2019 ASHS Annual Conference

Hydrogen peroxide (H₂O₂) is a common oxidizing agent used to disinfect recirculated irrigation water during the production of crops under controlled environmental systems (e.g. greenhouse). Despite this, limited research is available regarding leaf damage caused by foliar spray applications of H₂O_2, or the phytotoxic effects exhibited by plants grown in nutrient solutions containing H₂O₂. To characterize phytotoxic effects and define a concentration threshold for H₂O₂, three microgreen species [arugula (Brassica eruca sativa), radish (Raphanus sativus) and sunflower (Helianthus annuus ‘Black Oil’)], and three lettuce (Lactuca sativa) cultivars: ‘Othilie’, ‘Xandra’, and ‘Rouxai’, were foliar sprayed once daily with water containing: 0, 25, 50, 75, 100, 125, 150, or 200 mg⸱L^-1 of H₂O₂, from seed to harvest under greenhouse conditions. Leaf damage was assessed at harvest using two distinct methods; i) the percentage of damaged leaves per tray and ii) a damage index (DI). Applied H₂O₂ concentrations, starting from 25 mg⸱L^-1, increased the percentage of damaged leaves in every species except ‘Black Oil’ sunflower, which remained unaffected at any applied concentration. Symptoms of leaf damage, characterized by the DI, manifested in similar patterns on the surface of microgreen cotyledons and lettuce leaves. Establishing a concentration threshold for foliar spray with H₂O₂ required consideration of both metrics, as mean DI values and the percentage of damaged leaves were unique to each crop. Distinctive patterns in leaf damage, at higher H₂O₂ concentrations, allowed for rapid diagnosis of excessive H₂O₂ application. Fresh weights, dry weights and leaf areas, for all crops, were not significantly affected by daily H₂O₂ spray. Based on the aforementioned metrics, maximum recommended concentrations were: 150 mg⸱L^-1 (radish), 100 mg⸱L^-1 (arugula) for microgreens and 125 mg⸱L^-1 (‘Othilie’), 75 mg⸱L^-1 (‘Rouxai’) and 125 mg⸱L^-1 (‘Xandra’) lettuce. To evaluate the phytotoxic effects of H₂O₂ in solution culture, one cultivar each of Lettuce (Lactuca sativa), Cucumber (Cucumis sativus), and Tomato (Solanum lycopersicum) are growing in one type of nutrient solution, augmented with eight concentrations of H₂O₂: 0, 25, 50, 75, 100, 200, 300, or 400 mg⸱L^-1. Crops, grown under greenhouse conditions, are constantly exposed to H₂O₂ for a period of 30 days from Feb. to Mar. 2019. Phytotoxic H₂O₂ concentrations are determined using parameters quantifying root quality, plant mortality, and the growth of both roots, and above ground biomass, at each treatment concentration. Analyzed parameter data will be used to establish an upper threshold concentration at which H₂O₂ can be applied in solution culture.