Degradation of autotoxic chemicals and maintainence of electrical conductivity recover growth, yield and nutrient absorption of successive lettuce in recycled hydroponics.

Abstract

In recycled hydroponics, successive crop cultivation by maintaining electrical conductivity (EC) suffers lower growth performance due to accumulating autotoxic root exudates. In this study, the efficiency of alternate current electro degradation (AC-ED) was evaluated for degrading allelochemicals and recovering retarded lettuce yield cultivated in EC-adjusted repeatedly used nutrient solutions. From benzoic acid (BA)-added nutrient solution, BA was completely degraded after 24 h by applying AC-ED at 551 and 940 Hz frequency with 50 and 80% electrical duty. In lettuce bioassay, fresh mass was negatively affected without the AC-ED-treated solution. Finally, lettuce seedlings were hydroponically grown in a plant factory using a half-strength Enshi nutrient solution. Culture solutions were unchanged in non-renewed solutions. Nutrient elements were supplied based on the EC (1.42 dS m^-1) of culture solutions. The fresh weight of lettuce was gradually decreased in subsequent cultures. Nutrient absorption rate was reduced in non-renewed solutions though enough of all nutrient elements were available in the solution. In the final culture, the highest shoot fresh weight (SFW) was recorded in the renewed (83.0 g plant^-1) solution which was similar to the AC-ED-treated solution (81.0 g plant^-1) and the lowest (58.0 g plant^-1) was in the non-renewed solution. By applying AC-ED, 40% lettuce yield was recovered in the EC-adjusted solution without renewing. Therefore, it is recommended that the continuous application of AC-ED with the capacity of 551 Hz and 50% duty would be applied for recovering the retarded lettuce yield cultivated with repeatedly used culture solutions in recycled hydroponics.