Abstract
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.
Highlights
Weeds are associated with many crops and are a major threat to crop production in many cropping systems
They produce secondary metabolites known as allelochemicals, which belong to numerous chemical classes such as phenolics, alkaloids, fatty acids, indoles, terpens etc
Oleracea, B. campestris and B. rapa upon Obaid and Qasem (2005) reported that volatiles extracted decomposition. These results were credited to the phenolics from shoots of Convolvulus arvensis, A. gracilis, Portulaca released into the soil with water from decomposing oleracea and L. serriola caused a reduction in the growth of residues of P. hysterophorus
Summary
Weeds are associated with many crops and are a major threat to crop production in many cropping systems. Weeds cause a reduction in the growth and yield of crops by interfering with different metabolic processes (Hajizadeh and Mirshekari, 2011). Allelopathy is regarded as any process whereby secondary metabolites produced by plants, microorganisms, viruses and fungi influence the growth and development of agricultural and biological systems, including positive and negative effects (Torres et al, 1996). A large number of allelopathic weeds have been documented in the literature as they affect crop plants right from their emergence till maturity and cause considerable economic losses. Weeds allelopathy has been found responsible for perturbation of emergence and stand establishment, growth, yield and physiology of crop plants. An attempt has been made to elucidate the weeds containing potentially active allelochemicals, weeds allelochemicals mode of release and their harmful effects on germination, stand establishment, crop growth and development, crop yield and alterations in crop physiological processes.
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