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Mini-dissertation submitted in partial compliance with the requirements of the Master’s Degree in Technology: Homoeopathy, Durban University of Technology, 2012. / Lettuce is a cool season crop and requires an optimum growing temperature of 18°C. Lettuce is popular in summer as a salad ingredient but supply cannot always keep up with demand due to high spoilage rates, because of bolting and tip-burn as a result of heat stress while growing (Maboko &Du Plooy 2007). Heat stress reduces oxygen availability. This study sought to demonstrate that administration of homoeopathic Ozonum to hydroponic nutrient solution in a nutrient film technique design improves the growth ability and quality of lettuce during the summer season by increasing or improving its ability to absorb oxygen. The research was conducted at the Agricultural Research Council – Vegetable and Ornamental Plant Institute (ARC-VOPI) facility at Roodeplaat (near Pretoria), South Africa. The trial was conducted in a 40% shade net structure in the summer months of February to April 2012. Purpose The general aim was to measure the effect of homoeopathic Ozonum 6CH, Ozonum 15CH and Ozonum 30CH compared to a placebo control (96% alcohol only) on the growth and yield of leafy lettuce, Lollo Bionda Lobi cultivar grown in a closed hydroponic system in South African summer climatic conditions. Objectives:  Determine the increase in growth and yield of leafy lettuce as affected by Ozonum concentration in terms of: (i) leaf fresh mass; (ii) leaf dry mass; (iii) number of leaves; (iv) leaf area; (v) chlorophyll content; (vi) root fresh mass; (vii) root dry mass;  Determine the impact of heat stress in terms of bolting and tip-burn;  Determine the differences in specific plant leaf mineral content between the experimental groups and control group; and,  Determine the differences between dissolved oxygen (DO) concentrations in the nutrient solution of the verum group and control group. Methodology This was a placebo controlled randomized plant experiment. A total of 384 plants were laid out in a randomized complete block design with 4 treatments (Ozonum 6CH, Ozonum 15CH Ozonum 30CH and a control) and replicated 4 times. The total number of plants per plot was 24 and the middle 8 plants were selected as data plants while other plants were regarded as border plants. Thus, data was gathered from 96 plants per treatment in total. Leafy lettuce seeds of cultivar Lollo Bionda Lobi, were sown in polystyrene trays. The seedlings were transplanted 28 days after sowing into a gravel-film technique hydroponic system. Ozonum and the placebo was added to the nutrient solution on the first day and every 6 days thereafter, at the same time as the nutrient solution was changed. Various daily measurements were recorded. Plants were harvested 27 days after transplanting when final measurements of nutrient content and growth were made. Data Analysis Data was collected over time as repeated measurements; the time factor was included as a sub-plot factor in the analysis of variance (ANOVA). All data items collected were subjected to an appropriate analysis of variance using the GenStat statistical system (VSN International, 2011). The residuals were examined for deviations from normality and outliers causing skewness were removed. Fisher’s protected t-LSD (Least Significant Difference) was calculated to compare treatment means of significant effects (Snedecor & Cochran, 1980). Results The results for (i) lettuce yield criteria, (ii) DO concentration, (iii) leaf chlorophyll content and (iv) specific mineral content of lettuce leaves (Appendix B) indicate that there was no significant difference between Ozonum treatments and the control. The incidence of bolting was not observed in lettuce plants among the treatment or control groups. Results showed a reduced tendency in tip-burn percentage for treatment Ozonum 30CH although not significantly different to Ozonum 15CH or the control. Conclusion The conclusion derived from this study is that homoeopathic Ozonum was not effective at improving dissolved oxygen levels in the nutrient solution, or in stimulating growth in lettuce under South African summer climatic conditions (February to April). Further research is needed to explore the effect Ozonum 30CH may have on tip-burn.