Study On The Effects Of Different Combinations Of Rootstocks And Scions On The Growth And Salt Tolerance Of Handroanthus Spp.

With excellent landscape effects and high economic and medicinal values,Handroanthus impetiginosus and H.chrysanthus have been introduced and widely planted in southern China with promising market applications,but increasing soil salinization has hindered their application in coastal areas.Grafting plays an important role in enhancing plant adaptation to the saline environment and growth and development by changing the matter and information exchange between the root system and scion parts.Therefore,in this experiment,four different rootstock-scion combinations of impetiginosus/impetiginosus（P/P）,impetiginosus/chrysanthus（P/Y）,chrysanthus/impetiginosus（Y/P）,chrysanthus/chrysanthus（Y/Y）were obtained by grafting using the cutting method with salt-tolerant H.impetiginosus and salt-sensitive H.chrysanthus.Subsequently,experiments were conducted in the nursery to explore the effects of rootstock-scion combinations on the growth and flowering of the grafting plants.At the same time,physiological and biochemical parameters of grafted combinations under 0.5%NaCl stress were measured,and salt-tolerant genes were screened by transcriptomics,in order to preliminarily clarify the mechanism of different rootstock-scion combinations affecting the growth and salt tolerance of Handroanthus spp.grafted seedlings.The main test results are as follows:（1）Differences were found among the growth indicators,photosynthetic parameters,chlorophyll content,chlorophyll fluorescence parameters,and mineral element content of the four Handroanthus spp grafting combinations in the nursery.Plant height,biomass,net photosynthetic rate（Pn）,maximum light energy conversion rate（Fv/Fm）,elemental content of nitrogen（N）,phosphorus（P）and potassium（K）and chlorophyll content in leaves were significantly higher in the P/P and Y/P combinations with H.impetiginosus as rootstock than in the Y/Y combination,and the H.impetiginosus rootstock-scion combinations showed superior growth potential.The P/Y combination with H.impetiginosus as scion showed significantly better chlorophyll a content,Pn and N element content as well as plant height and dry weight than the Y/Y combination,indicating that the H.impetiginosus scion also had a significant effect on the growth potential of the grafted combination.The flowering period of both Y/P and Y/Y rootstock-scion combination was observed and found the combination with impetiginosus rootstocks presented 15-20 days earlier flowering period.（2）Under salt stress,the plant height,biomass,root activity,leaf relative water content and chlorophyll content of different rootstock-scion combinations of Handroanthus spp.were significantly higher than those of H.chrysanthus rootstock-scion combinations.For example,under salt stress,the contents of chlorophyll a and chlorophyll b in Y/P combination are 1.43and 1.35 times higher than those in Y/Y combination,respectively.Chlorophyll fluorescence parameters were significantly different in different rootstock-scion combinations,and the combinations of rootstock with H.impetiginosus were significantly better than that of the combinations with H.chrysanthus.For example,the Fv/Fm of Y/P was 1.11 times that of Y/Y under salt stress.（3）Salt stress significantly increased the relative conductivity,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）accumulation in leaves and roots of different rootstock-scion combinations of Handroanthus spp,but the relative conductivity,H₂O₂and MDA contents of the H.impetiginosus rootstock-scion combinations were significantly lower than those of the H.chrysanthus rootstock-scion combinations.For example,the H₂O₂and MDA contents in the leaves of Y/Y combination under salt stress were 1.49 and 1.26 times higher than those of Y/P combination,respectively.The activities of antioxidant enzymes（SOD,POD,CAT）and the contents of osmoregulatory substances（soluble sugars,soluble proteins,proline）in the leaves and roots of different rootstock-scion combinations increased significantly under salt stress,and the increase in the activities of antioxidant enzymes and the contents of osmoregulatory substances was greater in the rootstock-scion combinations with H.impetiginosus as rootstock.（4）Salt stress significantly reduced the K⁺content and increased the accumulation of Na⁺in the leaves and roots of different rootstock-scion combinations of Handroanthus spp.Compared with the H.chrysanthus rootstock-scion combinations,the Na⁺content in the leaves of the combinations with H.impetiginosus under salt stress was lower and the K⁺/Na⁺ratio was also significantly lower than that of the rootstock-scion combination with H.chrysanthus.For example,the K⁺/Na⁺ratio of the Y/Y combination under salt stress was 1.39 times lower than that of the Y/P combination.Compared with the H.chrysanthus rootstock-scion combinations,the combinations with H.impetiginosus rootstocks under salt stress had higher Na⁺content and lower K⁺content in the root system,indicating that salt-tolerant H.impetiginosus rootstocks could enrich Na⁺in the root system and translocate more K⁺to the leaves,which reduced ion toxicity and improved the salt tolerance of the rootstock-scion combinations.（5）Leaf transcriptome sequencing of P/P and P/Y combinations under NaCl stress showed that 5110 differentially expressed genes（DEGs）were found,and the number of DEGs in P/P combination was less than that in P/Y combination,but the up-regulated DEGs were more than them.According to GO function enrichment and KEGG pathway analyses,it was found that rootstocks with different salt tolerance affected the gene expression pattern of the leaves in H.impetiginosus scion.The key genes responding to salt stress were screened out from the differentially expressed genes.The results showed that under salt stress,salt-tolerant H.impetiginosus rootstock could alleviate the damage caused by salt stress by increasing the expression of ion transporter,osmotic adjustment protein and antioxidant enzyme genes,which had a positive effect on salt tolerance of the grafting combination.