## 4 Discussion
### Effects of the Type of Salt Stress on the Nutritional Characteristics  
In the current experiment, Na, Ca and K accumulation varied between EC-comparable stress types. Sodium, as expected, did increase in all salt-treated plants compared to the control, with plants growing with NaCl accumulating higher amounts of Na compared to seawater at comparable EC. Moreover, the accumulation of sodium in M-SA was so much higher than in M-SW as to be comparable to H-SW. Sodium accumulation in leaves represents one of most common responses to salinity in halophytes, especially for sodium-includer species such as _Tetragonia_[26; 35]. In these cases, plants accumulate sodium in their vacuoles to be used for osmotic adjustments [36]. The decrease in Na and K accumulation observed in lettuce is also in line with previous published studies [37]. Interestingly, in moderate-salinity conditions, the accumulation of K decreased in M-SA-treated plants only, whereas the effect of M-SW was comparable to the control. Similar results were obtained on different glycopytes too, i.e., on the common spinach (_Spinacia olerca_) [38]. Furthermore, in _Tetragonia_, the reduction in K concentration was stronger in NaCl-treated plants compared to that for seawater, and the accumulation of Ca in H-SW was the only one comparable to the control condition, whereas other treatments led to significant reductions. Such differences in plants treated with EC-comparable salt types might play a role in osmotic adjustment if efficiently compartmentalized at the cell level [39]. Since sodium could have a negative effect on human health, whereas K and Ca are among the main mineral elements lacking in the diet of over two-thirds of the world's population [40], seawater in the growing medium seems to provide better results than NaCl from a food quality standpoint. The concentration of phenolic compounds followed different patterns in salt-tolerant and salt-sensitive species, although no significant differences were observed between EC-comparable stresses. In _Tetragonia_ leaves, the concentration of phenolic compounds was always comparable to the control except for H-SA treatment, where the concentration significantly decreased overtime. In a previous experiment, we found that the phenolics concentration did significantly decrease with seawater irrigation at EC of 9.8 and 18 dS m\({}^{-1}\)[11]. This suggests that _Tetragonia_ may produce other specific chemical compounds (i.e., proline) to act as compatible solutes for osmotic adjustment, as shown in a previous study [26]. The decrease in phenolic compounds in response to salinity has been observed in other halophyte plants. For instance, a severe decrease in polyphenol concentration was observed in _Mesembryanthemum edule_ L. in response to increasing salinity [41]. However, it has been also shown that this response may vary also according to the species and within each species with the varieties. An interesting trial comparing two different _Cakile maritima_ Scop. accessions showed that the high salt-stress tolerance of one accession was concomitant with leaf enrichment in phenolic compounds, whereas the more-sensitive accession showed a decrease in these compounds, thereby suggesting that a difference in ability to accumulate polyphenols contributes to the difference in salt-tolerance of the two varieties [42]. This would also suggest that actually _Tetragonia_ should be considered less salt-tolerant than other halophytes. In lettuce, salinity stress led to an increase in phenolic concentration accordingly with increasing salinity: both moderate treatments resulted comparable to the control, whereas the high-salinity treatments assessed significantly higher concentration compared to control conditions. Similar results have been reported for several plants [43], including lettuce [44; 45], since phenolics are among the secondary metabolites whose concentration is modulated by plants as a function of osmotic regulation [46].