Salt tolerance of the halophyte Limonium delicatulum is more associated with antioxidant enzyme activities than phenolic compounds Souid Aymen A , Gabriele Morena B , Longo Vincenzo B , Pucci Laura B , Bellani Lorenza B,C , Smaoui Abderrazak A , Abdelly Chedly A and Ben Hamed Karim A,D A Laboratoire des Plantes Extrêmophiles, Centre de Biotechnologie de Borj Cedria, BP 901, Hammam Lif 2050, Tunisia. B National Research Council, Institute of Biology and Agricultural Biotechnology (IBBA), Pisa Unit, Research Area of Pisa, Via Moruzzi 1, 56124 Pisa, Italy. C Department of Life Sciences, University of Siena, Via A. Moro 2, 53100 Siena, Italy. D Corresponding author. Email: kbenhamed@yahoo.fr Abstract. In this work we studied the effect of salinity (ranging from 50 to 500 mM NaCl) on the physiological and the antioxidant responses of the local halophyte Limonium delicatulum Kuntze. We based our analysis on 12 biochemical assays that are commonly used to measure the antioxidant responses under stress such as oxidative stress markers, enzymes activities and polyphenolic compounds. Our aim was to study parameters that are strongly correlated with the growth response to salinity. Results showed two different growth responses depending on the concentration of NaCl in the medium. Under 50 to 200 mM, the growth was stimulated before it decreased significantly at 300–500 mM. L. delicatulum revealed a good aptitude to maintain photosynthetic machinery by increasing the concentrations of photosynthetic pigments, which is essential for the stabilisation of photosystems and the photosynthesis process under optimal NaCl concentration. Their breakdown at higher salinity decreased the photosynthetic performance of plants resulting in growth inhibition. Moreover, to reduce the damaging effect of oxidative stress and to tolerate the accumulation of salt ions, L. delicatulum induced the activities of their antioxidant enzymes more than their contents in polyphenolic compounds. Additional keywords: antioxidant enzymes, halophyte, Limonium delicatulum, polyphenols, salinity. Received 12 September 2015, accepted 6 March 2016, published online 13 May 2016 Introduction About 6% of the world’s total land area has been already plagued by increasing salinity, particularly in arid and semiarid regions (Flowers and Muscolo 2015). Salt excess in soils and water has detrimental effect on crop yields and results in substantial losses of arable soils, especially in the arid and semiarid areas (Rengasamy 2006). High concentrations of salt impose both osmotic and ionic stresses due to reduced water availability and to the accumulation of ions in cells, respectively, which have inhibitory effects on many physiological processes. To overcome such stress, plants have developed strategies that mainly lead to various morphological and physiological adaptations (Flowers et al. 2010). Physiological adaptations could be the result of salt exclusion or sequestration of salt ions in vacuoles and accumulation of compatible compounds into the cytoplasm, including amino acids, carbohydrates, and alcohols, to balance the osmotic pressure (Greenway and Munns 1980; Cayuela et al. 2001). Halophytes, which are naturally salt tolerant plants, display both types of mechanisms. They exclude salt well, but effectively compartmentalise in vacuoles the salt that inevitably gets in (Flowers and Colmer 2008). This allows them to grow a long period of time in saline soil. The elucidation of physiological and biochemical mechanisms are critical, before trying to introduce genetic and environmental improvements to this stress (Nakamura et al. 2001). Salt- tolerance mechanisms are quite complex, including osmotic adjustment, compartmentation of toxic ions (Strizhov et al. 1997), metabolite accumulation, ion homeostasis, redox control, and scavenging of reactive oxygen species (ROS) (Meloni et al. 2003). Salinity generates oxidative stress in plant tissues (Hernandez et al. 1993), though the origin of oxidative damage is still confusing (Gómez et al. 1999). It is likely that salt stress limits gas exchange and thereby CO 2 supply to the leaf (Foyer and Noctor 2005). One consequence is the over-reduction of the photosynthetic electron transport chain (Arbona et al. 2003). This induces the generation of ROS. The main ROS production sites are the chloroplasts, mitochondria and peroxisomes (Harris and Outlaw 1991). The most commonly produced ROS are singlet oxygen ( 1 O 2 ), hydrogen peroxide (H 2 O 2 ), superoxide (O 2 * – ) and hydroxyl ( * OH) radicals CSIRO PUBLISHING Functional Plant Biology http://dx.doi.org/10.1071/FP15284 Journal compilation Ó CSIRO 2016 www.publish.csiro.au/journals/fpb