Evaluation of drought tolerance in different citrus rootstocks

Abstract:【Objective】Citrus is one of the most important fruit crops in the world and also one of the main fruits in China. Rootstocks play prominent roles on stress tolerance of plants, and to some extent, determine the adaptability of Citrus. Drought is one of the most important abiotic stresses affecting citrus yield and quality. Systematic study on drought tolerance of citrus rootstocks has been rarely reported. The study on the physiological and biochemical responses of rootstocks to drought stress could provide a theoretical basis for drought tolerance breeding and rapid selection for tolerant lines.【Methods】The seedlings of ten different rootstock varieties including trifoliate oranges and their hybrids, mandarins and Citrus junos were used for drought stress treatment under controlled conditions. Twelve seedlings of each rootstock were divided into two groups, each group was further divided into the treatment and the control in 3 replicates.The first group was used to observe the symptoms of the plants under water stress, and the other was used to detect the physiological and biochemical responses to water stress. Mature leaves from both the control and the treatment were collected at 5, 10, 15 and 25 days of treatment. The samples were immediately immersed into liquid nitrogen and then stored at-80 ℃. Quantitative determinations of the contents of malondialdehyde(MDA), proline and soluble proteins, and the activities of superoxide diamutase(SOD), catalase(CAT) and peroxidase(POD) were conducted following the protocols previously described. The morphological and physiological characteristics related to water stress tolerance were investigated. The meth-the drought tolerance.【Results】There were significantly differences in both time course and the severityof leaf wilting under drought stress among varieties.‘Hongningmeng'(C. limonia Osbeck) began to wiltwithin ten days with a hazard index of 16.6%, and at the 50 thday of treatment, the hazard index reached100%. Leaves of‘Ziyangxiangcheng'(C. junos) and‘Shiikuwasha'(C. depressa Hayata) showed symptomof wilting on the 60thday;‘Z-021'(Poncirus trifoliata) wilted on day 70, and its hazard index was 10%.The other trifoliate oranges and their hybrids did not wilt until the 70 thday of treatment. The change trendsof the contents of proline, malondialdehyde and soluble proteins varied among different rootstocks. TheMDA contents in most of the rootstocks rose within the first 10 days of the treatment but that in‘KPJ-030'(C. reticulata),‘ZZ-030'(Poncirus hybrid) and‘ZZ-022'(Poncirus hybrid) decreased. The con-tents of MDA in‘Hongningmeng'‘Shiikuwasha'‘XC-009'(C. junos), and‘KPJ-030'changed dramati-cally. The results of variance analysis showed that MDA content was significantly different among differ-ent periods of treatment(P<0.05). The content of soluble proteins increased first and then decreased in allrootstocks except for‘XC-009', which presented a variation pattern of“rise-drop-rise”. Different root-stocks displayed great difference in changing pattern. The content of soluble proteins reached a peak val-ue on 5thday in‘Hongningmeng'‘Ziyangxiangcheng'‘Shiikuwasha',‘Z-021'‘KPJ-030'‘ZZ-030'and‘ZZ-022'. Compared to the control, protein content increased by 75.94% in‘Shiikuwasha', 63.88% in‘Z-021', 64.30% in‘Ziyangxiangcheng', 35.86% in‘KPJ-030', 30.78% in‘Hongningmeng', 9.60%in‘ZZ-022', and 8.69% in‘ZZ-030'. However, in‘Carrizo'citrange(C. sinensis × Poncirus trifoliata)and‘Z-006', the peak appeared at day 15 and was 78.58% and 69.43% higher than in the control, re-spectively. The content of proline in all the 10 rootstocks demonstrated a rising trend and increased by192.47% in‘Z-006', 189.50% in‘XC-009', 152.97% in‘Carrizo'citrange, 135.47% in‘Hongning-meng', 106.21% in‘ZZ-030', 104.41% in‘ZZ-022', 88.75% in‘Z-021', 68.68% in‘Shiikuwasha',and 55.57% in‘KPJ-030'. Basically, the change trend of SOD activity among the rootstocks was similar.The enzyme activity increased at the early stage and decreased at the late stage, and reached the maxi-mum level on the 15 thday. However, the range of change varied among rootstocks,‘ZZ-022'‘Z-006'and‘Carrizo'citrange increased greatly by 27.14%, 37.86%, 24.31%, respectively. However, SOD activity in‘Hongningmeng'‘KPJ-030'and‘Shiikuwasha'did not increased significantly. The activity of POD in‘ZZ-030'‘ZZ-022'‘Z-021'‘Carrizo'citrange and‘Ziyangxiangcheng'reached the highest level on the5 thday of treatment, and a less sharp increase was found in‘Shiikuwasha'and‘Hongningmeng'. POD ac-tivity in‘Z-006'and‘Ziyangxiangcheng'remained stable, but changed greatly in‘XC-009'. The activi-ty of CAT in all the rootstocks but‘Hongningmeng'rose first, peaked on the 5thday and then declined.Compared to the control, CAT activity in‘Carrizo'citrange,‘Z-021'and‘KPJ-030'increased by445.84%, 270.84% and 228.75%, respectively. However, the increase was less significant in‘ZZ-030'‘ZZ-022'‘Shiikuwasha'‘Ziyangxiangcheng'‘XC-009'and‘Z-006', with an increase of 75.71%,59.68%, 55.46%, 50.46%, 31.17% and 31.90%, respectively. However, in‘Hongningmeng', it kept in-creasing and reached the maximum at the end of the treatment.【Conclusion】According to the results ofthe physiological and biochemical investigations with comprehensive membership function and principalcomponent analyses, as well as the morphological observations, the drought tolerance of the 10 citrus root-stocks was significantly different. The trifoliate hybrid‘ZZ-022'was the most tolerant variety, while themandarin‘Shiikuwasha'was the most susceptible one.