Invasive species (weeds) contribute to great losses in crop productivity, and one of the strategies for controlling their distribution in the field involves the use of herbicides. However, the development of new formulations for the control of weeds is challenged by environmental issues, increases in the resistance of weeds to herbicides, and poor selectivity of herbicides towards invasive species. Here, by using pre-emergence experiments, we assessed the phytotoxicity of two (thio)urea analogues (2A10 and 2B2) against the weed species Bidens pilosa (a dicot), Urochloa brizantha and Urochloa decumbens (monocots). Similar to diuron (400 µM), which is a commercial urea analogue herbicide, the urea analogue 2A10 (>200 µM) was lethal to B. pilosa. Although 2A10 failed to disrupt the germination of U. brizantha seeds, this compound (≥600 µM) inhibited the accumulation of chlorophyll a and b and carotenoids and resulted in the development of seedlings that presented relatively short roots and small, chlorotic leaves. Moreover, the thiourea analogue 2B2 (≥600 µM) reduced the germination percentage of U. decumbens seeds and delayed their germination, and at a concentration of 800 µM, this analogue impaired root growth and blocked the formation of lateral roots. The presence of an oxygen atom in the urea moiety of the 2A10 structure is critical for its marked activity against B. pilosa seeds, as 2B2 bears a sulphur atom instead and marginally inhibits seed germination. Neither 2A10 nor 2B2 was toxic to the non-weed species Lactuca sativa (lettuce; a dicot), and the latter even exerted beneficial effects by stimulating leaf expansion. Therefore, the evaluated (thio)urea analogues are promising for the design and development of new phytotoxic compounds for the pre-emergent control of the spread of B. pilosa (2A10) or the post-emergence control of U. brizantha (2A10) and U. decumbens (2B2).
Keywords: 3,4-Dihydropyrimidin-2(1H)-ones/-thiones; Biginelli adducts; Diuron; Herbicide; Phytotoxicity; Weed control.