Phytotoxic effects of essential oils in controlling weed species Digitaria horizontalis and Cenchrus echinatus

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Abstract

In certain plant species, essential oils have been reported to inhibit seed germination and cause toxicity and have been associated with a loss of photosynthetic activity. This study aimed to evaluate the effects of essential oils and their major compound citronellal on the germination and growth of crabgrass (Digitaria horizontalis) and burrgrass (Cenchrus echinatus). Essential oils from E. citriodora and C. nardus and pure citronellal compound were used at concentrations of 1% and 10% and 20% to test seed germination and phytotoxic effects, respectively, in the plants. The treatments application was done when the plants were at the four-leaf stage. Seed germination was reduced drastically (97–99% reduction relative to untreated controls), with citronellal showing more drastic reductions than essential oils. In addition, evaluated the phytotoxic effects of essential oils on plant height and dry mass of shoots and roots were evaluated. The negative effects of the oils were seen 12 h following the treatment. The application of the oils at a concentration of 20% reduced the accumulation of dry mass of shoots and roots; however, the number of tiller only affected stomatal opening in burrgrass. Finally, the oils reduced chlorophyll and total protein content in the weeds by more than 80% and 90%, respectively. Therefore, these oils may have application as potential bioherbicides to controlling weeds.

Introduction

Recently, there has been increased pressure on developing alternatives to pesticides to preserve environment and public health. Therefore, there is a need to search for alternative, safe compounds, particularly plant products, that can be used without risks of contamination. Essential oils have emerged as a good option, and they are being studied with greater frequency (Dayan et al., 2009, Soylu et al., 2010).

Studies of essential oils have identified their chemical components and have revealed their phytotoxic effects. For example, they have been shown to act as herbitoxins (Martino et al., 2010; Brito et al., 2012; Verdeguer et al., 2009), insecticides (Hao et al., 2008, Ootani et al., 2011, Dias-Arieira et al., 2010) and fungitoxins (Souza et al., 2010, Veloso et al., 2012, Bigaton et al., 2013) In addition, they are known to be lethal against ticks (Agnolin et al., 2010, Campos et al., 2012, Olivo et al., 2013) they provide good post-harvest controls (Carnelossi et al., 2009), they have been shown to have antioxidant and antibacterial properties (Andrade et al., 2012), and they even prevent parasitic diseases (Carneiro et al., 2015). Finally, they have been shown to have allelopathic effects that interferee with the germination, growth, and development of other plants and microorganisms (Piccolo et al., 2007, Fortes et al., 2009, Filho et al., 2012). Therefore, with better characterization, they may serve as a basis for the production of herbicides that are more specific and less harmful to the environment than those currently used.

Essential oils have been reported as potential inhibitors of seed twinning, and they have been shown to have phytotoxic effects on Avena fatua, Cyperus rotundus, and Phalaris minor (Singh et al., 2009). In this context, the potential use of essential oils as bioherbicides, with mixtures of different classes of terpenoids, requires determining their specific chemical composition and understanding potential interactions among these constituents. One outcome of this work will be the potential to formulate essential oils for agricultural applications (Verdeguer et al., 2009).

Accumulation of substances with allelopathic effects can occur in all plant organs. Although there is a tendency for substances to accumulate in leaves, the release of these compounds can also occur by root exudation, leaching, or volatilization. Currently, plant sesquiterpenes are of interest as potential herbicides, mainly because of their direct effects on plant growth (Cantrell et al., 2007). Generally, both monoterpenoids and sesquiterpenoids appear to be responsible for these phytotoxic effects. For example, pure and combined compounds, such as limonene, α-pinene, (Z)-caryophyllene, terpinen-4-ol, bornyl acetate, citronellal, camphor, carvacrol, thymol, and geraniol (Ens et al., 2009, Vokou et al., 2003), can explain the herbicidal effects of essential oils.

Previous reports have verified that essential oils can have bioherbicide effects in different plant species (Ens et al., 2009). In addition, the oil of E. citriodora was observed to contain fungistatic activity against amaryllis (Hippeastrum hybridum), which directly influenced the development of the plant (El-Rokiek et al., 2009) by altering a variety of physiological processes that have significant effects on cell division and differentiation, phytohormone metabolism, respiration, photosynthesis, and enzyme function (Bramley et al., 1997; Duke et al., 2004).

As a consequence, there is a need for new studies that examine the phytotoxic effects of oils on various weed species present in crop fields. Therefore, this study aimed to evaluate the activities of the essential oils obtained from C. nardus and E. citriodora and the pure citronellal compound against the weed species Digitaria horizontalis and Cenchrus echinatus.

Section snippets

Plant material and steam distillation

Leaves of E. citriodora and C. nardus were collected from a rural region of the Gurupi, Tocantins state, Brazil (11°43′45″S, 49°04′07″W). Essential oils were extracted from the leaves of E. citriodora and C. nardus using steam distillation in a Clevenger apparatus, as described by Aguiar et al. (2014). Citronellal compound was purchased from Sigma-Aldrich (St. Louis, MO, USA) and stored at 4 °C until the phytotoxicity experiments were conducted.

Gas chromatography–mass spectrometry (GC–MS) analysis

We performed gas chromatography–mass spectrometry

Results and discussion

C. citriodora and C. nardus essential oils samples were analyzed by GC and GC–MS and their qualitative and quantitative compositions were established (Table 1). Major components found in the chemical characterization of essential oils from E. citriodora were citronellal (61.78%) and isopulegol (11.89%), while β-citronellol and 1, 8-cineol were present at lower concentrations. Analysis of essential oils from C. nardus detected a concentration of citronellal (36.53%) and geraniol (25.56%), while

Conclusions

Bioactivity results of essential oils from C. nardus and E. citriodora and citronellal compounds showed that the oils exhibited strong phytotoxic effects on seed germination, plant development, and reduction in chlorophyll and protein content. Therefore, these essential oils could be proposed as alternative herbicides to citronellal. However, further studies are required to determine the cost, applicability, and safety as potential bioherbicides.

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