Allelopathy refers to the direct or indirect effect of plants upon neighboring plants or their associated microflora or microfauna by the production of allelochemicals that interfere with the growth of the plant. The allelochemicals released from the plants act as a defense system against microbial attack, herbivore predation, or competition from other plants. Wild oat (Avena fatua L.) and canary grass (Phalaris minor Ritz.) being the most troublesome grassy weeds in wheat (Triticum aestivum L.) result in yield reduction by about 30%. These weeds not only cause yield reduction but also deteriorate the produce quality by seed mixing with grains and interfere the harvest operations. Although herbicides available in the market may offer effective control of these weeds but environmental damages occur herbicide resistance development among weeds and health concerns due to over and misuse of synthetic herbicides has led the researchers to focus on alternative weed management strategies. The study of allelopathy is a sub-discipline of chemical ecology that focuses on the effects of chemicals produced by plants or microorganisms on the growth and development of other plants in natural or agricultural systems. The effect can be either positive or negative on the growth of the surrounding plants. The word allelopathy is derived from two separate Greek words, allelon meaning of each other or mutual and pathos meaning to suffer or feeling. Even though the term ‘allelopathie’ was first used by Austrian scientist Hans Molisch in 1937, the chemical interaction between plants has been known for thousands of years. In 300 B.C, the Greek botanist Theophrastus mentioned the negative effects of chickpea on other plants and later Pliny, a Roman scholar noted the inhibitory effect of the walnut tree (Juglans spp.) over nearby crops. The allelochemicals are released from plant parts by leaching from leaves or litter on the ground, root exudation, volatilization from leaves, residue decomposition, and other processes in the natural and agricultural systems. Upon release, the allelochemicals can suppress the germination, growth, and establishment of the surrounding plants or modify the soil properties in the rhizosphere by influencing the microbial community. Since allelopathic substances play an important role in regulating the plant communities, they can also be used as natural biodegradable herbicides. The allelopathic effects of the eucalyptus species have been investigated extensively. Phenolic acids and volatile terpenes are the allelochemicals present in the leaves, bark, and roots of Eucalyptus spp. The foliage of the species also contains a variety of oils and resins that may have a direct or indirect effect on the neighboring plants, seeds, or microbes. Volatile terpenes that act as allelochemicals, such as 1,8-cineol, limonene, α- and β-pinene have been reported present in the species. In a greenhouse study, leachates from fresh leaves of bluegum eucalyptus (Eucalyptus globulus Labill.) at a concentration of 20% (w/v) and 40% (w/v) reduced the resprouting of purple nutsedge (Cyperus rotundus L.) by 57%-68% and Bermuda grass (Cynodon dactylon (L.) Pers.) by 82%-89%. In another study, the essential oils from blue gum eucalyptus reduced the growth of Bermuda grass by 66% at a concentration of 25% (v/v)). Eucalyptus essential oils at a concentration of 0.2% (v/v) and 0.5% (v/v) reduced the germination of Common amaranth (Amaranthus retroflexus L.) and common purslane by 80% and 90% respectively. One of the major allelopathic compounds present in eucalyptus leaf is 1,8-cineol. 1,8-cineol can decrease germination, reduce root growth, and inhibit mitosis.The plant extracts and essential oils from different parts of the plant consist of alkaloids, terpenoids, steroids, flavonoids, phenolic derivatives, and quassinoids. A study comparing the phytotoxicity of different parts of the plant showed that toxic activity was highest in the root bark and lowest in wood. In a container study, the oven-dried root bark of the tree of heaven mixed in the soil was shown to decrease the emergence, biomass, and survival of garden cress (Lepidium sativum L.). A major allelochemical present in the tree of heaven is a quassinoid compound called ailanthone. Fine fescue grasses are known to displace the neighboring plants by releasing allelochemicals through roots into the soil. Several studies have focused on the weed management potential of fine fescue grasses through their toxic root leachates. The major allelochemical released through the root exudates of fine fescue grass is a non-protein amino acid called m-tyrosine. M-tyrosine has been demonstrated to inhibit the germination and growth of large crabgrass (Digitaria sanguinalis (L.) Scop.), white clover (Trifolium repens L.), and common dandelion (Taraxacum officinale. Mtyrosine acts as a natural herbicide that affects the post-germination development and early establishment of the neighboring plants. It has been shown to impact cell division and cell elongation in several higher plant species. Japonica rice cultivar possessed stronger allelopathic potential against root growth of barn yard grass. Many phytotoxic compounds from several chemical classes, such as fatty acids, benzoxazinoids, indoles, phenolic acids, phenylalkanoic acids, and terpenoids have been found in rice extracts. The key allelochemicals found in allelopathic rice cultivars are tricin and momilactone B). These compounds have been shown to inhibit paddy weed growth and increase rice yield. 

Application of mulberry extract to both wild oat and canary grass seeds resulted in complete germination inhibition of both weeds, which can be attributed to strong allelopathic potential of the mulberry plant. Allelopathic potential of the mulberry has also been explored previously on pulses and radish, respectively. Inhibitory effects of mulberry leaf extract on pulses including peas, broad beans and lentils and reported suppression of germination and seedling growth. Sorghum has been intensively reported to possess strong allelopathic potential and obstruct the growth processes of other plant species, however, sorghum cultivars vary for their allelopathic potential and this might be a possible reason that sorghum did faintly affect the growth of weed species in this study. Recent allelopathic studies made great improvements with rapid progress in separation and structural elucidation techniques, active compounds can be detected, isolated, and characterized. Allelochemicals are produced by plants as secondary metabolites or by microbes through decomposition. Allelochemicals are classified into 14 categories based on their chemical similarities. The 14 categories are water-soluble organic acids, straight-chain alcohols, aliphatic aldehydes and ketones; simple unsaturated lactones; long-chain fatty acids and polyacetylenes; benzoquinone, anthraquinone and complex quinones; simple phenols, benzoic acid and its derivatives; cinnamic acid and its derivatives; coumarin; flavonoids; tannins; terpenoids and steroids; amino acids and peptides; alkaloids and cyanohydrins; sulfide and glucosinolates; and purines and nucleosides. Plant growth regulators, such as salicylic acid, gibberellic acid, and ethylene are also considered to be allelochemicals. Allelochemicals vary in mode of action, uptake, and effectiveness (Weston and Duke, 2003; Rice, 2012). The mode of action for many of the identified allelochemicals is still unclear. Many allelochemicals have mechanisms that are not used by any of the synthetic herbicides, giving researchers leads to new mode of action (Duke et al., 2002). While the efficacy and specificity of many allelochemicals are unknown or limited (Bhadoria, 2010), they are an appropriate alternative for synthetic herbicides. Many plant species have been listed worldwide for their allelopathic effects, and of these, we have listed a few with great potential for further research. In Pakistan, important broad-leaved weeds in wheat are lamb’s quarters, field bindweed, broad-leaved dock, wild cress, wild pea and sweet clover, while narrow-leaved weeds comprise two grasses, viz. wild oat and canary grass. These two grasses are responsible for major yield loss in wheat and it is more cumbersome to control them than all other weeds. Although effective chemical weed control methods are available, nonetheless, continuous use of the same type of herbicide also leads to the development of herbicide-resistant weed bio-types. Several re-ports indicate the resistance of wild oat and canary grass to clodinafop-propargyl,diclofop-methyl, fenoxaprop-p-ethyl, isoproturon, fluazifop-p-butyl, haloxyfop-methyl, sethoxydim and tralkoxydim in various countries across the globe. In addition, herbicides pollute the soil, water and aerial environments and may enhance the disease risks (Ronald, 2000). Hence, concern regarding use of herbicides is growing worldwide.

Demand for organically produced commodities in the world is also increasing. The area under organic crops is more than 24 million hectares distributed in 100 countries, while the global market for organic foods is more than $ 23 billion per annum and is growing rapidly. Scientists are looking for new ecological and natural approaches for weed management. The use of allelopathic plant water extracts for weed suppression offers a viable and pragmatic option. In our previous studies, sorghum water extract was found to be effective for weed suppression in many field crops including wheat, rice, maize and mungbean. The extent of weed control was 35–49, 40, 37–41, 18–50 and 44% in wheat, cotton, rice, maize and mungbean, respectively, which is far less than what is achievable by herbicide use. This necessitates exploring some other means of exploiting allelopathic potential. Many plants other than sorghum, such as sunflower (Helianthus annuus L.), brassica (Brassica campestris L.), eucalyptus (Eucalyptus camaldulensis D.), sesame (Sesamum indicum L.), rice (Oryza sativa L.) and tobacco (Nicotiana tabacum L.), etc., have also been found to possess allelopathic potential. This indicates that efficacy of sorghum water extract can be improved by combining it with water extracts of other allelopathic plants. Hence, we evaluated the possible integration of sorghum with eucalyptus and sunflower water extracts and found that mixed application of sorghum, eucalyptus and sunflower water extracts gave 70% more weed suppression than sorghum water extract alone in wheat. Based on a series of previous studies and availability of plants, the allelopathic plants. Sunflower has allelochemicals, viz. chlorogenic acid, isochlorogenic acid, α-naphthol, scopolin and annuionones. Soil from sunflower fields was rich in phenolics, which reduced the stand establishment, growth and yield of the crop following sunflower.