Study Of Physiological Mechanism On Grafting Increasing The Salt Tolerance Of Cucumber Seedlings

With the development of protected culture,soil secondary salinization has been more and more severe and has become the biggest obstacle of limiting the development of protected culture.Cucumber(Cucumis sativus L.) is one of the most important vegetables in protected culture which is salt sensitive.It is showed that grafting can improve cucumber tolerance to salt stress,though the physiological mechanism is not known.In our study,the cultivars with different salt sensitivity were evaluated from a great deal of cucumbers and rootstocks,and the physiological mechanism of different cucumbers and rootstocks to salt stress was determined.Basis am it,the effects and physiological mechanism of the connection of rootstock and scion on the salt tolerance of grafted cucumber plants were studied which supply theoretic basis for clarify grafting enhance salt tolerance of cucumber and reference for directing the grafted culture of cucumber. The main results in the present study are presented as follows:Salt resistance of 20 cucumber cultivars and 22 species of rootstocks were evaluated by using the cluster analysis.The results showed that:Among 20 cucumber cultivars, 'Zaoduojia','Huazao No.2','Zhongnong No.8','Jinyu No.1' and 'Jinyou No.3' held high resistance to salt stress,whereas 'Jinguan No.1','Xuyou No.1','Jinchun No.5','Jinchun No.4','Dongfangmingzhu','Xiangzao No.3','Tezao No.1' and 'Jinyou No.1' were moderately salt tolerant,but 'Baipihuanggua','Ningfeng No.8','Meinong No.4', 'Jinchun No.3','Jinza No.4','Jinza No.2' and 'Jinchun No.2' cucumber cultivars were sensitive to salt stress.Among 22 species of rootstocks,the rootstocks could be clustered to two groups,Cucurbit pepo L.cv.'Chunbaiyu','Zaoqingyidai' and 'Jizao xiuyu',and Cucurbita moschata Duch.cv.'Tiejiangjun F₁' were sensitive to salt stress whereas the other species of rootstocks were tolerant to salt stress,and salt resistant rootstocks could be clustered to three subgroups,Lagenaria siceraria Standl.var.clavata Makino cv. 'Xiaoganhugua','Tongxinhugua','Hanlongbiyu' and 'Chaofeng8848' showed the highest salt resistance,Cucurbit ficifolia cv.'Heizinangua' exhibited tolerant,whereas the other thirteen species showed more salt resistance.The salt resistance of cucumbers and rootstocks could be screened at germination stage and seedling stage,respectively.The effects of NaCl stress on the salt injury index,plant growth and physiological trait of different cucumbers were investigated by using 'Zaoduojia' and 'Jinchun No.2' as the test materials.The results showed that:The cucumber roots have not the mechanism of limiting the translocation of Na⁺ to leaf to alleviate salt stress.It is the essential cause that cucumber makes salt sensitive.Na⁺ accumulated in the leaf and cause ion toxicity,the content of K⁺,Ca²⁺,Mg²⁺ in leaf decreased markedly,Na⁺/K⁺ ratio and Na⁺/Ca²⁺ ratio of both cultivars increased significantly resulted in ion unbalance with increasing concentrations of NaCl.Despite the SOD and POD activities of both cultivars increased significantly and the free proline content of 'Zaoduojia' increased markedly,the membrane permeability,MDA content and the salt injury index increased markedly,and the shoot water content decreased significantly resulting in severe oxidation and osmotic stress.The chlorophyll content and photosynthetic rate decreased significantly,plant growth was restrained,but Fo,Fv/Fm,F PSâ…¡and qP of both cultivars was unaffected by salt stress.'Zaoduojia' exhibits a better protection mechanism against oxidative damage and lipid peroxidation by maintaining lower Na⁺ content,Na⁺/K⁺ ratio,Na⁺/Ca²⁺ ratio and higher Ca²⁺ content in leaf than salt sensitive cv.'Jinchun No.2'.The effects of NaCl stress on the plant growth and physiological trait of different rootstocks were investigated by using 'Chaofeng 8848','Chaojiquanwang' and 'Heizinangua' as the tested materials.The results showed that:It is essential that salt tolerance can be obtained by three rootstocks for limiting the translocation of Na⁺ from the root and stem to the leaf,making the lower Na⁺ content in the leaf to alleviate ion injury.The contents of Na⁺ in the leaf of three rootstocks were unaffected by at 50 mmol/L NaCl and increased significantly at 100 and 150 mmol/L NaCl.The K⁺,Ca²⁺ and shoot water content was gradually decreased under salt stress.The photosynthetic rate, transpiration rate,stomatal conductance of three rootstocks decreased markedly and plant growth was restrained with increasing concentrations of NaCl,but Fo,Fv/Fm,F PSâ…¡,qP of three rootstocks were unaffected by salt stress.The proline content,MDA content, SOD and POD activities of Chaojiquanwang and 'Heizinangua' increased,the root activity was decreased significantly,whereas those of 'Chaofeng 8848' were unaffected under salt stress.The degree of oxidation stress and osmotic stress of 'Chaofeng 8848' sufferred was the slightest whereas that of 'Heizinangua' was the most severe.'Chaofeng 8848' exhibits a better protection mechanism against salt stress by maintaining higher photosynthetic rate ability and root activity than 'Chaojiquanwang' and 'Heizinangua'. 'Chaojiquanwang' exhibits a better protection mechanism against salt stress by maintaining lower Na⁺/Ca²⁺ ratio,higher Ca²⁺ content in the leaf and photosynthetic ability than 'Heizinangua'.The ungrafted cucumber plants used as controls,the effects of grafting with different scions on cucumber growth and physiological trait were investigated by using 'Zaoduojia' and 'Jinchun No.2' as scion,and 'Chaojiquanwang' as rootstock under NaCl stress.The results showed that:Under NaCl stress,the reductions in the shoot and root dry weight, leaf area,stem diameter of grafted plant(?) were lower than those of ungrafted plants.The Na⁺ content,Na⁺/K⁺ ratio,Na⁺/Ca²⁺ ratio in the leaf and scion stem of grafted plants were lower whereas those of in the rootstock stem a(?)d root were higher than in ungrafted plants. The K⁺ contents,soluble sugar contents,shoot water contents,SOD,POD,CAT,APX activity,the stability of AsA/DHA,Photosynthetic rate,transpiration rate and stomatal conductance were higher,whereas the MDA contents in the leaf of grafted plants were lower than that in ungrafted plants.The reductions in leaf area and stem diameter of grafted 'Jinchun No.2' plants were more severe than those of grafted 'Zaoduojia' ones. The Na⁺ contents and Na⁺/K⁺,Na⁺/Ca²⁺ ratio in leaf were lower whereas K⁺,Ca²⁺,Mg²⁺, AsA and proline contents,POD,CAT,APX activities,the stability of AsA/DHA in leaves of grafted 'Zaoduojia' plants were higher than those of grafted 'Jinchun No.2' plants under the same NaCl stress.However,the shoot water content,MDA content, Photosynthetic rate,transpiration rate and stomatal conductance were not different between the grafted 'Zaoduojia' plants and the grafted Jinchun No.2 ones under salt stress.Grafted 'Zaoduojia' plants exhibit a higher salt tolerance by maintaining lower Na⁺ content,Na⁺/K⁺,Na⁺/Ca²⁺ and higher k⁺,Ca²⁺ and Mg²⁺ contents than grafted 'Jinchun No.2' ones.But grafting reduces the difference of salt tolerance between 'Zaoduojia' and 'Jinchun No.2'.The Ungrafted cucumber plants used as controls,the effects of grafting with different rootstocks on cucumber growth and physiological trait were investigated by using 'Chaofeng 8848' and 'Heizinangna' as rootstocks,'Jinchun No.2' as scion under NaCl stress.The results showed that:The reductions in the shoot and root dry weights, leaf area,stern diameter of grafted plants were lower than those of ungrafted plants under the same NaCl stress.Na⁺,MDA,H₂O₂ contents,O₂^- producing rate,Na⁺/K⁺ ratio, Na⁺/Ca²⁺ ratio were lower,whereas K⁺,solube sugar,shoot water content,SOD,POD, CAT,APX activity,the stability of AsA/DHA,Photosynthetic rate,transpiration rate and stomatal conductance in the leaf of grafted plants were higher than those of ungrafted plants at the same NaCl stress.Intercellular CO₂ concentration of grafted plants decreased whereas that of ungrafted plants increased significantly under salt stress.The reductions in shoot and root dry weight,leaf area and stem diameter of grafted 'Chaofeng 8848' plants were significantly lower than those of grafted 'Heizinangua' plants ones.MDA, H₂O₂ contents and Na⁺/K⁺ ratio were lower whereas proline contents,soluble protein content,SOD,POD,CAT,APX,GR activities,Photosynthetic rate,transpiration rate and stomatal conductance in leaves of grafted 'Chaofeng 8848' plants were higher than those of grafted 'Heizinangua' plants under the same NaCl stress.Therefore Then,the salt tolerance of grafted 'Chaofeng 8848' plants is higher than those of ungrafled 'Heizinangua' plants ones.It is the most possible physiological mechanism of grafting improves the salt tolerance of cucumber through root and stem limiting the translocation of Na⁺ to the leaf to alleviate salt damage.Whether grafting either using different scions or different rootstocks,the parts of limiting the translocation of Na⁺ to the shoot were mostly the stem and root of the rootstock,the effects of the rootstocks were are bigger than the scion on improving the salt tolerance of cucumber.