Allelochemicals from Thapsia garganica leaves for Lolium perenne L. control: the magic of mixtures

Luteolin 7-O-glucoside (1), 10β-acetoxy-8α-butyryloxy-11α-hydroxy-2β-((2-methylbutanoyl)oxy)-1βH,6αH,7αH,11βH-guaian-3-en-12,6-olide (2) and thapsigargin (3) herbicidal activities’ were evaluated in comparison with their binary and tertiary mixtures, against Lolium perenne. These allelochemicals were isolated from Thapsia garganica leaves methanolic extract. Experiments were carried out by irrigation and spray in pot trials. Each compound was tested at the concentration that inhibits 50% of L. perenne root growth (IC₅₀). Mixtures were prepared at the total concentration determined to inhibit 50% of weed root growth based upon the IC₅₀ value for each compound (1000 µM, 154 µM and 300 µM for 1, 2 and 3, respectively). The greatest herbicidal effect was observed in tertiary mixtures, followed by binary ones, and single compounds showed the lowest phytotoxicity. Moreover, spray treatment was more effective at inhibiting growth of L. perenne, compared with irrigation. For sprayed binary mixtures, the 2 and 3 mixture showed the best inhibitions in shoot (75.79%) and root (91.02%) growth, and fresh weight (89.28%). These values significantly improved those of the most active single compound, 1 (48.01%, 58.62% and 57.14%, respectively, following spray). On the other hand, compound 3, whose structure is related to guaianolide sesquiterpene lactones, was a common constituent of the most active mixtures, suggesting that it plays a more relevant role in the improvement of the phytotoxicity of mixtures. Results obtained for the spray treatment of the tertiary mixture of 1 (333.33 µM), 2 (51.33 µM) and 3 (100 µM) were even more prominent, since weed growth was completely inhibited. After irrigation with a tertiary mixture, the greatest inhibitions in shoot and root growth and fresh weight did not exceed 88.16%, 94% and 90.47%, respectively. The results reported highlight a synergistic behavior of the test allelochemicals which could be applied in the development of bio-herbicides.