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

In recycled hydroponics, successive crop cultivation by maintaining electrical conductivity (EC) suffers lower growth performance due to accumulating autotoxic root exudates. In this study, 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 hrs. by applying AC-ED at 551 and 940 Hz frequency with 50 and 80% electrical duty. In lettuce bioassay, fresh mass was negatively affected in without AC-ED treated solution. Finally, lettuce seedlings were hydroponically grown in plant factory using half-strength Enshi nutrient solution. Culture solutions were unchanged in non-renewed. Nutrient elements were supplied based on the EC (1.42 dSm) of culture solutions. Lettuce fresh weight 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 final culture, the highest shoot fresh weight (SFW) was recorded in renewed (83.0 g plant) solution which was similar to AC-ED treated solution (81.0 g plant) and the lowest (58.0 g plant) was in non-renewed. By applying AC-ED, 40% lettuce yield was recovered in EC adjusted solution without renewing. Therefore, it is recommended that continuous application of AC-ED with the capacity 551 Hz and 50% duty would be applied for recovering the retarded lettuce yield cultivated with repeatedly used culture solutions in recycled hydroponics.