| Allelopathy refers to the beneficial or harmful effects of one plant on another plant, both crop and weed species, by the release of chemicals from plant parts by leaching, root exudation, volatilization, residue decomposition and other processes in both natural and agricultural systems. Allelopathy has been used in agricultural practices, such as weed control, intercroppings, nutrient recycling, and low-external input farming practices, The allelopathic plant products are known to offer a vast array of secondary compounds which have the potential to be used directly as herbicide substitutes or as structural leads for new synthetic herbicides.The chemicals may reach other plants (receivers) through transport from the donor plants in the soil and may induce the inhibitory or stimulatory activity on the receiver plants. In addition to the physicochemical property of the allelochemical, phytotoxic activity may be affected by many factors. Current evidence illustrates the significance of soil microbes in influencing the bioavailability of allelochemicals. Soil microbial community may serves as the bridge in connection of donor and receiver plants, the donor plant could inhibit the natural plant growth and reproduction by changing the soil microbial community. Thus, to study root-root and root-microbe communications and root exudates transfer and activity in the rhizosphere will lead to a better understanding of the processes mediating plant-plant and plant-microbe recognition and communication.Accordingly, the author introduced a new method known as agar-soil-sandwich-method(ASSM) for the evaluation of rice allelochemicas activity in the soil. The results showed the phytotoxity of root exudates in the soil were that in rhizosphere soil > that in root zone soil >that in beyond root zone soil in order. It was also found that the soil samples from allelopathic rice accessions IAC47, PI31277 and Iguape Cateto always showed the stronger inhibitory activity on the target plant, Lettuce than that from their counterpart Lemont. The findings suggested that the exudates released from the roots of the donor plants might be absorbed by soil or decomposed by soil microorganisms. This also implied that there might exist stronger degrading microorganism in the rhizospheric soil of non-allelopathic rice accession Lemont and in turn led to decreased inhibitory effect mediated by rice exudates. .Microbial biomass C content and respirationin the rhizosphere soil ranged from 283．8～441．0mg/kg and 0．304～1．404μgC·g-1·h-1 respectively, Allelopathic and non-allelopathic rice accessions performed significantly higher content of microbial biomass C and respiration in the rhizosphere than no-rice check (CK1) , implying that the exudates might promote the rhizomicroorganisms. It was found that microbial biomass carbon contents in rhizosphere soils of Iguape Cateto(441．0 mg·kg-1 ) > that in IAC47(389．7 mg·kg-1 ) > that in PI312777(333．2 mg·kg-1 ) >that in Lemont(CK2) (283．8 mg·kg-1 ) > that in CK1 (129．3mg·kg-1 ) in order. The respiration rate of the three different rhizospheric soils as described above were that of the soil from PI312777 (1．404μgC·g-1·h-1 ) > that from IguapeCateto (1．019μgC·g-1·h-1 ) >that from IAC47 (0．671μgC·g-1·h-1 ) >that from Lemont(CK2) (0．488μgC·g-1·h-1 ) > that from CK1 (0．304μgC·g-1·h-1 ) in order.After investigating the number of cultural bacteria, actinomyetes and fungi variation in root-zone soil in the different time courses at rice seedling stage, As rice grew, The number of bacteria and actinomyetes in the root-zone of allelopathic and non-allelopathic rice shown an increasing tendency, especially in the late time courses at the rice seedling stage, but the change of fungi had no meaning.Using terminal restriction fragment length polymorphism (T-RFLP) analysis targeting the 16S bacterial communities in root-zone soil of allelopathic and non-allelopthic rice ribosomal DNA, differences in the bacterial community structure in the root-zone soil were detected in the form of different profiles of terminal restriction fragments (T-RFs). PCA scores of T-RFLP soil bacterial communities associated with allelopathic and non-allelopathic rice were different at 95% significance level. There were striking differences among the fingerprint patterns of the different root-zone soils,A total of 84 fragments were included in the analysis. Of those fragments, 72 ones were detected in the no-rice check (CK1) , 79 in all allelopathic rice soil samples (Igupae Cateto, IAC47, PI312777) , all fragments (84 ) in the soil sample from the non-allelopathic rice accession. It was also found that 72 fragments had a universal distribution, which were detected in all root-zone soil samples from rice accessions and no-rice check., the other 12 ones were unique in rice soil samples. This suggested that the root exudates from rice accessions including allelopathy and its counterpart appeared to have positive effect on rhizobacterial community. Among the 12 variable fragments, with the rice exudates released to soil, the microbial communities in the root-zone soil of rice accessions destabilized.eight fragments (136,138,427,438,431,467?483 bp) were found only in root-zone soil of rice (IAC47,PI312777,Iguape Cateto, Lemont), but Lemont showed larger peak areas than allelopathic rice accessions, and five fragments (145,154,168,169,452 bp) were found exclusively in root-zone soil of Lemont. The fragments matched the size of the theoretical T-RFs of species within the RDPⅡDatabase. It was found that there were many plant-mediated bacteria which functioned in degrading phenolic and some other compounds (allelochemicales) in root-zone soil of allelopathic and non-allelopathic rice accessions such as: Bacillus, Eubacterium, and the non-allelopathic rice showed stronger positive effect on them , and the five unique fragments only found in the soil sample of no-allelopathic rice Lemont were identified as much more stronger phenol-degradating microorganisms in the root zone soil of Lemont. Consequently, phenolic acids in the root-zone of soils Lemont might be reduced their real concentrations in the rhizospheric soil under non-allelopathic rice accession through the biodegradation. and therefore chemicals might not be accumulated to phytotoxic levels, even through there really existed allelochemicals in the exudates from the non-allelopathic rice , which was consistent with the result from the trial using ASSM method. Compard to CK, Bacteria in the root zone soil of allelopathic rice was not decreased, and seven unique T-RFs , which was as the same as non-allelopathic rice, were also detected and the reason was considered the induction by root exudates from rice. It was also believed that allelopathic rice root might excrete some kinds of chemical compounds to inhibit degrading bacteria, and in turn to overcome the problem that the putative allelochemicals were breakdown by those soil microbial flora. The findings strongly suggested that the allelopathic effect mediated by plant exudates was closely related to the selective function to the rhizosphenc microorganisms droved by the exudates. It is very important to further invesitigate why exclusively bacteria populations were found in rhizosphere soil of non-allelopathic rice and how they interacted, is the key to develop the potential of allelopathy and put it into agricultural practice as soon as possible.
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