Impact of Repetitive Salt Shocks on Seedlings of the Halophyte Cakile maritima

Abstract

Salinization is one of the main environmental constraints that threat global crop biomass production and thus food security. Hence halophytic species are currently widely studied in world because of their value for the development of saline agriculture. However, when applying salt two distinct protocols could be used salt stress and salt shock leading to different responses (Shavrukov, 2012). Gradual salt application (usually of 25 or maximum 50 mM increments of NaCl, until a final, predetermined salt concentration is reached) reflects the reactions expected for salt stress in saline field environments (Shavrukov et al., 2012). Various researchers refer to this method as progressive imposition (Almansouri et al., 1999), salt acclimation, gradual step acclimation (Rodriguez et al., 1997; Sanchez et al., 2008), or salt-adapting (Baisakh et al., 2006). Most of plants in natural ecosystems risk salt stress, but coastal halophytes growing on sand dunes are subject to inundation with seawater (Davy, 2006) and thus repetitive salt shocks. The main component of salt shock is osmotic shock inducing plasmolysis, especially in root cells (Munns, 2002). Desperate attempts by the cells to maintain equilibrium between external and internal water content results in the leakage of cell solution into open spaces between the cell wall and plasma membrane. These apoplastic solutes, containing high concentrations of Na , can freely flow through the open spaces in root cells and be transported to the shoot with minimal control by the plant. There is consequently rapid activation of many genes, in response to osmotic shock and damaged plasma membrane in root cells and to ionic stress in shoot cells (Shavrukov et al., 2012). High cellular NaCl concentrations are also supposed to increase formation of reactive oxygen species (ROS) (Hernandez and Almansa, 2002), which is considered as a primary event under a salt stress conditions (Noctor and Foyer, 1998). ROS could damage photosynthetic components, inactivate proteins and enzymes, and permeabilize membranes by causing lipid peroxidation (Price and Hendry, 1991; Meloni et al., 2003). Plants with high levels of anti-oxidants, either constitutive or induced, have been reported to have greater resistance to this oxidative damage (Shalata and Tal, 1998; Bor et al., 2003). Such correlation between anti-oxidant capacity and salt tolerance has also been demonstrated in a large number of plants, including salt-tolerant glycophytes and true halophytes (Broetto et al., 2002; Bor et al., 2003; Agarwal and Pandey, 2004; Ben Amor et al., 2005). Sea rocket or Cakile maritima (Brassicaceae) is an annual, succulent halophyte widely distributed in sandy coasts throughout the world (Clausing et al., 2000) and thus subjected to salt shocks. This plant displays potential for economical nutrient food (leaf comestible), for therapeutic utilization (Casal, 2004) and for it seeds contain up to 40% of oil (Ghars et al., 2005). Since at this time most of the studies on C. maritima were made using gradual salt application (Ben Amor et al., 2005; Debez et al., 2006; Ellouzi et al., 2014), we investigated the impact of repetitive salt shocks to mimic inundation with seawater and recorded changes in ionic and water status, growth parameters and some markers of oxidative stress in different organs