https://scholars.wlu.ca/etd/2215

Hydrogen peroxide (H2O2) is an oxidizing agent used to disinfect irrigation systems (e.g. nutrient solution and equipment) used in controlled environment plant production and produces no harmful by-products. The action of H2O2 is non-selective, and therefor H2O2 will oxidize all components of the irrigation water as well as tissues from the greenhouse crops being irrigated. The phytotoxic effect H2O2 has on common greenhouse crops is not well understood, requiring detailed characterization and determination of threshold concentrations. Thresholds within the thesis were based on applied H2O2 concentrations which negatively affected the marketability of crops. A combination of visual damage, declining plant growth, and the life stage of the crop at harvest were used to establish these thresholds. Two distinct studies, each of which involved a unique irrigation method, were designed to establish H2O2 threshold concentrations. In the first study, eight concentrations of H2O2, ranging from 0 to 200 mg⸱L-1, were applied to three microgreen species (radish, arugula, sunflower) and three lettuce cultivars (‘othilie’, ‘rouxai’, ‘xandra’). Applications were made once per day from seed to harvest using a backpack sprayer under greenhouse conditions. Phytotoxic effects were limited to visible injuries on cotyledons or leaves, as growth was not affected by any treatment. Injury from H2O2 manifested in similar patterns on leaves for each crop, other than sunflower, which was not damaged by H2O­2. However, the amount of damage on the leaf surface, and the extent of damage throughout the crop, were unique to each affected species, or cultivar. Visible injury occurred with any applied H2O2, though marketability of affected crops declined only at H2O2 concentrations near to or greater than those recommended to control pathogens. The second study investigated how H2O2 in hydroponic solution would affect the growth of ‘Picowell’ cucumber, ‘Maxifort’ tomato, and ‘Little Gem’ lettuce. Eight H2O2 concentrations ranging from 0 to 400 mg⸱L-1 were applied four times to the nutrient solution of each crop over a 20 d period under greenhouse conditions. Phytotoxic effects from H2O2 were unique to each crop, requiring species specific criteria to establish threshold concentrations. Roots for all three crops exhibited visible injury from H2O2 applications, with cucumber roots being most susceptible. This dictated that the concentration threshold for ‘Picowell’ cucumbers must consider root injury in the context of whole plant growth. Tomato roots exhibited less severe visual damage symptoms than cucumber roots and all above ground growth metrics significantly declined at one specific H2O2 concentration. Lettuce threshold criteria were unique in that they considered only marketable traits (e.g. fresh weight, leaf area) as H2O2 phytotoxicity was established over a full production cycle (7 weeks). Similar to the foliar spray experiment, maximum threshold concentrations were at, or above, H2O2 product label recommendations for pathogen control.

https://scholars.wlu.ca/etd/2215