**3. Allelopathy promotion plant growth**

Plants Growth promotion by other plants, as well as that of microorganism by plants and other microorganisms, is discussed. Agro system in mixed culture with wheat enhances growth and yield of wheat. Soil amended with shoots of *Solanum nigrum*, enhances the soybean growth and nodulation. Growth and yield of several legumes are enhanced by mixed culture with *Heliotropium peruvianum* [29].

The use of biostimulants, which are defined as substances or materials other than nutrients and pesticides that can be used to regulate the physiological processes in plants to stimulate their growth, Biostimulants promote plant growth and development throughout the crop's life cycle, from the seed stage to mature plants, by improving metabolic efficiency, resulting in increased yield and enhanced crop quality, and facilitating nutrient assimilation, translocation, increasing nutrient use efficiency continues to be a major challenge for world agriculture [30].

Thereby increasing plant tolerance to and recovery from abiotic stresses, allelochemicals are among biostimulants.

In laboratory and field has been reported to inconsistency between plant growth-promoting rhizobacteria effectiveness "PGPR" that stimulate growth of plant and yield.

In field this inconsistency results from PGPR applications can be solved by improved knowledge of interplay between host and introduced PGPR inoculant in rhizosphere under field conditions to reduced chemical fertilizer quantity can application of biofertilizer for maintaining threshold levels of crop productivity [31].

(PGPR) may be encourage plant growth by producing growth regulators; facilitating of nutrient uptake; mineralization accelerating; plant stressreducing; nodulationstimulating, providing nitrogen fixation; mycorrhizal promoting; suppressing plant diseases; and functioning as nematicides and insecticides. There are many PGPR are fluorescent pseudomonads (*Pseudomonas fluorescens*); and other bacteriaare known as well as (Bacillus sp., Azotobacter sp., Acetobacter sp., Azospirillum sp.).

Allelochemicals released by plants have promotory effects at low concentrations [32, 33]. Previous studies have elucidated the positive role of secondary metabolites, hormones and some other natural compounds produced by plants, in plant growth promotion [34].

Allelopathic effect in nutritional and ecological relevance in the soil system.

Relationship between soil characteristics and allelochemicals at two ways The level of phytotoxicity is affected in soil characteristic, and they are closely linked to each other and effect on retention, transport and transformation processes of allelochemicals in soil. We need to understand the interactions involved in soil allelopathy and to create new opportunities for a sustainable control of agroecosystems.

Allelopathy has offered a new alternative for the development of eco-friendly agricultural practices, with the dual purpose of enhancing crop productivity and maintaining ecosystem stability [35]. Allelopathy involves the positive or negative effects of a plant (donor), including microorganisms, on neighboring plants (targets) through the release of chemical compounds into the environment by (leaching, volatilization, root exudation, decomposition) **Figure 1**, mostly in the soil. According to [36], it is possible to distinguish between direct plantplant allelopathic interference (allelopathy in the narrow sense) and indirect allelopathy.

Influence of meteorological, soil and plant factors on the phytotoxicity of allelochemicals in soil [37]. Meteorological and plant factors can be effect in quantity and quality of allelochemicals released from plant (donor). When released to soil system, several of soil factors may be influence retention; transport and transformation processes of allelochemicals in soil and their presence in soil solution in order to be absorbed by other plant (target).

Allelochemicals effects on abiotic and biotic soil processes can effect in other plants. Aldrich [38] described these two kinds of allelopathy as true and functional allelopathy. Allelochemicals that released into soil can (1) effect directly on organisms, (2) can be that degraded or transformed with effect of soil microorganisms, (3) a third species may be induce to produce another compound which interferes with donor plants and (4) cause changes to soil abiotic factors that affect target plants.

It is impossible to separate direct from indirect allelopathic effects in field conditions and to assert that direct allelopathy is solely responsible for an observed phenomenon in the field, because many abiotic and biotic soil factors influence the fate of allelochemicals. Therefore, indirect allelopathic interactions, from an ecological point of view, are probably more important in plant communities than direct ones [36].

**Figure 1.** *Methods of released allelopathic compound in to environment.*
