## 5 Conclusions  
The hypothesis that, due to its different mineral composition, seawater would actually result in a less stressful growing medium compared to a NaCl solution at comparable EC was supported by our results. However, remarkable differences between the salt-tolerant and the salt-sensitive investigated species were observed. The most promising results were those related to growth: while for _Tetragonia_ no differences were observed between the two stresses at comparable EC, in the salt-sensitive lettuce H-SW treatment led to a higher biomass compared to its NaCl corresponding H-SA for FW, DW and yield. Additionally, water relations highlighted differences between the tested stress, with _Tetragonia_ decreasing water consumption in H-SA-treated plants compared to H-SW-treated plants, instead in line with control conditions. Such differences between H-SW and H-SA collaborate to indicate a major stress administered through the NaCl solution compared to seawater, even if the biomass produced with the two treatments was in the end comparable. Moreover, in lettuce, the WUE was significantly reduced in plants treated with H-SA only compared to the control, with H-SA also being the treatment associated with the lower WP, decreasing significantly not only compared to the control but also to the EC-equivalent H-SW treatment. Similarly, in _Tetragonia_, the H-SW-treated plants' WP was comparable to that of the control, whilst the H-SA-treated plants' WP was significantly reduced, confirming at high salinity the major severity of the NaCl stress compared to seawater. The use of seawater on lettuce increased the concentration of phenolic compounds, which are associated with an increase of biofortification effect in the edible parts. The results presented here confirm the need of a more complete salt-tolerance dataset for cultivated horticultural crops, since data on salinity thresholds are often obtained through screenings that do not reflect the complex salinity conditions of salt-affected soils: the highlighted differences in the salt response to salt stresses types in fact suggest a need for further studies investigating the specific salt conditions actually present in salt-affected soils.  
Conceptualization, W.G.N. and G.A.; data curation, W.G.N. and G.A.; formal analysis, W.G.N. and G.A.; funding acquisition, E.M., C.P. and S.M.; methodology, G.A.; project administration, E.M., C.P. and S.M.; resources, E.M.; supervision, E.M., C.P. and S.M.; visualization, W.G.N. and G.A.; writing--original draft, G.A.; writing--review and editing, W.G.N. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the POR FSE 2014-2020 Program of the Regione Toscana with the Jellyfish Barge: una serra galleggiante autosufficiente per cultivare il mare Project (UNIFI_FSE2017) and supported by Fondazione CR Firenze (project MEDUSA--Jellyfish Barge) and by PNAT S.R.L.  
**Data Availability Statement**: Not applicable.  
**Acknowledgments:** The authors gratefully acknowledge the LINV and PNAT S.R.L. for inspiring and supporting this research.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.