Leaf production and quality of sea beet (Beta vulgaris subsp. maritima) grown with saline drainage water from recirculating hydroponic or aquaculture systems

The application of greenhouse soilless culture (or hydroponics) and recirculating aquaculture system (RAS) is rapidly growing worldwide as these technologies provide controlled growing conditions for crop plants and aquatic organisms, thus enhancing productivity. The wastewater from RAS and hydroponics is generally rich in many essential plant nutrients and could be reused for crop irrigation, thus reducing the costs for both wastewater treatment and fertilizers. Many wild edible plant species are salt-tolerant glycophytes or halophytes and hence are suitable for cultivation with saline wastewater in cascade cropping systems or decoupled aquaponic systems. The goal of this work was to investigate the effects of drainage water from semi-closed substrate plant culture or saltwater RAS on leaf production and quality of sea beet plants (Beta vulgaris subsp. maritima) grown hydroponically in a greenhouse. Two experiments were conducted in autumn with plants cultivated in a floating raft system to compare five different nutrient solutions: standard nutrient solution (CNS, control; EC 2.80 dS m−1, Na 0.7 mM); the effluent from a semi-closed substrate culture of tomato used as such (tomato effluent 100%, TE100; EC 6.49 dS m−1, Na 34.9 mM) or diluted (50:50) with CNS (tomato effluent 50%, TE50; EC 4.50 dS m−1, Na 17.8 mM); the effluent from a saltwater RAS with gilthead sea bream, used as such (aquaculture effluent 100%, AE100; EC 42.00 dS m−1, Na 408.6 mM) or diluted (50:50) with CNS (aquaculture effluent 50%, AE50; EC 25.40 dS m−1, Na 204.6 mM). In both experiments, leaf production was significantly reduced in plants grown with AE50 (−46.8%, on average) and AE100 (−70.4%, on average) compared to CNS; on the contrary, no or minor differences were found between CNS, TE50 and TE100 plants. The reduction of crop yield was due to the higher salinity and not to abnormal concentration of some mineral nutrients in AE. In the first experiment, the use of TE and AE also resulted in higher leaf antioxidant capacity and concentration (both expressed on a fresh weight basis) of total chlorophylls, carotenoids, flavonoids, and phenols. In both experiments, leaf concentration of Na and oxalate (both total and soluble) significantly increased with the salinity of the nutrient solution. Due to the less favourable light conditions, leaf nitrate concentration was much higher in the second experiment than in the first one, regardless of the nutrient solution. In conclusion, sea beet could be grown using hydroponic wastewater with moderate salinity with no or minor effect on leaf production and quality. In contrast, the use of highly saline aquaculture effluents markedly reduced crop yield and negatively affected leaf quality due to increased concentration of sodium, oxalate, and nitrate. In general, sea beet leaves were high in oxalate and their consumption should be limited.