Allelopathy effect of some phenolic acids on roots of germinated Prunusdavidiana (Carr.) Franch seeds

Abstract:【Objective】One of the important factors leading to the continuous cropping obstacle in fruit production is allelopathy, and the chemical substances which cause allelopathy are called allelochemicals.Allelochemicals from the roots of plants are released directly into the soil. They are also released from dissociated matters of leaching residues via microbial transformation from the above-ground and finally stayin the soil. They accumulate and maintain activity in root zone as a result of detention, transformation andmigration, and create the problem of continuous cropping obstacle when they accumulate to certain concentration. Phenols are important secondary metabolites produced in plants. In the plant ecosystem, themain phenolic acids are ferulic acid, cinnamic acid, hydroxy benzoic acid and so on. Root exudates andextracts from branches, leaves and roots of peach have obvious autotoxicity to the plant, which has beendemonstrated by several reports. However, most of the reports studied mixed allelopathic effects of severalallelochemicals. Allelopathy effect of a specific phenolic acid in peach has been rarely reported, exceptfor benzoic acid. Phenols at high concentrations can inhibit the growth of plants, soil microorganisms andsoil enzyme activities, while at low concentrations to some extent promote plant growth and improve soilenzyme activity. Some phenolic substances also stimulate the growth and reproduction of soil microbes, in⁃dicating that there is a threshold concentration for the effect of phenolic compounds on plant growth.Prunus davidiana (Carr.) Franch is the main peach rootstock in northwest of China. P. davidiana (Carr.)Franch seeds were used as the material to test allelopathy effect of several phenolic acids on seed germina⁃tion characteristics, in order to bring some understandings on continuous cropping obstacles in peach tree.【Methods】Allelochemicals tested included phenolic acids, alkaloids, glycosides, terpenes, etc. The mainphenolic acid allelochemicals in stone fruits have been reported to include benzoic acid, benzaldehyde, phydroxybenzoicacid, gallic acid, and vanillin. P. davidiana seeds, which had been refrigerated for dor⁃mancy removal, were surface disinfected with 75% alcohol, and then immersed in distilled water for 12 hbefore they were taken out and placed in a Petri dish covered with a wet towel and placed in a 4 ℃ refrigerator for about 4 weeks. Then, after several different concentrations of vanillin, benzaldehyde, benzoic ac⁃id, p-hydroxybenzoic acid, gallic acid, ferulic acid were applied, the seeds were cultured for 14 days at 20 ℃ and a light/dark cycle of 8/16 h in an artificial climate chamber. The taproot length and lateral rootnumber were collected and the allelopathic effect at different concentrations used to calculate the sensitiv⁃ity index (SI). The absolute value of the inhibition effect were converted into the value of probability forthe regression equation, from which half inhibitory effect concentration (SI0.5) was estimated and served asthe allelopathic effect strength of the tested substance.【Results】Vanillin, benzaldehyde, benzoic acid, phydroxybenzoicacid, gallic acid, and ferulic acid displayed a significant inhibitory effect (P<0.01) on tap⁃root elongation at concentrations above 10, 10, 0.1, 10, 10 and 0.1 mg·L^-1, respectively. The lateral rootnumber was more than 20 in average. With the increase in concentration of different allelochemicals, lateral root number decreased. Vanillin, benzoic acid, p-hydroxybenzoic acid, gallic acid and ferulic acid atthe low concentration of 1 mg·L^-1 had a greater inhibitory effect on the number of lateral roots, with an SIvalue that could reach -0.2. At the same concentration of the corresponding allelochemicals, the SI valueevaluated by taproot length was between -0.08 to -0.15. The SI0.5 values obtained from the effects on taproot length and on lateral root number were similar. These two indexes can be used to evaluate the allelo⁃pathic effects on phenolic acids. With DPS software quantitative data bioassay module, the absolute valueof the allelopathic effect index (SI) was transformed to Probit value and nonlinear parameter estimationwas used to obtain regression equation. The slope of the regression curve was also correlated with allelo⁃pathic effects of different phenolic acids. When the number of lateral roots was used as parameter, theslope of the equation was consistent with the intensity of the sense effect calculated by SI0.5 method. However, when the length of the main root was used as the parameter, the result was not consistent, whichmight be mainly caused by the difference in response of seed germination of peach to low concentrationsof phenolic acids.【Conclusion】This study used P. davidiana taproot length and lateral root number asthe evaluation indexes, through comprehensive comparison, SI0.5 (i.e. allelopathy strength) of phenolic ac⁃ids tested was in the order of ferulic acid> benzaldehyde> benzoic acid> vanillin>gallic acid > p-hydroxy⁃benzoic acid.