved.