A COMPARISON BETWEEN HYDROPONIC AND AQUAPONIC GROWN SALANOVA LETTUCE INFECTED BY Pythium sp.

Introduction Soil-independent, closed environment agriculture (CEA) has potential to meet challenges in food production, such as climate change and soil deterioration by assuring yield, improving water and fertiliser efficiency, decreasing the incidence of pathogens and pests, among others. The use of recovered or recycled fertilisers instead of chemically pure mineral fertilizers would further contribute towards increasing the sustainability of vegetable production. By combining hydroponics (HP) with aquaculture, nutrient-rich aquaculture effluent is recirculated, thereby diminishing or removing the need for inorganic fertilisers. As additional benefit, cultivation in aquaponics seems to provide competitive presence of beneficial microorganisms that contribute towards mitigation of root rot infections1–3. However, detailed studies of this are rare. Pythium spp. are root pathogens that often cause root rot in soil-less grown vegetables. To test the hypothesis that aquaculture effluent contributes toward fighting against Pythium spp. infection, lettuce (Lactuca sativa, variety Salanova) was cultivated in either conventional HP or in the effluent of a commercial aquaculture farm before being inoculated with the pathogen. Materials and Methods Twenty-four lettuces were grown in rafts in a climate chamber (23-15 °C day/night, 65% relative humidity, 16-8h day/night, 0.12% CO2) and underwent two treatments in duplicate boxes of 6 plants each: (i) HP solution of demineralised water with added nutrients and NaCl to mimic the composition of the aquaculture effluent; (ii) aquaculture effluent from a commercial RAS rearing pike perch at 37 kg/m3 (AP). AP contained 782 mg/L Na+ and 830 mg/L Cl- (4.2 mS/cm) as the farm used NaCl to prevent disease in fish. K and Fe were added in AP as KOH and Fe EDTA, respectively, to meet lettuce requirements. pH at the end of the trial was 7.95 and 7.88 in HP and AP, respectively. On day 29 after seeding, 0.25 g Pythium sp.-infected millet was inoculated onto each rockwool cube of one replicate. After 60 days from seeding, chlorophyll, flavonoid, and nitrogen levels were estimated with Dualex® (ForceA, France)4. On day 61, the lettuces were harvested, shoots and roots were weighed separately and thereafter dried at 60 °C before determining dry matter. The elemental composition was analysed by XRF spectrometry (SPECTRO analytical instruments GmbH, Germany). R software5 was used to perform a two-way ANOVA with factors system (S; levels: HP, AP) and infection (I; levels: no, yes), followed by a Tukey's test. Results and Discussion All parameters, except shoot dry weight, root dry weight, and nitrogen balance index, differed between HP and AP systems (p<0.05 or smaller; Table 1). Plant growth in the AP system was lower than in the HP system, as found elsewhere6. There were no visible signs of infection on lettuces from either system. AP plants undergoing Pythium infection did not show differences from the non-infected ones, while the percentage of shoot dry weight on fresh weight in infected HP-grown lettuce was significantly higher. Infecting the plants also led to a not significant increase in the levels of flavonoids (stress response) and chlorophyll (possibly related to nitrogen status7), irrespective of the system. However, the nitrogen balance index was not significantly different, indicating that the nitrogen status of leaves was not sharply affected. Na, Ca, Mg, P, Cl, Mn, Cu and Zn contents were significantly higher in shoots of HP-grown plants, while there were no differences in K, Al, and Si; Fe and Ni were higher in the shoots of AP-grown plants (Table 2). The levels of Fe in shoot biomass seemed to be lower in the infected plants. The lack of replicates did not allow to detect statistical differences between the different treatments in the element concentrations in root samples. Conclusions The study showed that HP and AP nutrient solutions containing similar values of pH, nutrients and salinity could both sustain Salanova lettuce growth. Infection with Pythium sp. did not compromise the lettuce growth; however, the beneficial effect could not be confirmed. As there were no visible signs of infection on lettuces from either system, inocula containing more active spores should be tested to delve into the capacity of AP system to provide beneficial bacteria against pathogens. References 1. Somerville, C., Cohen, M., Pantanella, E., Stankus, A. & Lovatelli, A. Small-scale aquaponic food production - Integrated fish and plant farming. FAO Food and Agriculture Organization of the United Nations (2014). 2. Sirakov, I. et al. Potential for combined biocontrol activity against fungal fish and plant pathogens by bacterial isolates from a model aquaponic system. Water 8, 518 (2016). 3. Sanchez, F. A., Vivian‐Rogers, V. R. & Urakawa, H. Tilapia recirculating aquaculture systems as a source of plant growth promoting bacteria. Aquac. Res. 50, 2054–2065 (2019). 4. Cerovic, Z. G. et al. Nondestructive Diagnostic Test for Nitrogen Nutrition of Grapevine (Vitis vinifera L.) Based on Dualex Leaf-Clip Measurements in the Field. J. Agric. Food Chem. 63, 3669–3680 (2015). 5. R Core Team. R: A language and environment for statistical computing. R foundation for statistical computing, Vienna, Austria (2018). Available at: https://www.r-project.org/. 6. Yang, T. & Kim, H.-J. Characterizing Nutrient Composition and Concentration in Tomato-, Basil-, and Lettuce-Based Aquaponic and Hydroponic Systems. Water 12, 1259 (2020). 7. Schmautz, Z. et al. Tomato Productivity and Quality in Aquaponics: Comparison of Three Hydroponic Methods. Water 8, 533 (2016).