## 4 Discussion
### Planting Density  
Despite promoting lower individual growth, high-density treatments L220 and F220 produced higher yields at the level of the hydroponic unit. Doubling planting density from 110 to 220 plants m\({}^{-2}\) increased hydroponic unit productivity 52-57% for an average value of approximately 54-57 g FW m\({}^{-2}\) day\({}^{-1}\). Higher productivity values were observed by [PERSON] et al. [16], who reported average yields of 63-73 g FW m\({}^{-2}\) day\({}^{-1}\) under identical planting densities but by using different combinations of N and P concentrations. On the other hand, [PERSON] et al. [42] reported lower productivity values of 33 g FW m\({}^{-2}\) day\({}^{-1}\) using very different experimental conditions. These first studies with _H. portulacoides_ (Table 5) already suggest contrasting outcomes, most likely influenced by different culture conditions, genetic variability and the use of different ecotypes [64]. In other halophyte species, planting density seemed to have had little effect on the hydroponic performance (Table 5). For instance, [PERSON] et al. [41] observed that _S. europea_ displayed no difference in total productivity between 200 and 10,000 plants m\({}^{-2}\). This lack of a plant density effect could be ascribed to the high morphological plasticity of Salicornia plants [41; 65]. Still, some species of Salicornia seem to be more productive than others under similar nutrient conditions. For instance, in the same study presented above, _S. europea_ (200-10,000 plants m\({}^{-2}\)) produced between 105 and 124 g m\({}^{-2}\) day\({}^{-1}\) of harvestable biomass, meanwhile _Salicornia persica_ [PERSON] (100 plants m\({}^{-2}\)) [66], _Salicornia bigelovii_ [PERSON]. (260 plants m\({}^{-2}\)) [67] and _Salicornia dolichostachya_ [PERSON]. (38 plants m\({}^{-2}\)) [42; 68] produced on average between 50 and 70 g m\({}^{-2}\) day\({}^{-1}\). Salicornia species have annual life cycles, contrary to a perennial plant like _H. portulacoides_, which indicates major differences in life histories and growth strategies [69]. As such, comparisons between very different species are only meaningful from a horticultural perspective, where some species can be considered better than others in productivity and nutrient extraction rates, for instance. Other edible perennial halophytes investigated for their productivity under different plant densities (Table 5) were _S. portulacastrum_ and _B. maritima_ (92 vs. 184 plants m\({}^{-2}\)), which displayed no differences between density levels [39]. _Sarocorria ambigua_ (Michx.) M.A.Alonso & M.B.Crespo, a perennial Salicornioideae, displayed productivity values of 110 g m\({}^{-2}\) day\({}^{-1}\) when planted at a density of 100 plants m\({}^{-2}\)[70]. Yet, much lower values have been reported for the same species (11 g m\({}^{-2}\) day\({}^{-1}\)) at densities of 40 plants m\({}^{-2}\) under different aquaponic conditions [71]. Increasing planting density also affected DIN and DIP extraction efficiencies, as high-density units extracted more in total, with average extraction efficiencies of 28% DIN and 36% DIP. Under similar nutrient conditions and using the same fluorescent lighting, high-density and retention time, [PERSON] et al. [16] reported extraction efficiencies of 35% DIN and 32% DIP, values very close to the present study. However, using radically different hydroponic conditions can produce very different outcomes as shown in previous studies where _H. portulacoides_ displayed extraction efficiencies of 50% DIN and 45% DIP [42] and 65% DIN and 0% DIP [20]. This stresses the fact that culture parameters must be fine-tuned for the specific conditions of a hydroponic or IMTA system, based on the nutrient availability and nutrient extraction rates. Interestingly, [PERSON] et al. [39] observed an improvement in DIN extraction efficiency by _B. maritima_ when planted at a low density compared with high-density units; meanwhile, [PERSON] et al. [41] reported no changes in extraction efficiencies after a 50-fold increase in _S.