Mechanism Of Exogenous Salicylic Acid And Calcium Chloride Improving Salt Tolerance Of Wetland Plant Iris Pseudacorus L.

Wetland plants play a major role in the process of wastewater treatment in constructed wetlands（CWs）.The inhibition of salt stress on plants may reduce the performance of CWs.Therefore,the search for simple and cost-effective method is urgently needed to improve the salt tolerance of existing common wetland plants.Researches have shown that the rational application of exogenous growth regulators（such as salicylic acid,calcium ion and abscisic acid）can significantly promote the resistance of plants to cold,drought,high salinity,pests and diseases.Salicylic acid is a plant growth regulator,which can improve the tolerance of plants to salt stress,water stress,heavy metals and temperature stress by promoting plant growth.Ca²⁺,an essential element for cell composition,which plays an important role in the stability of cell structure and helps to improve the salt tolerance of plants.In this study,SA and/or CaCl₂ were used for root pretreatment to alleviate the salt stress（1%NaCl）in Iris pseudacorus L..The effects of different concentrations of exogenous substances on the growth index,physiological index and transcriptome of Iris pseudacorus L.were studied,and the best treatment concentration M3（0.1 mM SA+15 mM CaCl₂）was selected,which provided theoretical basis for enhancing the salt tolerance of wetland plants and expanding the application of constructed wetlands in the treatment of saline wastewater.The main results are as follows:（1）Salt stress inhibited the growth of Iris pseudacorus L..In the salt-stressed medium containing 1%NaCl,with the extension of exposure time（after 40 days）,salt stress had obvious inhibitory effect on the growth of Iris pseudacorus L.,which showed slow growth,decreased photosynthesis,toxic Na⁺,increased cell membrane permeability,excessive accumulation of MDA and so on.（2）Exogenous SA and CaCl₂ improved the salt tolerance of Iris pseudacorus L.from morphological and physiological aspects.Different concentrations of exogenous SA or CaCl₂ pretreatment can alleviate the growth of Iris pseudacorus L.under salt stress,which shows that the growth indexes such as leaf length,leaf width,root length,dry weight and fresh weight are improved,and CaCl₂is better than SA in leaf width and fresh weight.Exogenous SA and CaCl₂ also increased chlorophyll content and improved photosynthesis of plants.In addition,the two exogenous substances also played a significant role in increasing the content of K⁺,pro and the activity of antioxidant enzymes.The application of SA and CaCl₂ prevented the increase of cell membrane permeability to a certain extent,which showed the decrease of Na⁺,MDA content and relative conductivity.The optimum concentrations of SA and CaCl₂ are 0.1mM and 15 mM,respectively.（3）Exogenous SA and/or CaCl₂ have a certain timeliness and concentration range in alleviating salt stress.Exogenous SA or CaCl₂ has a certain concentration range,and both lower and higher concentrations will cause double stress to Iris pseudacorus L..Compared with single treatment,the salt tolerance of Iris pseudacorus L.was improved more effectively by the compound treatment of SA and CaCl₂,and the time of improving antioxidant enzyme activity was prolonged by the compound treatment,and the best combination was"0.1 mM SA+15 mM CaCl₂".After 40 days of salt stress,the activities of SOD,CAT and POD increased by 36.7%,37.8%and 73.8%respectively.（4）Exogenous SA and CaCl₂ improved the salt tolerance of Iris pseudacorus L.at gene level.Transcriptome analysis was carried out on the leaves of Iris pseudacorus L.treated with NaCl and SA+CaCl₂,and the differentially expressed genes after the two treatments were compared.RNA-seq analysis showed that after pretreatment with SA and CaCl₂,the expression of hormone signal gene,potassium transporter gene,oxidative stress gene and photosynthesis gene were all up-regulated,which indicated that the application of exogenous substances reveals the mechanism of salt tolerance of Acorus calamus at the gene level.（5）The application of exogenous SA and CaCl₂ improves the treatment efficiency of saline wastewater in constructed wetland.The results show that in the salt stress reactor（WCK2）,with the extension of salt stress（1%NaCl）,the leaves of Iris pseudacorus L.gradually lose water,shrink and turn yellow.The removal rates of COD,NH₄⁺-N,NO₃^--N and TP in constructed wetlands WS3（Plants were pretreated with 0.1 mM SA）,WC3（Plants were pretreated with 15 mM CaCl₂）and WM3（Plants were pretreated with 0.1 mM SA+15 mM CaCl₂）treated by exogenous SA and/or CaCl₂ are about 15.57%,24.89%,12.40%and 5.24%higher than those in WCK2,respectively.It can be seen that the application of exogenous SA and CaCl₂ improves the treatment efficiency of saline wastewater in constructed wetland.