Proceedings of the X International Scientific Agricultural Symposium “Agrosym 2019” 670 ALLELOPATIC EFFECT OF FIVE WEED SPECIES ON SEED GERMINATION OF SORGHUM CROPS Bogdan NIKOLOV 1* , Slaveya PETROVA 1,2 , Ekaterina VALCHEVA 2 , Irena GOLUBINOVA 3 , Plamen MARINOV-SERAFIMOV 3 1 Plovdiv University “Paisii Hilendarski”, Plovdiv, Bulgaria 2 Agricultural University, Plovdiv, Bulgaria 3 Instute of Forage Crops, Pleven, Bulgaria *Corresponding author: nikolov81bg@yahoo.com Abstract Aim of this study was to evaluate the allelopathic effect of five invasive weeds (Sorghum halepense L. Pers., Sonchus arvensis L., Cirsium arvense L. Scop., Xanthium strumarium L.and Aristolochia clematitis L.) in forage crops on two Sorghum crops (Sorghum sudanense (Piper) Stapf and Sorghum vulgare var. technicum (Körn.)). An ex-situ experiment was carried out as follows: 10 seeds of each test variety were placed in Petri dishes between filter paper and the dried weed biomass extract was added at concentrations of 0.1, 0.2, 0.4, 0.8, 1.6 and 3.2% w/v. Petri dishes were placed in a thermostat-operated device at a temperature of 22 ± 2°С for a seven-days period. Distilled water was used as a control. Number of germinated seeds, % of seed germination against the control, and weed inhibition rate (IR) were measured. Our study revealed that the alellopathic effect of the tested five invasive weeds on seed germination of S. sudanense and S. vulgare var. technicum varied according to the plant species (both weed and cultural) and the concentration applied. Most pronounced negative effect on the germination process of S. sudanense seeds have expressed the cold aqueous extracts from Sonchus arvensis L. and Cirsium arvense (L.) Scop. – IR varied from 4.67-5.0% up to 70.3% at the highest test concentrations (p<0.05). Maximal inhibition of seed germination of S. vulgare var. technicum in comparison with the control was found at 3.2% w/v extract of Sorghum halepense (L.) Pers. (IR=58.4%), followed by the two highest concentrations of Xanthium strumarium L. extract (IR=41.6-45.9%). Keywords: interaction, allelopathy, seed germination, inhibition rate Introduction Sorghum, originated in East Africa, is one of the first crops, which was taken into cultivation (House, 1985). At present, Sorghum sudanense hybrids are preferred as a fodder due to higher yields and tillering ratios and also their thin stems and higher leaf ratios (Uzun et al., 2009). Sorghum vulgare var. technicum is mainly utilized for producing brooms, washing brushes, knittings, paper, wallboard, fences, biodegradable materials for packaging due to their peculiar resistance (Popescu and Condei, 2014). Sorghum cultivar plants are more tolerance to drought and high temperatures, diseases, pests, various soil types and show higher water use efficiency and, higher production capacity per unit area (Undersander and Lane, 2003; Uzun and Cigdem, 2005). Main disadvantage of Sorghum crops is their high sensitivity against weed infestation in the first 30-40 days after sowing (Marinov-Serafimov and Golubinova, 2015). Drastic effects of weeds on germination and growth on Sorghum and other crops are due to their competiveness with cultivated crops for resources and allelopathic potentials. Through allelopathy, weeds may cause significant effects on the growth and germination capacity of other crops. Weeds may possess diverse allelochemicals, which can interfere with other plants in a number of ways by either retarding or enhancing the germination and growth of receiving plants (Oerke, 2006; Marinov-Serafimov and Golubinova, 2015).