Phytotoxicity of Cortaderia speciosa extract, active principles, degradation in soil and effectiveness in field tests

Abstract

Allelopathic plant extracts have been proposed as natural herbicides, and as an alternative to synthetic herbicides. However, further studies are still required on the active principles of these extracts as well as on their soil dynamics and weed control effects in the field. We carried out the bioguided isolation of active compounds of EtOH extract of the ornamental plant, Cortaderia speciosa, with biodegradation and field trial experiments. Six phenolic acids were found to be responsible for herbicidal activity against Solanum lycopersicum: ferulic acid, p-coumaric acid, caffeic acid, vanillic acid, p-hydroxybenzoic acid and gallic acid. The effective concentrations for 50% inhibition of germination (ECg50) were determined for each of these compounds in tomato (S. lycopersicum), and for mixtures of them in radish (Raphanus sativus), lettuce (Lactuca sativa), tomato (Solanum lycopersicum), onion (Allium cepa) and alfalfa (Medicago sativa). p-Coumaric acid was the most active in inhibiting the germination of S. lycopersicum (ECg50 = 0.31 mg/mL), followed by ferulic acid (0.39 mg/mL), vanillic acid (0.51 mg/mL), caffeic acid (0.73 mg/mL), p-hydroxybenzoic acid (0.79 mg/mL), and gallic acid (1.67 mg/mL). Some equimolar mixtures of phenolic acids showed synergistic effects on germination, the best combination being ferulic acid with p-coumaric acid (5:2 molar ratio), with an EC50 = 0.06 mg/mL. Biodegradation studies of C. speciosa extract in soil showed a half-life of 31.08 (± 0.90) h, according to HPLC quantification of phenolic acids in samples taken on different days. In field trials of C. speciosa extract, the weed cover area was significantly different from the control from Day 9 until the end of the experiment (Day 43). At concentrations of 50, 100 and 200 kg of C. speciosa extract/ha, weed suppression was 7%, 18% and 24% with respect to control at Day 43, indicating inhibition of the germination of spontaneous weeds. A treatment with a combination of phenolic acids (ferulic acid, p-coumaric acid, caffeic acid, vanillic acid, p-hydroxybenzoic acid and gallic acid, molar ratio 5:2:1:3:3:0.1) at 2.8 kg/ha strongly inhibited the appearance of weed, showing a surface cover 60% smaller than that of control at the end of the experiment. Weed biomass was reduced by 47%, 64%, 78% and 91% with respect to control by 50, 100, 200 kg of extract/ha and the 2.8 kg/ha mixture of phenolic acids, respectively. Biodegradation and field studies revealed that, despite degrading in a few hours, the phenolic acids inhibited weed emergence in the field.