The first bioherbicide was developed in 1981 and was based on the Phytophthora palmivora fungus. Bioherbicides can be in the form of cover crops, residues or plant extracts that are used to create a liquid bioherbicide, which can be beneficial towards soil conservation and climate change.
Some microbes such as fungi are sometimes used to develop bioherbicides, hence the term ‘mycoherbicide’. Bioherbicides are generally composed of biological agents such as plants and microbes that release toxins to control unwanted weeds.
Examples of biological agents are water extracts from sorghum and sunflower, which are incorporated into the bioherbicide. The toxins plants release are called ‘allelochemicals’, which are secondary metabolites produced naturally by the plant in the presence of biotic and abiotic stresses.
Plants or residues may contain several allelochemicals that induce phytotoxic responses in other unrelated plants, to survive in a competitive environment. The allelopathy phenomenon can sometimes also occur when neighbouring plants compete for nutrients, moisture and physical space to optimally grow and thrive.
In the case of bioherbicide development, biological agents are used to detrimentally affect the unwanted weed, whether it is to inhibit and damage cell division, elongation, cell membranes, photosynthesis and even the ability to absorb nutrients. Zero harm is caused to the present crop.
Plant families with bioherbicidal potential
The most popular plant family used for allelopathic and bioherbicide research, is the extended Lamiaceae family, which accounts for 43% of the total studied plant species. Researchers also achieved the most success with extracting allelochemicals from the Lamiaceae plant family, to use the secondary metabolites for bioherbicide development and research.
Two examples of plants in the Lamiaceae family are Thymus fontanesii (thyme) and Satureja calamintha (mint). This family typically is aromatic with culinary uses. Both these plants have a detrimental effect on seed germination of Xanthium strumarium L. (kankerroos) and Cyperus rotundus L. (rooi-uintjie), for example.
ALLELOCHEMICALS
Three main secondary metabolites – allelochemicals:
- Terpenes or terpenoids.
- Phenolics.
- Nitrogen-containing chemicals.
Each allelochemical group is composed of several classes. For example, the Terpene group has a subgroup called ‘monoterpenes’, which can inhibit or reduce seed germination. Monoterpenes are frequently used in bioherbicide development, due to its ability to reduce or inhibit photosynthesis and the chlorophyll content of the target plant as well.
Monoterpene esters (pyrethroids) compounds are found in chrysanthemum flowers and used in many organic and synthetic insecticides to control various insecticidal pests.
The mode of action (MoA) of allelochemicals:
The MoA is the mechanism in which an allelochemical negatively impacts the neighbouring plant, or the insect that has been feeding on the host. The following are examples of MoAs of allelochemicals:
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- Cell division, physical structure, elongation: Bioherbicides contain an amount of allelochemicals that inhibit/reduce the above-named parameters. Cineole (part of the terpene and monoterpene allelochemical group) is extracted from Salvia leucophylla (purple sage), which can inhibit mitosis at almost every stage. Mitosis is the formation of new cells that is necessary to develop a fully grown plant.
- Cell membrane permeability: The cell membrane is a barrier that controls and regulates the movement of hydrophobic molecules such as O2 and CO2. If an allelochemical inhibits or alters cell membrane permeability and functionality, the plant would experience oxidative stress and gain susceptibility to pathogenic infections.
- Photosynthesis: Scientists discovered that several allelochemicals can reduce leaf osmotic potential. Leaf osmotic potential is responsible for cell expansion and the opening/closing of leaf stomata. The stomata are microscopic openings on the leaf, which allows the plant to exchange gasses such as oxygen and carbon dioxide.
If the plant is under stress, whether it is drought or neighbouring plant competition, the stomata close to protect the plant from the threat. In the case of allelochemical toxins, the susceptible target plant is unable to create or expand cells while suffering reduced leaf osmotic potential, and eventually it dies off from poor photosynthesis. - Nutrient absorption and availability: Allelochemical toxicity affects plant physiology and morphology, but also the target plant’s ability to absorb nutrients efficiently. Toxic excretions reduce plant-available nutrients through affecting the mechanism of which nutrients are recycled and mineralised.
In other words, the nitrification process is influenced by the toxic allelochemicals. Nitrification is the biological transformation and oxidation of ammonia (NH4) to nitrite (NO2-) and then to nitrates (NO3-), which is an essential process within the soil to supply the plant with absorbable nitrogen.
Challenges regarding bioherbicide development
SELECTIVITY
Selectivity is when a herbicide only affects the target plant. The target plant must contain a specific structured morphology and certain metabolic processes for the herbicide to influence the plant.
Examples of selective products are broadleaf and grass herbicides. A broadleaf herbicide such as 2,4-D can only inhibit/reduce broadleaf weed growth, and NOT grass or nutsedge. 2,4-D contains an ‘Auxin-mimic’ mode of action, which causes the broadleaf weed to grow uncontrollably fast, and eventually it dies off.
The challenge for researchers is that bioherbicides are less selective than synthetic herbicides. It has been found that bioherbicides can inhibit weed growth, while affecting the non-target crop as well.
Also, specific plant extracts used for bioherbicide development can only influence a limited number of weeds, which may become an unrealistic weed-control method for large-scale crop production.
Conclusion
In the long term, bioherbicides are definitely beneficial towards soil conservation and climate change, due to the fact that less synthetic herbicides will be required.
Bioherbicides make use of allelopathy to detrimentally influence its neighbouring competition. However, due to selectivity issues, it is most likely that many allelochemicals within the bioherbicide can also negatively affect the crop, which can possibly reduce the yield.
For example, the use of plant extracts to develop bioherbicides can have little to no effect on a specific crop during the first year of use. However, in the second year, when a different crop is produced, the allelochemicals of the first year’s plant extracts may still have activity in the soil profile and negatively affect the second crop.
The producer must know exactly what the goal is when bioherbicides are used. If a specific weed must be controlled, then bioherbicides can be beneficial; but if a variety of weeds require controlling, then a combination of bioherbicides and synthetic herbicides should preferably be used. The use of cover crops is an excellent method of suppressing weed during the off-season or fallow years. When the cover crops die off, the remaining residues also aid in suppressing the weed for a longer period.
References
- Bezuidenhout SR, Reinhardt CF and Whitwell MI, 2012. Cover crops of oats, stooling rye and three annual ryegrass cultivars influence maize and Cyperus esculentus growth. Weed research, 52(2), pp.153-160
- De Mastro G, El Mahdi J and Ruta C, 2021. Bioherbicidal potential of the essential oils from Mediterranean Lamiaceae for weed control in organic farming. Plants, 10(4), p.818
- Hossen K, Das KR, Asato Y, Teruya T and Kato-Noguchi H, 2021. Allelopathic activity and characterisation of allelopathic substances from Elaeocarpus floribundus Blume leaves for the development of bioherbicides. Agronomy, 12(1), p.57
- Mushtaq W, Mehdizade M, Siddiqui MB, Ozturk M, Jabran K and Altay V, 2020. Phytotoxicity of above-ground weed residue against some crops and weeds. Pak J Bot, 52(3), pp.851-860
- Pushpa NSS, Baloch SK, Buriro M, Soomro AA, Khan MT, Jogi QU, Kandhro MN and Soomro FD, 2020. Allelopathic effect of weed species on germination and seedling traits of wheat varieties. J. Innov. Sci, 5, p.100
- Singh AA, Rajeswari G, Nirmal LA and Jacob S, 2021. Synthesis and extraction routes of allelochemicals from plants and microbes: A review. Reviews in Analytical Chemistry, 40(1), pp.293-311