Abstract Spartina alterniflora (Poaceae), commonly known as smooth cordgrass is originated from hydro- morph and salted marginal lands. In these regions, hypoxic and salt stress conditions strongly inhibit nitrification and promote denitrification. Distribution and yield of this species depend on its capacity to tolerate salinity and to use ammonium as the predominant source of nitrogen. S. alterniflora grown on medium containing varying ammonium:nitrate ratio were treated with 500 mol m -3 NaCl. In absence of NaCl, the highest growth rate was observed in the mixed medium and the lowest one in medium containing nitrate alone. However, growth was higher in ammonium than in nitrate medium, indicating that S. alterniflora was one of the scarce species which prefered ammonium as nitrogen source. The addition of 500 mol m -3 NaCl affected plant growth in both nitrate and mixed-media, but did not affect when ammonium was supplied. In absence or in presence of 500 mM NaCl, differences in growth between the treatments were due to difference in photosynthesis activity. In the same way the mean secre- tion of ions, especially Na + by shoots of S. alterniflora was increased by ammonium treatment. Ammonium treatment probably improved salt tolerance of S. alterniflora by increasing secretion activity of salt glands leading to a decrease in salt content of leaves mesophyll and consequently to avoiding toxic buildup of Na + in the apoplastic tissues of the leaves. Keywords Ammonium • nitrate • salinity • Spartina alterniflora Loisel 1 Introduction Under natural conditions of growth and development, plants are inevitably exposed to different types of stresses, such as drought, salinity, low and high temperatures, flooding and high radiation. In salted marginal zones, salinity reduces agricultural yield throughout the world (Yokoi et al. 2002; Cant et al. 2007). Salt stress inhibits plants growth by disrupting ion and water homeostasis as well as causing oxidative stress (Cant et al. 2007). The extent of growth and yield reduction due to salt stress depends on the type of species, duration and severity of salt stress. Application of nitrogen fertilizers to plants growing under saline conditions might have some beneficial effects on plant growth and yield since nitrogen is involved in biochemical and physiological processed related to osmotic adjustment (Sagi et al. 1997). Some studies showed that salinity affects nitrogen mineralization, and this effect is more marked on nitrification than on ammonification (Laura 1977; Sahrawat 1982; McClung and Frankenberger 1987). Under this condition, other stable nitrogen forms in soil, such as ammonium, are also available to plants (Cantera et al. 1999). The inter- action between nitrogen and salinity has been studied in several plant species, such as peanut, wheat, maize and barley (Lewis et al. 1989; Soltani et al. 1992; Cant et al. 2007). The response of plant growth to nitrogen fertilization under saline and non-saline conditions varies according to whether the nitrogen is supplied as nitrate or ammonium and also depends on plant species. Ammonium was a suitable nitrogen source under non-saline conditions for some species, whereas for other species (e.g. cotton), NH 4 + caused much less growth than did NO 3 - (Yokoi et al. 2002). However, NO 3 - alone may not be beneficial, especially under salt stress, where its rate of uptake is reduced in many plant K. Hessini, M. Gandour, W. Megdich, A. Soltani, and C. Abdely Laboratoire d’Adaptation des Plantes aux Stress Abiotiques, Centre de Biotechnologie à la Technopole Borj Cédria BP 901, Hammam-Lif 2050, Tunisia e-mail: hessini_kamel@yahoo.fr Chapter 10 How Does Ammonium Nutrition Influence Salt Tolerance in Spartina alterniflora Loisel? K. Hessini, M. Gandour, W. Megdich, A. Soltani, and C. Abdely M. Ashraf et al. (eds.), Salinity and Water Stress, 91 © Springer Science + Business Media B.V. 2009