Evaluation Of Acid Resistance And Studies On Adaptability To Acid Stress Of Citrus Rootstocks

The appriopriate soil pH for citrus is 5.5-6.5.In recent years,the area of acid soil has been expanding and the acidification is continuing due to acid rain caused by pollution and excessive fertilization.Rootstock is crucial for grafting,which plays important roles in tree growth,fruit yield,size,quality and the resistance to the stresses,and to some extent,directly determines the economic benefits of an orchard.Therefore,the quick selection and cultivation of citrus rootstocks with acid tolerance have become an eager demand for citrus industry.Five citrus rootstocks commonly used in citruculture with different genetic backgrounds KPJ-020(C.Depressa Hayata “Shiikuwasha”),KPJ-007(C.reticulata Blanco),KPJ-010(C.daoxinensis Blanco),Carrizo Citrange(C.sinensis×Poncirus trifoliata),Ziyang Xiangcheng(C.junos Siebold.ex Tanaka)were used as materials.The rootstocks were treated in nutrient solutions at pH 2.5,3.5 and 4.5(pH6.0 as control)in a Nutrient Film Technique hydroponic system for 60 days in Greenhouse.The symptoms of leave injury under the acid conditions were observed,the physiological and biochemical parameters such as the chlorophyll content,relative electrical conductivity,soluble sugar,soluble protein,proline,malondialdehyde,SOD,POD and CAT of leaves were measured at different time points under acid stresses.Membership function and principal component analysis were employed to comprehensively evaluate the tolerances.The two factors affecting the tolerances of the rootstocks were compared by orthogonal test and extreme difference analysis.The contents of both macro and micro nutrient elements in leaves of the rootstocks were detected and analyzed 60 days after treatment at different pH conditions.The main results were as follows:1.The injury rate,injury index and the highest injury level of leaves from the rootstocks treated 60 days under different pH value indicated that the injury index of Ziyang Xiangcheng was relatively low(20.00)and the highest injury level was the lowest(level 1)among these rootstocks,implying the most acid resistance.The injury index of Shiikuwasha was the lowest at pH 4.5 treatment,while the injury indexes at pH 3.5 and 2.5 were higher than those of Ziyang Xiangcheng,and the highest injury level were level 2,which also showed good tolerance.However,the highest level of injury was relatively higher in KPJ-007 under these conditions and a few plants died under pH 2.5.The injury index and the highest level of injury of KPJ-010 were close to those of KPJ-007.The results mentioned above indicated that KPJ-010 and KPJ-007 were susceptible to acid stress.2.The methods of membership function and principal component analysis were employed to comprehensively analyze and evaluate the results of the related physiological and biochemical parameters of the rootstocks treated with 10,30 and 60 days at pH2.5,3.5 and 4.5.Ten days after treatment at pH 2.5,the tolerance were: Ziyang Xiangcheng > KPJ-020 > KPJ-010 > KPJ-007 > Carrizo Citrange;at pH 3.5,KPJ-007 > Ziyang Xiangcheng > KPJ-020 > Carrizo Citrange > KPJ-010;at pH 4.5 treatment,Ziyang Xiangcheng > Carrizo Citrange > KPJ-020 > KPJ-007 > KPJ-010.Thirty days after treatment pH 2.5,the tolerance were: Ziyang Xiangcheng > KPJ-020 > Carrizo Citrange > KPJ-007 > KPJ-010;at pH 3.5,the order is: Ziyang Xiangcheng > KPJ-007 > KPJ-020 > Carrizo Citrange > KPJ-010;at pH 4.5: Ziyang Xiangcheng > KPJ-020 > KPJ-007 > KPJ-010 > Carrizo Citrange.Sixty days after treatment at pH 2.5,the tolerance were: Ziyang Xiangcheng > KPJ-020 > Carrizo Citrange > KPJ-010 > KPJ-007;at pH 3.5: KPJ-020 > Ziyang Xiangcheng > KPJ-007 > Carrizo Citrange > KPJ-010;at pH 4.5: KPJ-020 > Ziyang Xiangcheng > Carrizo Citrange > KPJ-007 > KPJ-010.3.Orthogonal test and extreme difference analysis were used to further analyze the two factors(treatment time(A)and pH value(B)),contributing to the differences of acid tolerance of the rootstocks.The results showed that treatment time is probably more important affecting the acid tolerance of KPJ-020(A > B),while the pH value is probably more essential contributing to the differences for the acid tolerance of the other 4 citrus rootstocks(B > A).4.The macro and micro nutrient elements of leaves from these rootstocks 60 days after treatment at different pHs were detected and compared.Results showed that both the macro and micro elements in leaves of KPJ-020 and Ziyang Xiangchen were less affected under these stresses,while those of KPJ-007 and KPJ-010 were affected more.Therefore,it suggested that KPJ-020 and Ziyang Xiangchen possess good tolerance to acid stresses,while KPJ-007(C.reticulata Blanco)and KPJ-010(C.daoxinensis Blanco)are susceptible to these stresses.Conclusion: The results based on the symptoms,the physiological and biochemical parameters and the nutrient elements analysis were consisted with each other.Short-term stress treatment in the Nutrient Film Technique hydroponic system with controlled conditions can cause injury symptoms of citrus,and can directly indicate the different tolerance to stresses.Therefore,the Nutrient Film Technique hydroponic system is an excellent system to efficiently evaluate the tolerance to stresses.The results mentioned above indicated that KPJ-020(C.Depressa Hayata)and Ziyang Xiangchen(C.junos Siebold.ex Tanaka)both possess good tolerance to acid stress with good growth and the physiological performance,while KPJ-007 and KPJ-010 are susceptible to acid stresses.Orthogonal test and extreme difference analysis indicated that treatment time is more important to affect the acid tolerance of KPJ-020(A > B),while the stress condition at pHs is probably more essential to contribute the differences for the acid tolerance of the other 4 citrus rootstocks(B > A).The results implied that the mechanisms of the tolerance to acid stress between KPJ-020 and the other 4 rootstocks are probably different.