Bioherbicidal properties of sunflower (Helianthus annuus L.) and its activities in weed management

Abstract

The use of chemical herbicides to suppress weeds possess risks to environment, health, water contamination, and soil microorganisms. It is therefore imperative that research into more eco-friendly alternatives is conducted. Furthermore, there are more than 470 biotypes of weeds that are resistant to chemical herbicides. Sunflower (Helianthus annuus) is one of the most important oil crops globally. It produces strong allelochemical compounds that have been shown to affect some crops and weeds. This study was conducted in order to investigate bioherbicidal properties of sunflower on germination and growth of some crop and weed species. The study consisted of five experiments. The first experiment was conducted using petri dishes in order to evaluate effects of sunflower growth stage on the bioherbicidal activity of aqueous sunflower shoot and root extracts on seed germination and early growth of Brassica napus, Secale cereale, Cephalaria syriaca, Phalaris minor, Pisum sativum, Triticum aestivum, Avena fatua and Helianthus annuus. Extracts were prepared from dried shoots and roots of sunflower sampled at one week of growth, and at one and two months of age and also at crop maturity. Aqueous two month shoot extracts caused a significantly higher reduction (81.27%) in seed germination and seedling growth than one week (54.44%), one month (55.67%) and mature stage extracts (62.75%) of most seed species. On the other hand, one month root extract had a more negative impact on seed germination (54.17%) and seedling growth than other root extracts. Also the dry matter yield of sunflower shoot and root extracts at different growth stages was studied. Two month shoot (18.9%) and one week root extracts (14.22%) gave higher yields than other extracts of each type. Analysis of phytochemicals from sunflower shoot and root extracts at different growth stages indicated that tannins, terpenoids, saponins, and phenolics were present in both shoot and root extracts at most growth stages while phlobataninns were only present in root extracts at different growth stages except one week. A second experiment was carried out in order to investigate the allelopathic potential of different concentrations of aqueous shoot extracts prepared from shoots of plants at the mature stage on seed germination and seedling growth of two monocot (S. cereale, P. minor) and two dicot (B. napus, C. syriaca) plant species. Seed germination and seedling growth of B. napus and C. syriaca were completely inhibited at 5% and 10% (w/v) concentrations respectively.The effect of sunflower shoot extract on mitotic index and cell elongation was investigated. Aqueous sunflower shoot extract significantly reduced root cell elongation (control: 8.2 μm, test: 6.2 μm) of A. fatua. However, statistical analysis indicated that aqueous sunflower shoot extract did not significantly affect mitotic index. No significant difference in distribution of cells between mitotic phases (prophase, metaphase, anaphase, and telophase) was recorded. The third experiment involved pot experiments for examination of the allelopathic effects of two month sunflower shoot aqueous extract and incorporated two month sunflower ground shoot on seed germination, shoot and root length, shoot and root dry weight, chlorophyll a and chlorophyll b of B. napus, A. fatua, C. syriaca and P. minor grown in soil. Soil calcium, magnesium, potassium, pH and electrical conductivity were also measured. In general, two month sunflower ground shoot caused more significant reduction in seed germination and early growth than shoot aqueous extract. The fourth experiment involved identification and quantification of phenolic compounds present in sunflower shoot and root extracts. Four phenolic compounds, syringic acid, protocatechuic acid, 4-hydroxybenzoic acid, and ferulic acid, present in root extracts from one week old sunflowers were identified and quantified. Additionally, twelve phenolic compounds were identified and quantified in sunflower shoot extracts from one month, two month and mature stage plants. These were gallic acid, syringic acid, vanillic acid, protocatechuic acid, catechol, 4-hydroxybenzoic acid, p-coumaric acid, sinapic acid, ferulic acid, caffeic acid, chlorogenic acid, and trans-cinnamic acid. Sunflower two months shoot extract had the greatest concentration of phenolic compounds (0.026 mg/ml) compared with those from other growth stages. The effects of total and individual phenolic compounds on seed germination and seedling growth of B. napus, C. syriaca, T. aestivum and S. cereale were investigated. Total phenolic compounds caused the greatest reduction in seed germination and seedling growth followed by chlorogenic acid and caffeic acid. For the fifth experiment, the effects of a positive control, the herbicide trifluralin, on seed germination and growth of B. napus and C. syriaca were examined at different concentrations in petri dishes. Seed germination of both species was significantly decreased by most concentrations (P < 0.001). The inhibition percentage of seed germination ranged between 21 and 32%. The highest reduction of seed germination (32.28%) was from the highest trifluralin concentration (900 ppm). The effects of an aqueous shoot extract from two month old sunflower plants, the total phenolic compounds identified within these extracts and trifluralin on sugar content, protein content, proline content, DNA content, gibberellic acid (GA) content, indole acetic acid (IAA) content, and abscisic acid (ABA) content of B. napus, C. syriaca, T. aestivum and S. cereale seedlings were examined. Trifluralin reduced sugar content significantly more than sunflower extract did (P < 0.001). Both trifluralin and sunflower extract significantly increased protein content compared to the control treatment (P < 0.001). Total phenolic compounds significantly reduced proline content in S. cereale and C. syriaca; the effect of trifluralin was only significant for C. syriaca (P < 0.001). Trifluralin had the greatest effect on DNA content of T. aestivum and S. cereale and total phenolic compounds the least in comparison to the control treatment (P < 0.001). Sunflower extract had the greatest effect on GA contents. ABA was reduced by application of total phenolic compounds and sunflower shoot extracts in S. cereale and C. syriaca. Sunflower shoot aqueous extract and total phenolic compounds significantly reduced IAA in all studied plant species but trifluralin only reduced IAA in T. aestivum, B. napus and C. syriaca (P < 0.001). A pot experiment was carried out for examination of the effects of sunflower ground shoot, its total phenolic compounds, and trifluralin on seed germination and seedling growth of S. cereale, T. aestivum, B. napus and C. syriaca. Trifluralin had a significantly greater effect on seed germination of most plant species. The inhibition percentage ranged between 26 and 100% for shoot and root length and shoot and root dry weight, while total phenolic compounds had the least effect (P < 0.001). In conclusion, sunflower shoot extract from two month old plants has a significantly greater influence on seed germination and seedling growth than other extracts. Also, the application of total phenolic compounds causes significantly more reduction in seed germination and early growth. Two month sunflower ground shoot has a greater effect on most parameters measured than aqueous sunflower shoot extract in pot experiments. Furthermore, the effects of sunflower two month ground shoot, total phenolic compounds and trifluralin in pot experiments indicated that trifluralin has the greatest effect on seed germination and seedling growth while total phenolic compounds have least effect on seed germination.