INCREASING GLOBAL population demands high crop production to meet the food requirements. Sustainable agriculture ensures the food security, food safety and environmental protection simultaneously. Allelopathy is one of the key processes being used in organically managed agro-ecosystems. It is a natural ecological phenomenon in which plants or micro organisms influence the growth and functioning of others in their vicinity by releasing specific chemicals known as allelochemicals. These chemicals are released in environment by volatilization, leaching, decomposition and root exudation.
Allelopathy has two dimensions; inhibitory and promotory. Inhibitory role of allelopathy has been intensively studied and has been utilized effectively in organic weed management. But its growth stimulatory effects are less explored and this novel aspect was neglected so far. Allelochemicals are naturally released in aerial and soil environment but the use of allelopathic crop water extracts as foliage application has shown good results regarding weed control. Allelochemicals are concentration dependent. Important secondary metabolites identified as allelochemicals are phenolics, alkaloids, flavonoids, terpenoids, momilactone, hydroxamic acids, brassinosteroids, jasmonates, salicylates, glucosinolates, carbohydrates and amino acids. These chemicals have inhibitory and promotory effects and also imparts resistance against several abiotic stresses by improving physiological and biochemical adaptations of plants.
They increase crop growth at low concentrations while suppress the growth at high concentrations. Most of the allelochemicals are water soluble and water is the best medium and carrier for such chemicals. Recent research has shown that water extracts of sorghum, brassica, sunflower, rice, wheat, barley and moringa having promotory allelochemicals, improve growth of different arable crops and vegetables, when applied at low concentrations.
Crops with promotory allelochemicals
Laboratory as well as field research trails have revealed that;
l Moringa (Moringa oleifera) leaf extracts are growth and yield (20-35 per cent) enhancer in different vegetables and sugarcane, when applied at diluted (30 times) rates.
l Very low concentrations of alfalfa leaf extracts are promotory for alfalfa root growth and proliferation.
l About 52, 42 and 42 per cent increase in maize yield has been reported under foliar application of 3 per cent moringa, 3 per cent sorghum and 3% brassica water extracts, respectively.
l Foliar application of 2 per cent brassica, sunflower, moringa and rice water extracts promote wheat grain yield significantly.
l Foliar application of sorgaab (sorghum extract) has promotory effects on growth and yield of crops as allelochemicals present in it show stimulatory behavior at diluted concentrations.
Growth promotion by low concentrations of secondary metabolites has been proved by different researchers and different mechanisms have also been proposed which are responsible for it. But it is very hard to determine a definite mechanism due to diverse nature and complicated responses of such chemicals under natural conditions. Secondary metabolites at low concentrations help to induce germination by breaking seed dormancy, promote root growth by improving moisture availability and temperature regulation, enhance mineralization of nutrients and improve their uptake. They also induce tolerance against biotic and abiotic stresses to ensure plant defense and survival. As antioxidant, potential of phenolics (e.g., flavonoids, lignins and tannins) under sub-optimal conditions have been elucidated by several researchers. They regulate maturity, senescence water relations, translocation of assimilates and quality. Plant hormones have a positive role to play in physiological processes like seed germination, root growth, chlorophyll accumulation, photosynthesis, transpiration, leaf expansion, translocation and genetic encodings. Hence they have positive effects on different physiological, chemical and biochemical processes at low rates and resultantly on crop growth and development.
Allelopathy can be employed for crop growth promotion and yield maximization in modern agriculture avoiding the environmental hazards of chemical growth regulators. Scientists have done a lot of work on synthetic plant growth regulators but did not get much success regarding their adoption in farming community especially in developing countries like us. This failure is mainly due to high prices of such growth promoters. Whereas, allelopathic water extracts are cost effective, eco friendly and efficient. They have no negative impact on environment and are already present at the door step of farmer. We just have to optimize their concentration which can perform best in the field. This work is in augmenting progress at Department of Agronomy, University of Agriculture Faisalabad and encouraging research findings are reinforcing the concept and its application in todays agriculture.
In near future the use of allelopathy for crop growth promotion will be the most acceptable and applicable option because of its numerous and long term advantages. In the long run it can ensure the provision of wholesome and nutritious food around the clock.
The writers Ali Ahsan Bajwa, Dr. Muhammad Farooq and Zahid Ata Cheema, are associated with Allelopathy Lab, Department of Agronomy, University of Agriculture, Faisalabad.