The green revolution in the 1960s and increased demand for food. The agricultural sector and farmers also tend to spend more time outside agricultural work. But the weeds are a major constraint limiting crop yield in agricultural systems and in organic systems in particular [1]. Weeds reduce crop yield by 5% in the most highly developed countries, 10% in the less developed countries and 25% in the least developed countries. Hence, herbicide insecticides, fungicides and others use has been increased in the last few decades. A herbicide use has been increased many fold. According to the statistics of the total pesticides used worldwide, herbicides accounts for 37%, while 24%, 9% and 29% are insecticides, fungicides and others. In which 332 herbicide resistant biotypes including 189 species (113 broad-leaved and 76 grasses) have been reported in more than 0.30 million fields worldwide. Although herbicides are efficient for weed control, continuous use has caused the development of resistance in weeds against several herbicides [2]. Furthermore, herbicides also pollute the soil, water and aerial environments and herbicide residues in food have deteriorated food quality and enhanced the risk of disease [3]. Therefore, there is growing public as well as scientific concern about the use of herbicides.

Allelopathy is a phenomenon in which one plant inhibits growth of other plants through release of allelochemicals. It is a phenomenon in which the seconddary metabolites, produced by plants, microorganisms, viruses, and fungi stimulate or suppress the growth and development of agricultural and biological systems (excluding animals). The crops inhibit weeds—A very promising, it has emerged as an alternative to synthetic herbicides for weed management in field crops [4,5]. It can reduce herbicide use to obtain eco-friendly and costeffective weed control. Earlier, confirmed, a large number of crops and trees has been found to possess allelopathic potential [6]. International research on the allelopathy of wheat is active [7]. Research on wheat allelopathy has progressed rapidly from the initial phase of evaluation of wheat allelopathy to the identification of wheat allelochemicals, the degradation of these compounds andfurther, to the identification of genetic markers associated with wheat allelopathy [8,9].

Wheat (Triticum aestivum) has been extensively studied for its allelopathic potential. The common allelochemicals from crop plants are generally secondary metabolites. These include phenolics, terpenoids, alkaloids, coumarins, tannins, flavonoids, steroids and quinines. The allelopathic activity of wheat has been attributed to hydroxamic acids and related compounds and phenolic acids [22]. In general, the allelochemicals of plants to the environment is as follows: 1) exudation from roots, 2) leaching from plants by fog, mist and rain, 3) decomposition of residues [23]. In which, crop plant root exudation has been chemically investigated from as early as 1921 when Lyon and Wilson studied the liberation of organic substances through the roots of growing plants. Further suggest that organic substances can interfere with basic processes of receiver plants as photosynthesis, cell division, respiration and protein synthesis [24] and indirectly provoke other forms of stresses. Another important effect of these allelochemicals is the activation of cellular antioxidant system in response to uncontrolled production and accumulation of reactive oxygen species [25]. Rovira has written a review covering plant root exudation. Allelopathic secondary metabolites such as methoxyphenylacetic acid have been observed to move from one plant to another through their root systems and as one of the important world crops, the root exudates from wheat have become a focus of allelopathic study [26-29].

Allelopathy in wheat is complex and determined by many known, partially known, and unknown factors. It has been described as a dark grey system (grey theory) [36]. Allelopathic potential in wheat and genetic, chemical and ecological factors and a synergistic relationship.

Heritability and its degree of influence on yield were more evident in the vegetative growth stage compared to the reproductive stage. Research indicated that allelopathic effects exhibited high heritability (55% - 95%) throughout the life cycle of wheat. In which, Heritability was highest in the tillering stage and weakest in the seed filling stage [37]. Allelopathic potential varied and was discontinuous throughout the wheat life cycle. Implying that multiple genes participated in the expression of allelopathy. In addition, that genotype · tillage interaction was reported in tests involving diverse genotypes. In a comparison of a conventional cultivar (Janz) and a novel experimental line (Vigor 18) bred for high leaf vigour, found that the latter grew best in unploughed soil. They suggested faster root growth, different exudates promoting more beneficial rhizosphere microflora, or modified shoot responses as possible mechanisms for the superior growth of Vigor 18. Hence, vigorous genotypes may present an opportunity for increased productivity under reduced tillage [38]. The genetic background for allelopathic potential in crops and the inner genetic mechanisms for allelopathic potential are gradually becoming known through the introduction of molecular biological techniques [39]. For example; Wheat-rye translocations involving 1 RS are also widely used in wheat breeding programmes [40] and have been shown to improve root characteristics, it to be used in breeding programmes to improve weed suppression ability of wheat. The most triticale cultivars, weed biomass could be reduced by 18% - 28%. If early vigour is also improved to the level of triticale, weed biomass could be reduced by 40% - 60%. In spring wheat, an improvement of PAAr by 20% resulted in an average reduction in weed biomass of four near isogenic breeding lines by 19% [41]. Some of the wheat-rye substitution lines showed a PAA value as high as 63%. Further screening it may be possible to find lines with perhaps 70% root inhibition or more. PAA values of that magnitude in new wheat cultivars could provide a viable alternative to the use of herbicides or at least allow reduced use of herbicides [42].

Autotoxicity is an intraspecific allelopathy occur when a plant species releases chemical substances that inhibit or delay germination and growth of the same plant species. Application of the concepts of allelopathy and autotoxicity in depicting the chemical interactions between varieties [43]. First appeared in an early report, who claimed that the roots of wheat, oats, and certain other crop plants exude chemicals inhibitory to their own seedlings. Since then, autotoxicity has been identified in many field crops, including alfalfa, rice, barley, wheat, asparagus and cucurbit crops [44,45]. Study display that, aqueous extract of wheat differed in varietal autotoxicity and varietal allelopathy, inhibiting wheat germination by 2% - 21%, radicle growth by 15% - 30%, and coleoptile growth by 5% - 20%. These results suggest that careful selection of suitable wheat varieties is necessary in a continuous cropping system in order to minimize the negative impacts of varietal allelopathy and varietal autotoxicity [46]. Confirmation under controlled environments showed that wheat residue water extracts were autotoxic to the germination and seedling growth of wheat. In solution culture, Protic, et al., found that wheat seedling root volume, total root area and active root area were decreased with an increase in the quantity of the wheat residue amended into the nutrient solution [47,48].

Wheat has caused allelopathic inhibition to the growth and yield of crops such as rice, barley, rye (Secale cereale L.), cotton (Gossypium hirsutum L. Merr) and soybean (Glycine max L. Merr). Wheat straw was also allelopathic to a number of forage crops including sorghum (Sorghum bicolor L. Moench), sunflower, pearl millet (Pennisetum glaucum L.), clusterbean (Cyamopsis tetragonoloba L.), cowpeas (Vigna unguiculata L. Walp.) and mulberry [49,50].

Allelopathy has been a core theme of the agricultural research, it has been a complex and variable topic. The main reason for the variability of allelopathic crop materials involves effectiveness of cooperation and the antagonism mechanism. Modern agricultural research investigating it on the basis of genetic level mainly focusing on the relationship between each genetic factor which has achieved certain results, yet there still have been more potential problems to be further solved. Wheat is the stable food for the mankind, therefore, conducting more research on wheat allelopathy in regards to the effects to the yield and quality of crops is significant in order to contribute to the human life as a whole.

This study was supported by Funding from Innovation and Technology Support Programme, the Government of the Hong Kong Special Administrative Region No. ITS/313/11 and Seed Funding Programme for Basic Research from The Hong Kong University No. 201111159043.