Studing On Drought Tolerance Of Rootstock And Its Correlation With Root Morphology In Citrus

Citrus is mainly grown in mountainous and hilly areas in China.Due to uneven rainfall and poor irrigation facilities,The drought has become an important environmental factor affecting citrus production.Selection and use of the drought-resistant citrus rootstocks is the most economical and essential way to solve this issue.The rootstock,as the underground part of the fruit trees,is very important on the absorption of water and nutrition for the fruit tree and protection of the tree from environmental streses.The ability of root up taking nutrients and water directly affects the growth and development of whole plants.Previous researches have shown that the morphology configuration of the roots is dominated by the genetic background,and greatly related to its ability to absorb water and nutrients.The genes controlling root morphology configuration in monocotyledonous plants have been identified to have significant effect on plant drought tolerance.At present,the research on drought tolerance in citrus is mainly focus on the changes on growth and morphology,also the physiological and biochemical response mechanism of above-ground part of plant under drought stress.There is no report on relationship between the growth characteristics and architecture of root system and drought tolerance of the plant.In this research,the rootstocks of different citrus species were used as materials to evaluate the drought tolerance and study the characteristics of the root system,and their relationship with drought resistance.In the study,10 kinds of citrus rootstocks with different genetic backgrounds were used as experimental materials,including: Hongningmeng(Citrus limonia Osbeck cv “Hongningmeng”),XC-009(C.junos),Z-006(Poncirus hybrid),Carrizo citrange(C.sinensis × Poncirus trifoliata cv Carrizo citrnage),Ziyangxiangcheng(C.junos cv “Ziyangxiangcheng),Shiikuwasha(C.depressa Hayata cv “Shiikuwasha”),Z-021(Poncirus trifoliata),KPJ-030(C.reticulata),ZZ-030(Poncirus hybrid),ZZ-022(Poncirus hybrid).The seedlings and grafted trees were subjected to drought treatment,and the morphology and parameters of physiology and biochemistry were measured and analyzed.The tolerance to drought stress was evaluated by using the principal component and membership function.At the same time,the root growth and morphological structure of different root stocks were observed.The results showed that there were significant differences in drought resistance among the 10 rootstocks,which could be divided into 3 categories: resistant rootstocks(R),including Carrizo citrange,ZZ-030,Z-006,KPJ-030;medium rootstocks(M),including ZZ-022 and Z-021;susceptible rootstocks(S),including Ziyangxiangcheng,Shiikuwasha,XC-009,Hongningmeng.According to comprehensive evaluation results derived from analysis the date of physiological and biochemical measurements,including the contents of malondialdehyde(MDA),soluble protein(SSC),proline(PRO),and the activities of superoxide diamutase(SOD),catalase(CAT),peroxidase(POD),the drought-resistant of those rootstocks were in the order: ZZ-022 > Z-006 > ZZ-030 > Ziyangxiangcheng > KPJ-030 > Z-021 >Carrizo citrange >Hongningmeng > XC-009 > Shiikuwasha.The results of physiological and biochemical indexes were highly correlated with the results of drought indexes.However,there were obvious differences in the correlation between physiological and biochemical indexes with comprehensive drought tolerance.SSC and PRO were significantly correlated with drought tolerance,with correlation coefficiencies at 0.715 and 0.716,respectively.POD and MDA were positively correlated with drought tolerance,but not significant;The correlation coefficiencies were 0.556 and 0.458.Different size of the plant and morphology of leaf among the 10 seedling rootstocks may affect the judgment of drought tolerance of the rootstock.To reduce the effects of drought-resistant evaluation influenced by differences of transpiration due to the volume of plants above-ground parts,Newhall navel orange(C.sinensis)was grafted onto the rootstocks to evaluate the drought resistance.According to the results of comprehensive analysis of photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr),intercellular CO2 concentration(Ci),SOD,POD,CAT,PRO,MDA,SSC,soluble sugar(SS).The drought resistance of 10 grafted seedlings were ranged as,ZZ-030 > Carrizo citrange > ZZ-022 > Shiikuwasha > Z-021 > Z-006 > Ziyangxiangcheng > Hongningmeng > XC-009 > KPJ-030.This result is very similar to the result evaluated with seedlings: Among the analyzed parameters,PRO demonstrated significant correlation with the result of comprehensive evaluation with(correlation coefficiency of 0.898);In addition,the SSC,SOD,Pn,CAT were also positively correlated to drought tolerance,but less significant(correlation coefficiencies of 0.456,0.605,0.370,0.380,respectively).Root morphology of the 10 citrus rootstocks was observed and compared.The parameters of length of taproots,total length of roots,total surface area,volume and average diameter of the roots were determined,and analyzed by comprehensive membership function.Results showed that there were significant differences on root morphological configuration among the 10 rootstocks.The ZZ-022,ZZ-030 and Hongningmeng demonstrated the highest values of comprehensive membership function.Those rootstocks possess the higher values on total root length,surface area,volume and average diameter of the root system;the number and density of lateral roots.The “Shiikuwasha” had lowest value.The variations on the morphology of the roots among different rootstocks indicated that the structure features of the root system are dominated by genetic factors.Meanwhile,results showed that there was highly significantly positive correlation between the length of roots and drought tolerance at(correlation coefficiency of 0.836).The average diameter as well as volume of root system was significantly and positively correlated with(The correlation coefficiencies of 0.6866 and 0.7148,respectively).The results of this study indicated that there were significant differences on the drought tolerance among different genotypes of citrus rootstocks,suggesting the significance of genetic influence on drought tolerance.The results of comprehensive analysis based on physiological and biochemical indexes were highly correlated with the results of drought tolerance indexes.Among those parameters,PRO and SSC were highly positively correlated with the drought resistance.The morphological configuration of root system was significantly different among different germplasms,showing obvious genetic diversity.The length of taproot,average diameter and volume of the root were positively correlated with drought tolerance.