In the experiment, which was conducted from May to September 2023 in a greenhouse in the laboratory field of the Biotechnical Faculty in Ljubljana, the effect of grafting and treatment with biostimulants on the mitigation of mineral nutrient deficiency in tomatoes was investigated. The experiment included grafted and ungrafted plants of the tomato variety ‘Rally F1’, which were grafted onto two rootstocks, the varieties 'Vigoterra' (R×V) and 'Optifort' (R×O). The ungrafted plants of the variety ‘Rally F1’ represented the control plants (R). For the hydroponic experiment, the plants were planted in pots filled with perlite and placed on tubs filled with the Hoagland-Arnon nutrient solution. The experiment was set up in 3 or 4 replicates. The treatments with different concentrations of nutrient solution were: 100% nutrient concentration (100 %+0), reduced nutrient concentration in the nutrient solution (60 %+0) and reduced nutrient concentration in the nutrient solution + biostimulant treatment (60 %+B). The plants were treated once a week with a biostimulant (foliar application) based on hydrolyzed proteins (Delfan Plus) at a concentration of 0.1%. For treatments R, R×V and R×O, we have two plants per tub. In total, there were 10 tubs of nutrient solutions. The technically ripe fruits were harvested from Aug 7 to Sept 8 2023. Total there were 5 harvests. In addition to counting and weighing the marketable and non-marketable fruits, six samples per treatment were taken to measure the morphological characteristics and colour of the fruit skin as well as the total soluble solids content. Despite the reduced amount of nutrients in the nutrient solution, the plants had vigorous growth, dark green leaves and no symptoms of nutrient deficiency. During the experimental period, the air temperature and relative humidity were extremely high, resulting in massive cracking of the tomato fruit skin. As a result, the percentage of unmarketable fruit was extremely high, 70-% for R×O and 95-% for R. The reduction of nutrients in the nutrient solution contributed to lower yields in both grafted and ungrafted plants. Treatment with biostimulants had no effect on reducing the amount of unmarketable fruit in either grafted or ungrafted plants. The reduction of nutrients in the nutrient solution affected the morphological characteristics of the fruits in the ungrafted plants, which had a higher fruit mass, mainly due to the lower number of fruits per plant. The reduced nutrient solution resulted in less coloured fruits with an a*/b* index of 0.7 for R×V and 0.96 for the non-grafted plants. Treatment with biostimulant had no significant effect on fruit colour.
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