ALLELOPATHY
INTRODUCTION
The term ‘Allelopathy’ is detected from the Greek – derived compounds ‘allelo’ and ‘pathy’ meaning mutual harm or suffering. In this phenomenon, one organism produces certain types of specific biochemicals, which affect the germination, growth, survival and reproduction of neighbouring organisms(Molish, 1937). The word allelopathy is rooted in allelon, which means “from each other”; pathetic, which means “suffering” (Molisch, 1937). Thus, it seems to us in the phenomenon that work to generate adverse environments for neighbouring plants by declining seed germination and growth of biochemical plant initiators, which are known as natural inhibitors so that they have a very effective role in inhibiting the growth of plants and weeds (Waseem et al., 2020).
In plant system, these types of biochemicals are called Allelochemicals, which are present in different parts of plants such as leaves, fruits, flowers, pollen, roots and stem. These chemicals interact and affect the physiological and biochemical processes of neighbouring plants. Allelochemicals are non- nutritional secondary metabolites, which ate released by plants in different conditions and in different processes. Allelochemicals play a significant role in monitoring abundance and dispersion of plant species in a community and also aid in the success of invasive plants. Therefore, allelopathy is considered as one of the indirect factors of regular cropping interventions in agricultural sector.
Use of herbicides and allied chemicals for crop protection negatively influence health, productivity and quality of crops. Hence, in recent years scientists are in favour of bioherbicide formulation which is a potent approach for crop health management (Arnab Jash et al., 2019). Our present study reviews the different plant allelochemicals and their importance in crop industry.
NATURE OF ALLELOPATHY
Commonly cited effects of allelopathy include reduced seed germination and seedling growth. Like synthetic herbicides, there is no common mode of action or physiological target site for all allelochemicals. However, known sites of action for some allelochemicals include cell division, pollen germination, nutrient uptake, photosynthesis, and specific enzyme function. Allelopathic inhibition is complex and can involve the interaction of different classes of chemicals, such as phenolic compounds, flavonoids, terpenoids, alkaloids, steroids, carbohydrates, and amino acids, with mixtures of different compounds sometimes having a greater allelopathic effect than individual compounds alone.
Different plant parts, including flowers, leaves, leaf litter and leaf mulch, stems, bark, roots, soil, and soil leachates and their derived compounds, can have allelopathic activity that varies over a growing season
ALLELOCHEMICALS PRESENT IN PLANTS
Allelochemicals are secondary metabolites produced by plant. There arev arious types of allelochemicals.Various allelochemicals produced by plants include; Phenolic compounds, Alkaloids, Terpenoids, Glucosinolates and isothiocyanate, Benzoxazinoides and other miscellaneous compounds.
APPLICATIONS OF ALLELOPATHY
Allelopathy has both beneficial and harmful effects.Autotoxicity,soil sickness or biological invasion etc. include its negative effects. While its positive effects include crop protection,weed control and so on.Recent researches are majorly focused on the use of allelochemicals as growth regulators,herbicides, insecticides, antimicrobial crop protection products and so on.
Role of allelopathy in crop industry
Allelopathy offers natural weed control, insect pests and diseases.Allelochemicals exuded in plant rhizosphere improve nutrient acquisition. Allelochemicals promote plant growth at low concentration and suppress at high concentration. Allelopathic crops are an effective substitute for synthetic herbicide as it does not have toxic effects. So, application of allelochemicals at low concentration is an effective method to promote growth and enhance crop productivity.
Use of allelopathy helps to get rid of chemical pesticides and insecticides, thereby helps to get rid of environmental pollution.
Allelopathy in Intercropping
Weeds cause major loss in crop production due to various reasons like cost of weed management, weed crop competition etc. Recent studies proved that intercropping of allelopathic crops can be useful for weed management. Release of allelochemicals, weedcrop competition, shade effect are the factors that are beneficial. It also helps in increasing crop production.
Allelopathy for management of phytopathogens
Allelopathic chemicals can control phytopathogens. Crude extracts of plants can be sprayed against pathogen in air.
Development of herbicides
Allelochemicals with herbicide activity include phenolics and terpenoids. One research study reported that, phenolic compounds show their allelopathic effect by inhibiting symbolic association with rhizobium and legume.
Allelopathy in weed management
Allelopathic water extracts are successfully used in weed management.Allelochemicals, at high concentration, interfere with cell division, hormone biosynthesis,mineral and water uptake, membrane permeability, photosynthesis and respiration.
In addition to water extracts,allelopathy control weeds through incorporation of allelopathic crop residues into soil. Inclusion of allelopathic crops in crop rotation, intercropping, cover crops also control weeds effectively.
Allelopathy and insect management
Many plants have natural defence mechanism against insect pests like Azadirachta indica that produce Azadirachtin. They inhibit the growth of different insect species. So, allelochemicals from such plants can be effectively used as insecticide with the advantage of biodegradation,economic affordability, environmental safety and easy handling.Allelochemical water extracts from sorghum, sunflower, mulberry is very effective in controlling aphids, sucking insects in Brassica spp.
Allelopathy and disease management
Allelochemicals have shown positive role in controlling fatal pathogens. Water extracts of different cereals, sweet clover, lentils suppress fungus Sclerotina sclerotium in beans.Two potent allelochemicals of rice, Momilactone A and Momilactone B have shown antifungal,antibacterial,antioxidant and anti-cancerous activities in-vitro.
Allelopathy in resistance against abiotic stress
Allelopathic chemicals induce resistance to plants against stress and help them to grow vigorously. The production of allelochemicals differ according to the plant, the type of stress, intensity and ambient surroundings.Generally, biosynthesis of allelochemicals is high at high temperature. They help to avoid,tolerate, and mitigate all catastrophes in an efficient way.
Allelopathy in stress signalling
Plants respond to their environmental stress efficiently through sensors regulated by feedback mechanism. Plants use secondary metabolite messengers under suboptimal condition to trigger defence mechanism and to protect plant ultra structure from hazards.
Allelopathy in crop nutrition
Allelochemicals facilitate crop nutrition by altering nutrient forms, microbial populations,availability modes and uptake channels. Nitrogen is the most important primary macronutrient needed for optimum plant growth. Allelochemicals are involved in biological nitrification inhibition that reduce nitrogen loss and improves nitrogen use efficiency.
Allelochemicals bind on nutrient radicals to form chelates that hold nutrients and improve their stay in rhizosphere to minimize loss.
Hormonal balance and enzymatic activity
Allelochemicals regulate the production of plant hormones Gibberellins and auxins are affected by secondary metabolites, which in turn affect cell enlargement in plants.
Application as seed treatment
Seed germination percentage, germination power, and germination index, radical and plumule length, fresh and dry weight etc. are generally affected by allelochemicals depending on their concentration. Low concentration of allelopathic water extracts as seed treatment before sowing improve all these parameters.
FUTURE PROSPECTS OF ALLELOPATHY
Even though allelopathy was considered as an evil in previous times, according to recent studies, there are many allelopathic chemicals like phenolics, alkaloids, terpenoids etc. That possess several applications in mulching, intercropping etc. Production of herbicides, abiotic stress mitigation etc. play a great role in sustainable agriculture and it also facilitate to conserve available resources. Allelopathic applications offer multiple solution to decreasing food availability under rising global population. Due to its wider application, allelopathy has many future prospects.
Allelochemicals replaces hazardous chemical and mechanical approaches used in crop production. Allelopathic water extracts are safe, efficient, eco- friendly and easy to handle. Researchers focus on screening of more potent cultivars that produce more allelochemicals in water extracts. Allelopathy researches are done to ensure food security, environmental safety, resource conservation and economic stability.
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