The possibility of using of an aqueous extract from stevia (Stevia rebaudiana Bertoni) leaves as a biostimulant of plant growth in photoculture

Relevance. Plant growth biostimulants based on natural raw materials are ecologically safe and harmless to humans. Stevia (Stevia rebaudiana Bertoni) contains in its leaves sweet diterpene steviol glycosides, which are similar in structure and properties to gibberellins, as well as a number of other valuable biologically active compounds. Water extracts from stevia leaves are capable to accelerate growth, to increase the yield and quality of plants. However, further study and development of application technologies of the extracts for specific crops and different growing conditions are required. The purpouse of this work is to investigate the effect of an aqueous extract from stevia leaves on the yield and quality of lettuce and small radish under photoculture conditions. Materials and methods. The objects of the study were lettuce var. Typhoon and small radish var. Riesenbutter. The plants were grown in photoculture at the biopolygon of the FSBSI Agrophysical Research Institute (St. Petersburg) at an illumination of 10–20 klx, a temperature of 22–26°С (day) and 18–20°С (night). Watering was carried out with water, top-dressing was carried out with Knop's nutrient solution three times a week. Peat with mineral additives was used as a substrate. The original extract was prepared by extraction from stevia dry leaves powder with hot water (80°С) in a ratio of 1 g of leaves: 100 ml of water. Foliar treatment of plants was carried out twice at dilution of the original extract from stevia leaves 1:10, 1:50, and 1:100 (extract : water) at the rate of 0.20-0.25 ml / lettuce plant and 0.09-0.1 ml / radish plant; control plants were treated with water. The data were processed according to standard procedures using the Excel 2016 software. Results. Foliar treatment with an extract from stevia leaves did not significantly affect the size and weight of radish root crops, but increased the yield due to a decrease in the number of bolting plants. A significant decrease of bolting was observed in all treatment options (by 34-51%). When using a dilution of 1: 100, the yield of marketable root crops increased by 20% and the yield per sq. m. – by 25%. Foliar treatment with stevia extracts increased the mass of lettuce plants, as well as their biological value. The content of photosynthetic pigments, sugars, and vitamin C significantly increased, and a tendency towards a decrease in the content of nitrates was observed. According to data, we recommend for foliar treatment are dilutions of the original extract 1:50 and 1: 100. The effect of using of an extract from stevia leaves is similar in action to some biostimulants from plants with hormone-like properties, which determines the possibility of using it as a safe plant growth stimulant.