In the trial conducted from April to May 2022, the effect of adding a biostimulant to the leaves of lettuce plants with full and deficit irrigation on the quantity and quality of the yield was investigated. In the laboratory field of the Biotechnical Faculty of the University of Ljubljana, the pot experiment was set up in four treatments with six replicates each. The first factor investigated was irrigation to 100 % of the soil field capacity (100 % PK) and deficit irrigation (50 % PK). The second factor was treatment with a biostimulant (treated +B, untreated +0). The biostimulant Delfan Plus was applied to the plants four times at a concentration of 0.1 % during the growth period. The plants were harvested at the time of technological maturity and measurements were taken of above-ground plant mass, marketable plant mass, stem length, root weight, SPAD measurements and nitrate content. The mass of the above-ground part of the plant was not affected by the biostimulant treatment. The average aboveground mass of the plants was 403 ± 16 g at 100 % PK+B and 404 ± 25 g at % PK+0. With deficit irrigation, the average aboveground mass of the plants was 207 ± 12 g at 50 % PK+B and 194 ± 9 g at 50 % PK+0. The biostimulant application decreased the marketable mass of the lettuce plants in treatment with 100 % irrigation, which was 260 ± 11 g at 100 % PK+0 and 228 ± 16 g at 100 % PK+B. At plants with deficit irrigation, the biostimulant treatment increased the marketable mass of the plants, which was 132 ± 8 g at 50 % PK+0 and 151 ± 14 g at 50 % PK+B. Treatment with the biostimulant had no effect on lettuce rosette diameter, stem length and nitrate content, regardless of irrigation regime. The biostimulant had an effect on reducing root mass and increasing relative chlorophyll content, but only in plants with deficit irrigation.
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