A large number of weeds are known to be associated with crops and causing economic losses. Weeds interfere with crops through competition and allelopathy. They produce secondary metabolites known as allelochemicals, which belong to numerous chemical classes such as phenolics, alkaloids, fatty acids, indoles, terpens etc. However, phenolics are the predominant class of allelochemicals. The allelochemicals release from weed plants takes place through leaf leachates, decomposition of plant residues, volatilization and root exudates. Weeds leave huge quantities of their residues in field and affect the associated, as well as succeeding crops, in various cropping systems. Liberation of allelochemicals from weeds affects the germination, stand establishment, growth, yield and physiology of crop plants. They cause substantial reduction in germination and growth of the crop plants by altering various physiological processes such as enzyme activity, protein synthesis, photosynthesis, respiration, cell division and enlargement, which ultimately leads to a significant reduction in crop yield. In crux, allelopathic weeds represent a potential threat for crop plants and cause economic losses.

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Aquatic weeds are higher plants found in the aquatic ecosystem and in anaerobic rice fields, where they have no economic benefits. The continuance of aquatic weeds is more widespread than terrestrial weeds because in aquatic ecosystems there is very little fluctuation in the environmental conditions compared with terrestrial ecosystems. Scientists have been working to address the harmful allelopathic effects of aquatic weeds on the aquatic ecosystem, but limited information is available on the allelopathic influence of aquatic weeds on agro-ecosystems through the release of phytotoxic compounds. Phytotoxic chemicals released by different aquatic weeds into irrigation water and/or directly into rice ecosystems might have a significant inhibitory influence on germination, growth and yield field crops, soil properties and nutrients availability, population and community structure, and weed invasion. However, aquatic weeds might also be used as a potential organic alternative to chemical weed-control, due to the higher susceptibility of terrestrial weeds to the phototoxic chemicals released by aquatic weeds. Natural alternatives to chemical weed control are need of time and are crucial for a sustainable weed control. Chemical weed control is challenged, due to recent increases in herbicides resistance from weeds and to the harmful side-effects of herbicides on the environment. This review is focused on the influence of aquatic weeds on agro-ecosystems, with examples of common weeds in aquatic ecosystems and invasive aquatic weeds found in anaerobic rice.

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