Changes of phenolic acids in the soil of replanted apple orchards surrounding Bohai Gulf

Apple replant disease(ARD) is widely found in the most,if not all,apple production region through the world.It is a complex soil-borne disease syndrome that affects tree growth,suppresses the yield and quality of apple fruits when orchards are replanted.The causes of replant problem of apple trees are considered as a combination of abiotic and biotic factors,including phenolic acids,fungi,bacteria,actinomycetes and nematodes etc.At present,there is a bulk of research conclusions on replant disease and some of them suggest that phenolic acids are closely link to replant disease.Meanwhile,however,few references are referred to the changes in phenolic acids in the soils of apple replant orchard.In view of it,the composition and content of phenolic acids in the soils of apple orchards were tested in several regions around Bohai Bay,including Changyi,Qixia,Penglai of Shandong province and Dalian,Funing,Suiyuan of Liaoning province as well as Changli and Qingxian of Hebei province.The results showed that the composition and content of phenolic acids in apple orchards infested with replant diseases varied with regions and soil thickness.The contents of phenolic acids gradually decreased from spring to fall in apple orchard soils.However,the contents of phenolic acids in non-replant soils were much lower than those of replant soils.In replant soils the composition and content of phenolic acids remarkably differed in diverse seasons,especially in spring,while the types of phenolic acids were markedly different from the summer and fall.The possible reason on the difference was that the composition and content of phenolic acids secreted by plants in different seasons varied.Besides,the distribution of phenolic acids in different soil layers varied with seasons.In summer the phenolic acids mainly distributed in shallow soil.In contrast,it mostly relocalized to the deep layer of soil in autumn.The possible reason on this change was that artificial irrigation and natural precipitation resulted in an eluviation of the phenolic acids in soils,which dispersed large numbers of phenolic acids to move to the deep layer of soil.As a result,phenolic acids content increased with the soil depth in replant orchard soil in autumn.There were no significant differences in contents of p-hydroxybenzoic acid,catechin,caffeic acid and ferulic acid between the replant and non-replant soils from different regions.Furthermore,contents of pyrogallic acid,chlorogenic acid and phlorizin were much higher in replant soils than those in non-replant soils.Taken together,pyrogallic acid,chlorogenic acid and phlorizin were summarized to be the key phenolic acids that brought about apple replant disease in Shandong,Liaoning and Hebei.