Mitigation of benzoic acid-driven autotoxicity in waste nutrient solution using O and O/HO treatments: Seed germination and root growth of Lactuca sativa L.

Benzoic acid (BA), a secondary metabolite released through root exudates, is considered to be the most common inhibitor that leads to plant autotoxicity, even at low concentrations in closed hydroponic systems. In this study, to mitigate BA-driven autotoxicity, the effects of O and O/HO oxidation treatment (O concentration: 1, 2, 4, 8 mg L, HO concentration: 4, 8 mg L) on waste nutrient solution (WNS) were investigated in terms of BA degradation, the rate of germination inhibition (GI), and the rate of root growth inhibition (RI). In the case of O treatment, the BA degradation rate improved up to 14.1% as the O concentration increased, while alleviation of GI was insignificant (94.6-100%), confirming that a single O treatment was unsuitable for mitigating autotoxicity. On the other hand, O/HO treatment increased BA degradation by up to 24.8%, thereby significantly reducing GI (up to 7.69%) and RI (up to 0.88%). Both the highest BA mineralization rate and phytotoxicity mitigation was observed at BA (4-4) (BA mineralization: 16.7%, GI: 12.82%, RI: 11.69%) and BA (1-8) (BA mineralization: 17.7%, GI: 7.69%, RI: 0.88%) at each HO concentration. In addition, the operating costs were evaluated by a chemical and electricity cost analysis at the different treatments. As a result, the operating costs of BA (4-4) and BA (1-8) were calculated to be 0.40 and 0.42 $ L mg of mineralized BA, respectively. After consideration of the mineralization rate, autotoxicity mitigation, and operating cost, BA (1-8) was suggested for the optimal treatment condition and our findings would contribute to the alleviation of BA-driven autotoxicity.