Effects Of UV-B Radiation On The Accumulation Of Phenolic Acids And Related Physiological Metabolism In 84K Poplar Plantlets

Poplar(Populus L.)is considered as one of the new model plants in woody plant research because of its fast growth,easy reproduction and strong stress resistance.In recent years,the ozone layer has been seriously damaged with the increase of environmental pollution which results in the gradual enhancement of UV-B radiation reaching the earth’s surface.UV-B radiation,as a typical environmental stress factor,has a significant effect on the growth and physiological metabolism of plants,so it is important to study the defense response of plants to UV-B radiation.Based on the above,84K poplar plantlets were subjected to UV-B radiation,and six defensive phenolic acids(ferulic acid,caffeic acid,gallic acid,protocatechuic acid,chlorogenic acid and isoferulic acid)were detected by UPLC-MS/MS.The optimal irradiation time was determined,and the accumulation of target phenolic acids was discussed from the perspectives of photosynthetic pigments,ROS system and biosynthesis enzyme gene expression.Finally,the morphological changes as well as the contents of lignin content,total biomass and photosynthetic pigment in 84K poplar plantlets after UV-B radiation relief were verified.The main findings of this paper are showed as follows:1.Effect of UV-B radiation on the accumulation of phenolic acids and photosynthetic pigments in 84K poplar plantletsSix phenolic acids(ferulic acid,isoferulic acid,gallic acid,protocatechuic acid,chlorogenic acid and caffeic acid)were analyzed by UPLC-MS/MS after 84K poplar plantlets exposed to UV-B radiation,results showed that the contents of six phenolic acids were enhanced after UV-B treatment,and the yield of chlorogenic acid was 46513.18±2230.03 ng/g at 8 h,which was 15 times higher than that at 0 h.In addition,it is a 24 h cycle that 16-24 h was found to be the best period of UV-B radiation for photosynthetic pigment accumulation,and the content of total chlorophyll was 33.333 ± 0.282 mg/g at 24 h,which was 1.18 times higher than that at 0 h.The results showed that suitable UV-B radiation could improve the accumulation of 6 phenolic acids and photosynthetic pigments in 84K poplar tissue culture seedlings.2.Effect of UV-B radiation on ROS system and biosynthesis gene expression in 84K poplar plantletsWith the increase of UV-B radiation duration,the contents of H2O2,MDA and OFR in 84K poplar plantlets were higher than that before UV-B radiation,and the activities of SOD,POD and CAT were enhanced in different degrees that showed that UV-B radiation triggered the ROS system of 84K poplar plantlets.In addition,qRT-PCR was used to analyze the expression of three genes(PAL,C4H and 4CL)closely related to phenolic acid biosynthesis in 84K poplar plantlets.It was found that the expression of PAL,C4H and 4CL increased significantly at 16 h,and there was a positive correlation between the transcription level of these genes and the contents of phenolic acids.The transcriptional abundance of PAL,C4H and 4CL was 3.98,7.73 and 5.59 at 16 h,respectively.The results showed that the synthesis and accumulation of phenolic acids in 84K poplar plantlets exposed to UV-B radiation were ascribed to the up-expression of these key biosynthesis genes.3.Effect of UV-B radiation relief on the morphological changes,growth and photosynthetic pigment content in 84K poplar plantletsResults of phenotypic analysis showed that the leaves of 84K poplar plantlets after UV-B radiation were yellowing at 24 h,and the micromorphology of the stems of 84K poplar plantlets after UV-B radiation relief was larger with increased xylem than that without UV-B treatment.Results of physiological analysis showed that the contents of lignin,total biomass and photosynthetic pigment in 84K poplar plantlets were increased after proper UV-B radiation The content of lignin increased 15.08%after UV-B treatment(8 h)than that without UV-B treatment.And,16-24 h of UV-B radiation was beneficial for the biomass accumulation.The content of biomass after UV-B treatment(16 h)increased 11.49%than that without UV-B treatment.And,8-24 h of UV-B radiation was beneficial for the chlorophyll accumulation.The content of total chlorophyll increased 31%after UV-B treatment(16 h)than that without UV-B treatment.Results showed that the development of secondary xylem was enhanced after the appropriate UV-B treatment,which was beneficial for the growth of 84K poplar plantlets.In conclusion,8-16 h of UV-B radiation(3w/m2)was the best treatment for phenolic acid accumulation in 84K poplar plantlets.UV-B radiation of 16-24 h was the best radiation duration for the accumulation of photosynthetic pigment in the tissue culture of poplar seedlings at 84K.UV-B radiation can trigger the ROS system of 84K poplar tissue culture seedlings,and the changes of the transcription level of three genes closely related to the biosynthesis of phenolic compounds were positively correlated with the changes of phenolic acid compounds content.UV-B radiation was removed and cultured for another 20 days could promote plant growth and development.In addition,the accumulation mechanism of phenolic acids in 84K poplar plantlets under UV-B radiation was preliminarily clarified in this study.