The Effect of Nutrient Source and Beneficial Bacteria on Growth of <i>Pythium</i>-Exposed Lettuce at High Salt Stress

High salinity, nutrient imbalance, and pathogens are some of the challenges of closed soilless cultivation systems, e.g., those combining hydroponics (HP) with aquaculture effluents (AE). Plant growth-promoting microorganisms (PGPM) can support plants to cope with stressing agents. To address these topics, lettuces were grown in soilless systems (20 boxes) at an electrical conductivity of around 4.2–5 mS/cm, following a full factorial design with two nutrient sources and five bacterial treatments. The nutrient sources were either organic (AE) or inorganic (HP); the treatments were either commercial PGPM or sludges of an aquaculture farm or of an urban wastewater treatment plant. Finally, half the plants were exposed to pathogen <i>Pythium</i> sp. After 61 days of culture, most of the differences between HP- and AE-plants could be attributed to the composition of the nutrient solutions. Nutrient imbalances, salinity, and the pathogen exposition did not cause severe damage, except for tip burn. Fresh weight was significantly higher in HP (177.8 g) than in AE (107.0 g), while the chlorophyll and flavonoid levels tended to be higher in AE. The leaf sodium and chlorine concentrations were higher than the values found in similar studies; however, AE plants contained a lower content of sodium and chlorine (35.0 and 21.5 mg/g dry weight) than the HP ones (44.6 and 28.6 mg/g dry weight). Many macro- and micronutrients in the AE-grown plants tended to be higher when the commercial PGPM or the sludges were administered, supporting the idea that those treatments contain a flora that helps to extract nutrients from organic sources. The study demonstrated that lettuce can be successfully cultured at relatively high salt concentration. To further investigate beneficial services such as nutrient extraction, salinity mitigation, and pathogen protection, we suggest administering bacterial communities of known composition, or single microbial strains. The study also showed that PGPM can be found in sludges of different origins; isolating beneficial strains from sludge would additionally transform its management from a burdensome cost to a source of beneficial services.