Repair And Regulation Mechanism Of Xylem Embolism Driven By Cortex Photosynthetic In Salix Matsudana

Embolism destroy the water delivery function of vessels,reduce the hydraulic conductivity of xylem,lead to stomatal closure of leaf and block photosynthesis.Therefore,synchronous repair in situ is very important to maintain the continuity of water conduction and then to ensure the gas exchange of the leaf.Studies have shown that the corticular photosynthesis of branches plays a key role in maintaining the hydraulic function of branches.Especially in the period of deciduous,corticular photosynthesis is the only source of carbon assimilates,and it is the main driving force of the situ repair of xylem embolism.However,the role of corticular photosynthesis in the repair of xylem embolism and its driving mechanism are still unclear.In this study,the current year twigs of Salix matsudana were taken as the experimental material,the chloroplast distribution in cross section and its photochemical efficiency were illustrated,on the basis of that,the difference of photochemical characteristic and structure between twig and leaf were analyzed.Moreover,the contribution and the regulation mechanism of corticular photosynthesis to the repair of xylem embolism were studied by light and dark treatment after different time.The pathway and driving force of in-situ repair of xylem embolism were analyzed.The main results are as follows:?1?Cortex chloroplast was the main contributor for the twig photosynthesis.The max photochemical efficiency of cortex tissue was 1.53 times of that of vascular tissue and 2.10times of that of pith tissue,respectively.The value of F694/F737 in cortex tissue was 0.53,which was 1.51 times of that in leaf,and the distribution ratio of light energy of cortex chloroplast to PSII was higher than that of leaf.Compared with leaf chloroplast,the cortex chloroplast had a higher ratio of chlorophyll b/a,which was conducive to capture and transfer of light energy.The proportion of starch grain to chloroplast space and the ratio of grana lamella to total lamella of cortical chloroplast were significantly higher than that of leaf chloroplast.So,by adjusting the composition of photosynthetic pigments and specializing the structure of sub-organelles,cortex chloroplast adapted to the physical and chemical environment of low light and acidification of the branches and made maximum use of the limited light energy inner the twigs.?2?Osmotic regulation is the main driving force for the in-situ repair of xylem embolism.After 2 h,4 h and 6 h of soaking treatment,the volume of xylem sap was increased by 17.28%,21.63%and 21.74%,and the percent loss of conductivity?PLC?was decreased by 16.99%,23.65%and 25.22%compared with the initial,respectively.This suggests that water can be transported into the xylem to repair the embolism.In the process of embolism repair,the contents of soluble sugar and inorganic ions(K⁺,Ca²⁺,Mg²⁺and Na⁺)in xylem sap were increased significantly.The osmotic potential of xylem sap was decreased by 46.82%,77.46%and 99.42%compared with the initial after 2 h,4 h and 6 h of soaking treatment,respectively.The establishment of osmotic differential pressure is the main driving force to drive water flow to embolism vessels.?3?Corticular photosynthesis drives radial water transport to promote embolism repair.When the embolized branches of 50%conductance loss in Salix matsudana,the PLC of the branch with light treatment was decreased by 10.67%,9.11%and 16.73%than that of dark treatment after 2 h,4 h and 6 h of soaking time.The contents of starch and soluble sugar in bark and xylem of branches with light treatment were higher than those with dark treatment.Corticular photosynthesis can provide non-structural carbohydrates such as soluble sugar for branches in situ.Corticular photosynthesis can regulate and control the starch response of woody parenchymal cell and transmembrane transport of osmotic adjustment substances by providing non-structural carbohydrates for woody parenchymal cells,and then increase the transposition of osmotic adjustment substances to embolism conduits,reduce the osmotic potential of embolism conduits and improve the efficiency of xylem embolism repair.?4?The genes related osmotic regulation mediates embolism vessel refilling.The pathway of carbohydrate metabolism,ion transport,aquaporin and ATP hydrolysis were upregulated during the process of xylem embolism repair driven by corticular photosynthesis.The genes of?-amylase?smAMY6,smAMY7 and smAMY8?,aquaporin?smPIP1;3?,sucrose synthase?smN-100 and smSPP1?,sucrose transporter?smSUT1?,carbohydrate transporter?smMFS1?,cation transporter?smMRS2,smKUP2 and smKUP3?,anion transporter?smSLC17A?,amino acid transporter?smAAT2?,ABC transporter?smCAF16 and smABCB1?,and ATP hydrolase?smATPN?are the key genes in the process of refilling.