The science of Allelopathy is stimulatory or inhibitory effects of one plant on another, mediated through release of allelochemicals. Allelochemicals are secondary metabolites, produced as by-products during different physiological processes in plants. These by-products appreciably promote growth at low concentration and inhibit at high concentration. Moreover, these chemicals are diverse in nature and structure, thus lack common mode of action. Important secondary metabolites identified as allelochemicals are terpenoids, momilactone, alkaloids, phenolics, flavanoids, glucosinolates, and hydroxamic acid.

Ever rising global population is threat to agricultural sustainability and food security. This situation made the food security, a challenge for scientists and farming community. There is increasing concerns for potential use of allelochemicals in sustainable agriculture because diseases, abiotic stresses, insect pests, imbalance crop nutrition and weeds are major threats to crop production. The allelopathy may be playing an important role to solve these problems.

There are various implications of allelopathy in agriculture. Allelopathy is helpful in organic weed management. Application of allelochemicals at high concentration interferes with membrane permeability, stomatal oscillations, respiration, mineral uptake, protein metabolism, cell division, hormone biosynthesis. This leads to reduction in reduction in weed density and biomass. For instance sorghum water extract (concentrated sorgaab) is natural herbicide that can be use to control Fumaria Indica, Chenopodium album, Phalaris minor, rumex dentatus in wheat crop.

Allelochemicals can be employed for insect management. Insects have developed resistance against chemical insecticides. These chemicals have detrimental effects on environment and cause hygienic problem. Allelochemicals are alternative of synthetic chemicals, and they advantages owing to environmental safety, easily handling, and biodegradation. For instance Neem produced salannin, nimbin and azadarachtin, these inhibit growth of whitefly and green cicadellid. Neem oil is antifeedant against strawberry aphid. Secondary metabolites from bhang, chilies, olives, garlic are effective against chickpea beetle. Water extracts of mustard, sunflower, sorghum, and mulberry are effective against sucking insects of Brassica and aphid.

Allelo-chemicals can also be used to control diseases by controlling causal organisms of diseases like fungi, bacteria, virus and nematodes. Like as water extracts of sweet clover, cereal, canola, and lentil are effective against fungus (sclerotinia sclerotiorium). Other allelochemicals such as Momilactone A, Momilactone B show anticancer, antioxidant, antibacterial and antifungal properties.

Allelopathy may be effectively employed against abiotic stresses by producing resistance against these stresses. Allelochemicals released in case of stress facilitate the nutrients uptake by altering nutrient form and microbial population.  Nitrogen use efficiency is enhanced by allelochemical by process of biological nitrification inhibition. Sorgoleon reduce the population of nitrifying bacteria (nitrosomonas and nitrobacter) and promote BNI. Allelochemicals released in nutrient stress, interact with microbes and facilitate in nutrient solublization. Under low phosphorus level, plants release phosphatases, which improve phosphorus availability by hydrolysis. Citric acid and oxalic acid also improve nutrient uptake of K, P Fe and Mg under deficit conditions. Allelochemicals are also helpful in reduction of nutrient toxicity through chelation. Allelo-chemicals help in signalling during stress condition, which helps in mitigation of stress effect. Moreover, allele-chemicals under salinity, heat and drought stress produced reactive oxygen species which in turn activate antioxidant defense system.

Allelochemicals can also be used as seed treatments which in turns improved the plant growth and development. Seed treatment by using moringa extract enhances the growth and yield of different vegetables and sugarcane. When allelochemicals are applied in low concentration for seed treatment, they enhance germination power, germination index, plumule length, rdical length. Allelochemicals through root application also improved nutrient uptake.  Allelopathy can replace hazardous mechanical and chemical approach in Agriculture. It is a good approach to achieve resource conservation, economic stability, food security, environmental safety and agricultural sustainability. Research efforts should be focused on screening more allelopathic plants and to identify promoting allelochemicals in plant water extracts.

This article is collectively authored by Maria Batool, Dr. Imran Khan, Dr. M. Umer and M. Umair Hassan Department of Agronomy, University of Agriculture Faisalabad.