Wheat Allelopathy And Its Rhizospheric Ecology

Allelopathy was one of the most important chemoecological phenomenon, which refers to one plant(donor) impact the growth and development on its neighboring plants (accepter) through the release of secondary metabolism chemical compounds into the environment via volatilization, leaching, excretion, and decomposition. Using wheat allelopathy to control weeds in agroecological system, a low dosage, high selective and without "3R" problems(resistance, rampancy and residue) in weed control, will be a key technology for controling weed friendly in 21st century.Recently, the reaserch for wheat allelopathy was mainly focus on the screning of allelopathic germplasm, identification of allelochemicals, and biological characteristics of allelopathy, such as genetics diversity, QTLs location, physiological response, and its genetic control. Little work was done on the ecology of wheat allelopathy in rhizosphere. In this paper, we aimed to elucidate the ecological characteristics of wheat allelopathy in rhizosphere, 6 screening methods were employed to evalute the allelopathic potential of 90 wheat acessions, their agronomic characters and genetics diversity were tested as well. For the more, allelopathy and resoure competetion between wheat and weeds, the physiological characteristics of weed control, microbal population in rhizospheric soils, allelochemicals in the plant and rhizospheric soils, and allelochemicals effect on the growth of accepter plants, soil microbe and enzyme activities were documented. The result was mainly concluded as below.(1) Six different methods, such as soil-agar-sandwich method(SASM), aqueous extract screening bioassays(AES), relay seeding in agar(RSA) and field investigation, were employed to evaluate the allelopathic potentials of 90 wheat acessions introduced from both home and abroad. The result offered three strong allelopathic wheat acessions: Jin 2148, 115/Qinghaimai and 92L89, and 3 weak ones: 9813, 8401 and 10103 for wheat allelopathy reseach. It was also confirmed by experiments that SASM was a simply feasible method for screening allelopathic wheat acessions.(2) Multiple correlation ananlysis was calculated among wheat agronomic characters, yield characters and the IRs of lettuce root length growth showed that IRs were negative correlation with plant height, spike length, leaf length, leaf breadth, arista length. In reverse, IRs were positive correlation with spike number, plump grains, number of grains, setting rate, thousand grains weight and cauline thickness. But all the correlation were not significantly at p<0.05. It suggested that the charater of allelopathy in wheat was not notable linkage to its agronomic and yield characters, and new wheat varieties possess both high yield and strong allelopathic characters can be achieved by hybridize breeding.(3)In order to estimate allelopathy potential resonably, allelopathy-competition separation method (ACS), a methodology containing four gradient nitrogen application was introduced to evaluate allelopathy and resource competition effects included in the biointerference. The result showed that the allelopathic effect on the growth of Alopecurus aequalis increased with the decline of nitrogen application levels for all the tested wheat acessions. Strong allelopathic wheat acessions suppressing the weed mainly by allelopathic effect, for 115/Qinghaimai, allelopathic effect occupied 48.5% and resoure competition effect was 25.4%. For 92L89 allelopathic effect occupied 41.9%, resoure competition effect was 27.5%. While weak allelopathic wheat acessions Kangl0103 was in reverse, allelopathic effect occupied 17.8% and resoure competition effect was 27.7%. Different ecological strategy in weed suppression was found between strong and weak allelopathic wheat acessions.(4) The physiological response for Alopecurus aequalis was determined by treating the plant with water extracts from allelopathic wheats. The result showed that allelopathic wheat siginificantly suppressed the growth of Alopecurus aequalis, performing low root number, root length, fresh weight of root and low root vigor. Simultaneously, It initiated the protection enzyme system in Alopecurus aequalis, higher enzyme activities of SOD, POD and CAT were detected in the tissue of root and leaf the weed. The higher MDA content in the weed implied that wheat water extracts doharmful to root and leaf of the weed, and root was more sensitve than leaf especially for 115/Qinghanmai. The MDA content in the root tissuse treated with 115/Qinghanmai was 10.88 to 25.53 times as higher as that in control, acompanying with higher flavonoids c and total phenols contents in the tissues of Alopecurus aequalis leaf and root. Further experiments by sand culture showed the same trend in the physiological response for Alopecurus aequalis. higher MDA content was found in the leaf, suggested that obvious lipid peroxidation occurred in the leaf, and reduced the content of ZR, GA and IAA, which pulled down the growth of weed both in root and shoot of the plant, demonstrating the allelopathic effect of wheat.(5) The allelopathic potential declined after the soils were sterilized, it indicated that allelopathy of rhizospheric soils was related to the microbial flora in the soils. The total number and population components of rhizospheric microbial flora showed significant difference among the three wheat cultivars. Among the three major microbial species, bacterium was the predominant one, accounting for 80.4%～83.9% of the total microbial population. The following -up was actinomycete with 15.6-19.1%, and the last one of fungus with only 0.4%～0.6% of the total microbial number. In addition, the cultivation of allelopathy wheat was likely to enhance the abundance of microbal components of azotobacteria, denitrifying bacteria, aerobic cellulose decomposing bacteria, thiobacillus.BIOLOG analysis showed that the value of Average Well Color Development (AWCD) differed significantly among wheat cultivars. According to the AWCD incubated for 72 h, Principal Component Analysis (PCA) identified 3 principal component factors (PCF) in relation to carbon sources, accounting for 55.8%, 31.9% and 12.3% of the variation respectively. Amino acids and carbohydrates were the two main carbon sources separating the 3 principal component factors. Furthermore, soil enzyme activities were found among the rhizospheric siols cultivated with different allelopathic potential wheats.We extracted DNA from the soils, combing the technology of T-RFLP, five common bacteria were found in the rhizospheric soils of 115/Qinghaimai and 92L89, they were putative to be Bacillus, Azoarcus, Blastomons, Listeria, Desulfobacter latus. This bacteria involved in nitrogen fixation, nutrient cycle, pestcise degradation and the rehabilitation of soil contaminated. Allelopathic wheats mostly promoted the population of bacteria, Actinomycete and azotobacteria, which enhance the adaption ability for wheat.(6)Thirteen species of allelochemicals were identified by GC/MS from leaf, root and rhizospheric soils of wheat, they were phenolic acid, carboxylic acid, amine, terpene, aether, ester, purine, hydrocarbon, phenol, alkaloid, ketone and hydroxamic acid. It was found phenolic acids, such as p-hydroxycinnamic acid, p-hydroxybenzoic acid, in all the wheat leaf, root and rhizosperic soils. The simulant experiment in laboratory showed that the absorption and desorption process of p-hydroxycinnamic acid in the soils can be perfectly fitted by the equation of Y(μg) = b₀ + b₁V + b₂V².The bioassay in laboratory showed that p-hydroxycinnamic acid has the suppressing effect on the growth of Alopecurus aequalis. Furthermore, p-hydroxycinnamic acid and cinnamic acid obviously changed the components of microbal flora and enzyme activities in the soils, which enhanced bacteria population, inhibited fungi population, and for actinomycete, it was inhibited at high concentration and enhanced at low concentrtion. At the same time, both p-hydroxycinnamic acid and cinnamic acid promoted the activities of urease, protease, sucrase and Phosphatase. Microbiology ananlysis by the technology of T-RFLP found employed to were found the soils treat with the two phenolic acids was benefitial to the establishment of Brevibacillus, Sulfobacillus, Bacillus stearothermophilu, Eubacterium, Burkholderia, Desulfuromonas, Micrococcus, Campylobacter and the cycling of substances in the soil.