3.2 Sodium and potassium sequestration in leaf following NaCl treatments
Plants of SV and SA were grown for 4 or 8 weeks before their exposure to a range of salt concentrations. Exposure of SVto NaCl concentrations higher than resulted in plant death before the end of the experiment; so higher NaCl concentration treatments were not used for SV . Exposure of SA to NaCl concentration up to did not result in considerable mortality. The concentration of Na+ in control leaf tissue was considerably higher inSA than in SV (Figure 3A-B). This difference disappeared after exposure to salt, due to a quick accumulation of Na+ in the leaf of SV . The accumulation of Na+ in leaves of SA was much lower at external NaCl concentrations between 0 and . Na+ accumulation increased sharply in SV leaves over the experiment (Figure 3A), whereas leaf Na+ content in SA increased less, even at higher external concentrations of NaCl (Figure ). The Na+ levels measured after 12 days NaCl treatment in Spartina exposed to 400 and NaCl was nearly similar to that of SVsubjected to only NaCl (Figure 3A-B). At NaCl, SV accumulated more NaCl in the leaf than SA under all salt concentration range (100-550 mM). This is owing to the exclusion of NaCl to the leaf surface for Spartina. This exclusion mechanism represents a second barrier ofSA defense against high NaCl concentrations besides the sequestration of salt in the vacuole. Earlier study performed on halophyte Aeluropus littoralis , a species that can tolerate up to NaCl, showed that an increase in leaf epidermal thickness was mainly due to an increase in cell size following salt accumulation (Barhoumi et al., 2007).
SA and SV differed also in their K+concentrations in the leaf. Herein, the concentration of K+ in leaf tissue of plants watered with salt-free medium was higher by about 30% in SA leaves (Figure 3C-D). Following salt treatment, the K+ content of the leaf in SV decreased considerably, especially after 4 and 8 days treatment at 100 mM NaCl. However, in SA , there was an initial increase in K+ with the increase in NaCl concentration; after 12 days of treatments, the K+concentration in the leaf gradually decreased with an increase in the NaCl concentration (Figure 3D). This also reflected by the ratio k+/Na+ (Figure 3E-F), where we observed a dramatic decline in this ratio for SV but very less and mostly maintain stable with time course in SA , especially at NaCl concentrations higher than 250 mM (Figure 3E-F).