Allelopathy is a prominent ecological phenomenon playing a vital role in ecosystem sustainability and productivity. Scientists have recognized that it could be improvised to get benefits in crop production. Weed management, disease control, pest management, stress tolerance and growth promotion are its major fronts in agronomic preview. It is necessary to alter the innate tendency of crops to produce and release allelochemicals for better crop management. Interest is developing among researchers to modify cultivars for high allelopathic potential. Natural weed management is inevitable because of resistance against herbicide from major weed flora and environmental concerns related to herbicide use. So, the utilization of technology for improving allelopathic potential of crop cultivars is affirmative.
Breeding efforts: Breeding is touch older but still the most effective way of genetic improvements. Currently it is required to breed smothering crops having high allelopathic activity against weeds minimizing the use of chemical weedicides. The improved cultivars offer more resistance to weeds and better able to perform in integration with herbicides. Intensive agriculture requires such complementary natural approaches that can provide sustainable controls alone as well as in addition to conventional measures being used. Within a species different cultivars have varying degree of allelopathic potential. So, to start a breeding programme for allelopathic potential enhancements, screening of more feasible ones is very much significant.
Genetic variability is well evident among cereals, legumes and some other potential allelopathic crops. They affect associated weeds at different rates and thus acceptance as weed management tool is purely dependent on comparative efficacy. For instance, scientists have bred a smothering plant through crossing dwarf and perkinensis genotypes of Brassica compestris. This new line of Brassica provided very effective weed control when inter-planted in maize and soybean fields. It controlled weeds during critical growth periods without damaging the host crops. Rice is another important allelopathic crop on which breeding focus has been diverted by scientific community. In different research studies potential accessions were screened successfully to be employed in breeding programs. Some accessions were very efficient in suppressing Jungle rice, Red rice and many noxious weeds of rice. So, it is wise to select those genotypes which already have higher amounts of allelochemicals and then breed them with those having lesser potential. In another study the influence of rice root exudates of many genotypes on weeds was observed. Indica rice was proved more allelopathic and suppressive as compared to Red rice and Japonica rice. It is attributed to its high allelochemical contents. The activity was evaluated on the basis of weed biomass and weed density parameters. The examination of 27 rice lines proved some genotypes highly suppressive in terms of weeds root growth and they were superior up to 90 per cent. Similarly Javanica rice offers 30 per cent more suppression of barnyard grass than Japonica cultivars. Such variability is the base of encouraging breeding for allelopathic cultivars. It is more optimistic and feasible to introduce new cultivars of same crop rather than breeding different species. Highly suppressive rice cultivars are commercially at disposal in China and USA.
Weed suppression through allelopathic crops is majorly through the release of allelochemicals and somewhat through competition. In field conditions distinction between allelopathy and competition is very difficult. However, laboratory bioassays and greenhouse trails on genotypic variation under same light, water, and nutrients offer better understanding of the phenomena.
Interestingly, present cultivars are more allelopathic as compared to ancient ones. It may be due to changing climatic patterns and natural modifications in crop cultivars making them able to trigger biosynthesis of secondary metabolites for antioxidant activities. So, breeding old with modern cultivars is another successful option. Eco-variations and interaction between genotypes and environment can make phenotypic selection a bit complex. Varietal differences for weed control are unpredictable under spatial as well as temporal fluctuations. Thus screening, identification and isolation of potential genotypes should be done after considering environmental changes, cropping patterns and ecological trends. Breeding cultivars to impart allelopathic potential is an important and pragmatic tool for sustainability of modern day farming.
Biotechnology and genetic engineering tools:
Introduction of desired traits in individual plants and then successful transfer of such characteristics to offsprings are no more a dream just because of legacy of biotechnology. Genetic engineering is flourishing at an impressive pace in agriculture. Success stories like BT cotton, round-up ready maize, glufosinolate resistant rice and other genetically modified diseases, pests and weed resistant crops are symbol of excellence of agro-biotechnology. Working on the same lines the novel idea is emerging to develop transgenic crops having high allelopathic potential. Such crops will be able to withstand biotic and abiotic stresses better due to improved genetic makeup offering quick activation defensive mechanisms like allelochemicals synthesis and release, efficiently.
Although, the use of biotechnology to improve crops allelopathy is rare and still at primary stages but is promising. A large pool of winter wheat accessions was genetically screened to assort allelopathic active genes. The selection is being used for quantitative mode of inheritance. Genotypes with high allelopathic potential produce genotypes having uniform distribution of allelopathic activity after cross with those having lower activity. Elite work has been done to map the allelopathy genes in wheat. Hydroxamic acids are potent allelochemicals of wheat and gene responsible for their expression has been discovered and can be transferred. Genetic engineers are now able to define and identify quantitative trait loci (QTLs) which are actually the exact sites of genes responsible for a particular trait on chromosomes. QTLs for hydroxamic acid are present on chromosomes 4A, 4B, 4D, and 5B of wheat genotype. Mapping of QTLs in a double haploid population was also done successfully which was obtained from the cross of two cultivars, one being strongly allelopathic and other being less allelopathic. For mapping these QTLs, scientists use:
Amplified Fragment Length Polymorphism (AFLP)
Restriction Fragment Length Polymorphism (RFLP)
Simple Sequence Repeat Markers (SSRM)
These markers are used as portraying tools for identification of quantitative as well as qualitative loci. Lot of work has also been done on rice QTL mapping.
Genes responsible for regulation and biosynthesis of allelochemicals can be identified through isolation, discovery, activation tagging, purification of plant enzymes, purification of and related bioactive metabolites and through gene knockout libraries. Particular genes responsible for the biosynthesis and regulation of allelochemicals like momilactones, phenolic compounds and benzoxazinoids has been reported. Antisense knockout techniques and over expression of genes can be used to change the quantity and quality of secondary metabolites of allelopathic plants.
Allelopathy can be successfully engaged for weed management in modern agriculture. Allelopathic potential of many arable crops have been identified but the cultivars differ in terms of allelopathic expression and weed suppressing ability. Such variations within the species make it necessary to screen cultivars with high allelopathic potential and then breed them with high yielding cultivars to integrate desired features. Breeding allelopathic cultivars should also consider the climatic changes so that a perfect set of genetic, environmental and management options can be developed.
In addition to conventional breeding, modern biotechnology and genetic engineering tools are getting popularity for improvement of allelopathic crop potential. Fortunately, transgenic plants with desired allelopathic potential can be developed through genetic engineering. Identifying QTLs and genetic makeup of allelopathic genotypes is revolutionary tier in plant sciences. Furthermore, expression of multiple desired genes into crops and gene silencing techniques are bringing crop production systems to a sustainable stand. It is expected that improved allelopathic profile of cultivated crops will enable us to ensure the organic weed management and thus enhanced crop productivity.
The authors are associated with the Department of Agronomy, University of Agriculture, Faisalabad, Pakistan.