Root morphology of citrus rootstocks and drought tolerance evaluation of their grafted plants

Abstract:【Objective】Drought stress is one of the important constraints in citrus production. Root sys- tem determines the adaptabilities of plants to soil conditions. This research aimed to investigate the char- acteristics on morphological structure of root systems of different citrus rootstocks, and the correlation between root morphology and drought tolerance.【Methods】Eleven citrus rootstock accessions includ- ing trifoliate oranges (Poncirus trifoliata) and their hybrids, mandarins (Citrus reticulata) and Xiangcheng (C. junos), were used as the materials. Parameters of root morphology such as main root length, total root length, total root area, average root diameter, root volume and tip number were mea- sured with seedlings at 30, 45 and 75 days after sowing‘. Newhall’navel orange (C. sinensis) was graft- ed onto those rootstocks. Parameters including Pn (net photosynthesis rate), Gs (stomatal conductance), Tr (transpiration rate), Ci (intercellular CO2 concentration), activities of SOD (superoxide dismutase), POD (peroxidase) and CAT (catalase), and contents MDA (malondialdehyde), proline (pro), soluble pro- teins, and soluble sugars were determined in the leaves of the grafted plants on 0, 10 and 25 days after drought stress treatment.【Results】There were significant differences in root morphology among the rootstocks at different time points. The most obvious differences were observed on 45 days after sow-ing. Results of comprehensive evaluation on morphological parameters of root system ranked Volkamer (C. volkameriana) at first place, followed by Z-006, ZZ-30, KPJ-030, XC-009 and ZZ-020, and Shiiku- washa (C. depressa) was at the last. Cluster analysis divided these rootstocks into 3 groups based on these parameters. Volkamer and Z-021 were clustered into group 1, with the best structure of root sys- tem according to the results of comprehensive evaluation. Z-006 (P. trifoliata), ZZ-030, KPJ-030 (C. re- ticulata), XC-009 (C. junos), ZZ-020 (Poncirus hybrid) and Ziyang xiangcheng (C. junos) were classi- fied into group 2, in which the structure of root system was ranked as good. Carrizo (C. sinensis × P. tri- foliata), Canton lemon (C. limonia) and Shiikuwasha were in group 3, which had the worst structure of root system among all of the analyzed rootstocks. Under normal conditions, the highest values of Pn and soluble sugar content were observed i‘n Newhall’/Ziyang xiangcheng; the highest values of Ci and solu- ble protein content were i‘n Newhall’/Carrizo; the highest values of MDA, Gs, SOD activity, POD activ- ity and CAT activity were observed in‘Newhall’/ZZ-030,‘Newhall’/Z-006,‘Newhall’/Shiikuwasha,‘Newhall’/XC-009 and‘Newhall’/Z-021, respectively. The physiological and biochemical parameters were greatly affected by drought stress and varied among different scion/rootstock combinations, which suggested that the genotype of rootstock is an important factor affecting the drought tolerance of the grafted plants. Membership function and principal component analysis were applied to evaluate the drought tolerance of the grafted plants based on the physiological and biochemical measurements. The drought tolerance according to this analysis was in the order of‘Newhall’/ZZ-030>‘Newhall’/Carrizo citrange>‘Newhall’/ZZ- 022>‘Newhall’/Shiikuwasha>‘Newhall’/Z- 021>‘Newhall’/Z- 006>‘Ne- whall’/Ziyang xiangcheng>‘Newhall’/Canton lemon>‘Newhall’/XC-009>‘Newhall’/KPJ-030. The correlations between morphological structure of root system and drought tolerance of grafted plants were investigated. The results demonstrated that drought tolerance of the grafted plants was significant- ly and positively correlated to the parameters of total root surface area and root tip number.【Conclu- sion】The results of current study suggested that the number of root tips and the total root surface area are the important parameters of root morphology in relation to the drought tolerance of plants. They could be the target traits for breeding and selection of rootstocks for good drought-tolerance. In addition, the trifoliate orange hybrid ZZ-022 demonstrated good potential as a superior rootstock due to its good root system and drought tolerance.