ECOLOGIA BALKANICA 2019, Vol. 11, Issue 1 June 2019 pp. 167-177 A Case Study of Allelopathic Effect of Parsley, Dill, Onion and Carrots on the Germination and Initial Development of Tomato Plants Ekaterina Valcheva 1 , Vladislav Popov 1 , Plamen Marinov-Serafimov 2* , Irena Golubinova 2 , Bogdan Nikolov 3 , Iliana Velcheva 3 , Slaveya Petrova 1,3 1 - Agricultural University, 12 Mendeleev Boul., 4000 Plovdiv, BULGARIA 2 - Institute of Forage Crops, 89 Gen. Vladimir Vazov Str., 5800 Pleven, BULGARIA 3 - University of Plovdiv “Paisii Hilendarski”, 24 Tzar Asen Str., 4000 Plovdiv, BULGARIA * Corresponding author: plserafimov@abv.bg Abstract. Vegetables are important worldwide, but their production faces many problems. One of them is the yield decline due to soil sickness, allelopathic effects of other crops, weeds and trees, and also the autotoxicity when grown continuously for several years. For the purpose of organic farming, allelopathy can be an important element in balancing the relationship between density and weeds, pests, diseases and cultivars. Relationships among the vegetable species and varieties in particular mixed stand are not sufficiently explored, which is a sufficient reason to conduct such a study. The present study focuses on the allelopathic relationships in agrophytocenoses in order to assess the possibilities of mixed crop cultivation of tomato plants with other vegetable crops and spices. Vegetable species may have a negative, neutral or positive effect one to another when grown in mixed agrophytocenoses. To explore the extent of this impact, test plants from tomatoes have been treated directly with plant extracts from other vegetable plants - parsley, dill, onion and carrots. The study showed that the applied concentrations of parsley, carrot, dill and onion extracts had stimulating, inhibiting or indigenous effect on tomato seed germination, growth and accumulation of dry biomass. Seedling vigor index of plant development (SVIcm) and biomass synthesis (SVIg) depended on the type of the extract applied more than the concentration applied (p<0.05). Seed germination was less affected than root and shoot growth in all species (p<0.05). Length of the seedlings was significantly influenced (positively or negatively) by the allelopathic plants and the effect was stronger with the increment of the extract concentration (p<0.05). Most pronounced negative effect was found at the 1% extract of fresh onion biomass – 34% reduction against the control (p<0.001). Stimulatory effect was strongest at the 1% extract of fresh carrot biomass – 37% increment against the control (p<0.001). Key words: allelopathy, mixed crops, organic farming, vegetables. Introduction Some plants have allelopathic potential by releasing allelochemicals to their surroundings that have either deleterious or beneficial effects on other plants in the vicinity. The term allelopathy includes both harmful and beneficial biochemical interactions between all types of plants but also with microorganisms, through the release of chemicals from plant parts by leaching, root exudation, volatilization, residue decomposition and other processes in both natural and agricultural systems (MOLISH, 1937). Allelopathy can affect many © Ecologia Balkanica http://eb.bio.uni-plovdiv.bg Union of Scientists in Bulgaria – Plovdiv University of Plovdiv Publishing House