The Chemical Substance Basis Of Allelopathic Rice Weed Control Circle

The allelopathic rice PI3123777 and nonallelopathic rice Lemont were selected as culture material in the weed inhibitory circle method which rice and barnyard grass(target plant)cocultured in pot tests.The rhizosphere soils of rice(inner soil)and barnyard grass(outer soil)at 12-cm distance from rice seedlings,the effective inhibition distance on target weed(inhibitory rate>50%),were sampled to explore the chemical components,contents and their inhibitory effect.The details of results were showed as follows:1.The inhibitory rates of PI inner soils on the root length of barnyard grass were decreased significantly after extracted by water,compared with the untreated soils.There were some effect on the root length of barnyard grass when the PI inner soils after extracted by 70%ethanol,compared with the untreated soils.The inhibitory rates of water extracts and 70%ethanol extracts from the PI outer soils on the root length of lettuce were 42.6%and 21.4%,respectively.These results indicated that both water-soluble and ethanol-soluble compounds in the PI outer soils contained the putative allelochemicals.2.The different types of phenolic acids in the rhizosphere soils were extracted with water,water in an autoclave,0.25 M citric acid/citrate(pH=7.0)and 0.5 M NaOH,respectively.By Folin-Ciocalteu method,the contents of total phenolic acids had no significant different between the inner and outer soils,both in PI and in Le.However,the contents of available phenolic acids in soils were significant higher in PI than that in Le.HPLC analyses indicated that total amounts of 8 putative phenolic acids was significantly high in outer soil of PI than that of Le.In the meantime,total amounts of 5 benzoic derivatives(i.e.p-hydroxybenzoic,protocatechuic,salicylic,,vanillic and syringic acid)was higher in PI outer soils than that in PI inner soils.On the contrary,total amounts of 3 cinnamic derivatives(i.e.cinnamic,p-coumaric and ferulic acid)was less in PI outer soils than that in PI inner soils.However,there was no difference between the outer and inner soils of Le.In addition,total amounts of 3 cinnamic derivatives with reversible adsorption and non-reversible adsorption in PI outer soils were 11 and 2 times larger than that in PI inner soils,respectively,whereas,no significant difference in total amounts of 5 benzoic derivatives.3.GC/MS analyses showed that there were a large number of oxygenated monoterpenoids in the inner and outer soils of two rice cultivars.The amounts of total terpenoids and monoterpenoids in the PI soils were more than that in the Le soils.Furthermore,the percentage of oxygenated monoterpenoids in the outer soils of PI and Le was 84.3%and 69.2%,respectively.Meanwhile the percentage of oxygenated monoterpenoids was higher in the PI outer soils than that in the PI inner soils,but there was no difference between outer and inner soils of Le.4.The 70%ethanol extracts of outer soils of two rice cultivars were isolated by silica gel column chromatography and ADS resin chromatography with bioassay-guided isolation.Other components in four active elutes were identified by GC/MS sequentially.Long-chain fatty acids existed mainly in the fraction eluted at 80%chloroform in methanol,but the relative amount in outer soils of Le was lower than that of PI.Five steroids,six flavonoids and eight alkaloids were found in the fraction eluted at 60%chloroform in methanol,40%chloroform in methanol and ADS resin sorption,respectively in the PI soils.However,these components didn't exist in the Le soils.5.Based on the above results,5 oxygenated monoterpenes,6 fatty acids/esters,2 steroids,3 flavonoids,one indole alkaloids and one amine alkaloids were chosen as candidates for laboratory bioassay.The inhibitory effect of oxygenated monoterpenoids on shoot length of lettuce was significantly stronger than that on root length.The 6,10-Dimethyl-3,5,9-undecatrien-2-one and 3,7-Dimethyl-7-hydroxy-1-octan(detected in PI inner soils),and 2,6-Dimethyl-l-octene-7-ol(detected in PI outer soils)were more bioactive than the terpenoids in Le soils,showing 1.2%,13.9%and 14.2%of the inhibition rates on plant heights of lettuce.With respect to fatty acids,Myristic acid,Stearic acid,Palmitic acid and Oleic acid inhibited significantly on root length of lettuce at 10"3-10-4 M.In addition,Cholic acid,Biochanin A,3-Hydroxy-7-methoxy flavone and Indolo[2,3-a]quinolizine,1,2,3,4,6,7,12,12b-Octahydro-3,12b_dimethyl-had a negative effect on the root length of lettuce.Further,the interaction of these compounds which have negative effect on lettuce growth were studied by the combination the equimolar mixture of 8 phenolic acids(PAM)with each of other classes,and then with the mixture of effective compotents.The following mixtures were tested:2,6-Dimethyl-1-octene-7-ol +equimolar mixture of PAM,Myristic acid + PAM,Cholic acid + PAM,3-Hydroxy-7-methoxy flavone + PAM,and Indolo[2,3-a]quinolizine,1,2,3,4,6,7,12,12b-octahydro-3,12b-dimethyl-+ PAM,and finally,mixture of all compounds.The bioassay results showed when mixed with the all other compounds at low concentrations(<100 M),the inhibitory effect of PAM(10-4 M,5 × 10-5 M)on shoot length and root length was significantly enhanced.In summary,allelopathic rice released more cinnamic derivatives(cinnamic acid,p-coumaric acid etc.),which were mostly adsorbed to soils avoided direct decomposition by microorganisms.When they reached the receptor plant roots,benzoic derivatives with higher activity(vanillic acid,p-hydroxybenzoic acid etc.)content increased,which enhanced weed suppression.In addition,the soils in weed inhibitory circle of allelopathic rice contained other allelochemicals such as oxygenated monoterpenoids,fatty acids and so on.These substances can significantly improve the inhibitory effect of phenolic acids on receptors in low concentration.Therefore,in the weed inhibitory circle experiment,the soils at effective distance from the allelopathic rice contained mainly phenolic acids with high concentration and activity,and then the comprehensive effect of oxygenated monoterpenoids,flavonoids,fatty acids,steroids and other secondary metabolites,significantly increased phenolic compounds for weed suppression.Especially,2,6-Dimethyl-1-octene-7-ol,Cholic acid etc.promoted the weed-suppressive activity of phenolic acids.The joint action of all putative allelochemicals is the mechanism of the formation of weed inhibitory circle in allelopathic rice cultivars.