Effects Of Exogenous Phenolic Acids On Morphogenesis And Physiological Characteristics Of The Fine Roots Of Cutting Seedlings Of Populus × Euramericana ‘Neva’

Soil degradation of continuous cropping plantation is a worldwide problem.China is the fastest country in developing plantation and it has the largest area of plantation in the world.Because of limit of the timberland resources,serious soil degradation and low forest land productivity have been appeared in plantation in China resulted from the long-term continuous cropping intensive management mode and this problem has severely affected the sustainable management of the plantation.To study and clarify the mechanism of continuous cropping plantation soil fertility decline is the major theoretical issues to realize continued operation of artificial forests.As plant allelopathy and allelopathic effect research gradually thorough,artificial forest allelopathic effect becomes a new hot spot in the field of plantation soil fertility decline mechanism research.Plant root is the first point that bears the effect of allelopathic.Allelopathic influence on plant roots has been an important topic in the research of allelopathic effect.So far,the allelochemicals on root growth and its influence of the anatomical characteristics and physiological characteristics are lack of necessary research both at home and abroad.Based on this,this article attempts to illustrate the allelochemicals influence on poplar growth and physiological activity,for further revealing the sense of continuous cropping poplar artificial forest effect mechanism,through the allelopathic effect of poplar fine root order characteristics,anatomical characteristics and physiological characteristics.Cuttage seedlings of populus nigra cultivar I-107(Populus × euramericana ‘Neva’)were cultured in 1/2 Hoagland nutrient solution added with phenolic acids of six levels(0X(CK),0.25 X,0.5X,1.0X,1.5X,2.0X)according to actual content in soil of continuous poplar plantation.Root physiologic indexes were studied respectively at the 7th day,14 th day,21 th day and 28 th day after treated with phenolic acids.Using the improved complete-root sampling method,fine roots(<2mm)of sample trees were collected and carried on the classification according to the root order to build poplar fine root classification system;Using fine root order at all levels to produce paraffin section,to study the anatomical characteristics;Using WINRHIZO root system analyzer(Canada)and corresponding software to obtain different root order parameter characteristic.Measure a series of physiological indexes: thefine root injury degree,MDA content,root activity,and the key enzyme of phenolic acids metabolism: PAL,PPO,C4 H,4CL activities,to study the effects of exogenous phenolic acids on fine roots membrane system and the enzyme activity.ELISA method is used to study the influence of root endogenous hormone levels under the gradient concentration exogenous phenolic acids.The results showed that:(a)Morphologic characteristics: fine root length,fine root area,fine root volume and root dry & fresh weight were significantly inhibited by increasing concentrations of phenolic acids while mean root diameter was promoted by increasing concentrations(P < 0.05);Provided that certain morphologic characteristics tended to be promoted by phenolic acids of low level(0.25X),while most morphologic characteristics tended to be inhibited by phenolic acids of high level(1.5X,2.0X),and most morphologic characteristics had no obvious changes by phenolic acids of 0.5X(P>0.05).It’s suggested that the influence of phenolic acids on the growth of poplar cuttage seedlings was imbalanced and closely related to the concentration of phenolic acids.Lateral roots near the tip of the main root were obvious inhibited by phenolic acids.While phenolic acids level rose,the general shape of root system was ranged from column aspect,inverted pyramid to umbrella aspect.Root tips necrotized under high level phenolic acid treatments.Root color was ranged from white(0X,0.25 X,0.5X),brown(1X,1.5X),deep brown(2X).Fine root shape could be divided into cylindrical(0X,0.25X),inverted Pyramid shape(0.5X,1X)and an umbrella shaped(1.5X,2X)three forms.(b)Fine Roots Morphological and Anatomical Properties: the morphological parameters,such as: root orders dry & fresh weight,root orders length,root orders area,root orders volume,etc.,were significantly inhibited by increasing concentrations of phenolic acids while root orders diameter was promoted by increasing concentrations(P < 0.05).Provided that certain morphologic characteristics tended to be promoted by phenolic acids of low level(0.5X)while most morphologic characteristics tended to be inhibited by phenolic acids of high level(1.5X,2.0X),and most morphologic characteristics had no obvious changes by phenolic acids of 0.5X(P>0.05).The biomass allocation appeared to decrease with increase of root orders.The anatomical properties showed that the cross section diameter of all fine roots increased with root classes and the same trend was found with the area ratio of stele(or vascular cylinder)to cross section.Root order transverse section can reflect root sequence offine root diameter.The anatomical properties of the 1,2 root classes was characterized as primary structure in general and their diameter were small,while,the secondary structure was formed from the 3th root classes and their diameter were large.The column diameter of advanced root sequence(three,four,five grade)was significantly greater than the lower root sequence(grade 1,2)(P<0.05).This may be related to the senior root orders secondary growth.The research on effects of phenolic acids on physiologic characteristics and morphologic developments of poplar roots were essential to reveal the degradation mechanism of poplar forestland productivity.The results showed that exogenous phenolic acids affected fine roots of Poplar seedling cuttings by the concentration,performance for2X>1.5X>1X> 0.5X>0X.(c)Root Physiologic Characteristics: Except no obvious changes by phenolic acids of0.25X(P>0.05),fine root malonaldehyde(MDA)content and root injury degree were both significantly enhanced by phenolic acids(P<0.05);root activity reduced significantly while the level of phenolic acids rose(P<0.05).It shows that phenolic acids of high concentration seriously damaged poplar fine roots cell membrane structure,and seriously influence the root cells normal physiological function.The root activity enhanced by low level phenolic acid0.25 X.By studying and measuring the activity of the key enzymes of phenylpropanoid pathway,such like: L-phenylalanin ammonia-lyase(PAL),polyphenol oxidase(PPO),Cinnamate-4-hydroxylase(C4H)and 4-coumarate: CoA ligase(4CL).The results showed that the change rules of the PAL,C4 H and 4CLwere basic consistent with the phenolic acid treatment: the activity enhanced by low level phenolic acid(0.25 X,0.5X,1X)and inhibited significantly by high level phenolic acids(1.5X,2.0X)(P<0.05).The PPO activity enhanced by low level phenolic acid(0.25X)and inhibited significantly by high level phenolic acids(1X,1.5X,2.0X)(P<0.05),while had no obvious changes by phenolic acids of 0.5X.Plants may be increasing the key enzyme of the phenolic acids metabolism process to adapt to the phenolic acid environment: burning off redundant phenolic acids,and catalytic generating a lot of phenolic acid out of the body,in order to improve the resistance of plants.Effected by exogenous phenolic acids,IAA,ZR and GA contents in fine roots decreased at first,and then increased,but ABA contents in fine roots increased at first,and then decreased.The max/min content of endogenous plant hormones appears in different time.Treatment of exogenous phenolic acids decreased IAA/ABA 、 ZR/ABA and GA/ABA ratio.Effects of exogenous phenolic acids on changes in the content and the ratio of endogenous plant hormones duringfine roots of poplar seedling cuttings may the main reason that phenolic acid inhibits or delays poplar seedling fine root germination and morphogenesis.Affected by exogenous phenolic acids,the morphologic,biochemical and physiologic characteristics of the fine roots of poplar seedling cuttings changed.And these changes in fine roots had influenced the absorption and transportation function of roots,and then affected the plant root growth and development as well as water and nutrient metabolism,and consequently showed indirect and somewhat significant effects on the growth and development of the whole plant.This changing process was suggested to be one of the main pathways for phenolic acids to perform allelopathic effects.