PROMOTING GROWTH OF LETTUCE PLANTS (Lactuca sativa L.) WITH SUBLETHAL OZONE DOSES APPLIED TO CULTURE MEDIUM

Abstract

Plants have developed a set of morphological, biochemical and physiological mechanisms in response to O₃ environmental changes. This study evaluated the effect of sub-lethal, O₃ doses in the growth media on lettuce (Lactuca sativa L.) plants. Based on the phenomenon called hormesis, we postulate that there is a threshold dose of O₃ that positively modifies plant metabolism that increases growth and productivity. Ozone doses between 0.53 and 59.40 mg L^-1 applied weekly were evaluated in plants in a hydroponic float system and compared to a control without O₃. Measured variables were root length, stem diameter, stem height, leaf number, leaf length, wet and dry root biomass, and wet and dry shoot biomass week every week. Plants completed their development with concentrations between 0.53 and 5.94 mg O₃ L^-1 without any growth damage. O₃ doses of 20, 40 and 60 mg L^-1 were lethal to five weeks-old plants because they caused a rapid leaf tissue darkening and necrosis after 20 min of a single application. In contrast, 2.66 and 3.96 mg L^-1 significantly (P ≤ 0.05) increased stem diameter, wet root and shoot biomass and dry root and shoot biomass gaining 16, 23, 15, 89 and 11%, compared to the control, after 10 weeks of growth.