In a graduation study conducted from March to June 2022 on the laboratory field of the Biotechnical Faculty, the effect of foliar application of a biostimulant on the quantity and quality of lettuce (Lactuca sativa L.) yield was tested. Delfan Plus, a biostimulant based on a protein hydrolysate, was included in the trial. A rosette type of the lettuce cultivar 'Tourbillon' was used as the test plant. The effect of the biostimulant was determined by growing lettuce on a soil covered with black PE mulch and on an uncovered soil. In the trial there were 4 treatments: covered soil with black PE mulch, bare soil, foliar application of biostimulant (treated plants) and without biostimulant (untreated plants). We had three replicates for each treatment. The measurements on the plants were carried out at the time of technological maturity of the plants, on June 17, 2022, 57 days after planting the plants in the field. The mass of the whole plant, the mass of the marketable part of the plant, the plant height and the diameter of the rosette, the height of the stem were measured, and samples were prepared for the analysis of nitrate content. The results showed that foliar application of the biostimulant had no effect on plant biomass. Plant yields were higher on the covered soil (41 t/ha) than on the uncovered soil (29-33 t/ha), with no effect of biostimulant application. The morphological characteristics of the lettuce rosettes were also not affected by the biostimulant. We found a positive effect of the biostimulant treatment on the survival of the lettuce plants after transplanting. No treated plants died on the covered soil and 2.5% of the treated plants died on the uncovered soil. The mortality of the untreated plants was 7% on the covered soil and 8% on the uncovered soil. The nitrate content of the lettuce rosettes was not affected by the biostimulant treatment. The nitrate content of treated and untreated plants was below 150 mg NO3–/kg fresh weight.
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