Effect Of UV Induction On Secondary Metabolism And Oxidative-antioxidant System Of Rhamnus Erythroxylon Pall.Leaves

UV irradiation,as an abiotic stress factor,has important effects on physiology and secondary metabolism of plants.However,different plant responses differently under the exposure to UV irradiation.Rhamnus erythroxylon Pall.leaves can be used as medicine,and as a substitute for tea in some parts of China due to its strong fragrance.In the previous study of our lab,the chemical constituents of R.erythroxylon Pall.leaves and their seasonal variations were studied,which demonstrated the secondary metabolites were abundant in June.In this study,we used different UV（UV-A,UV-B,UV-C）to treat the fresh leaves in June（2、4、6、8 h）,to assess the effect of UV induction on the level of its secondary metabolites（flavonoids）and their synthesis pathways,as well as the effect on its oxidative stress-antioxidant defense system（oxidation-antioxidant system）,exploring the effect of ultraviolet induction on the secondary substance accumulation of R.erythroxylon Pall.leaves and its preliminary mechanism.This study provides theoretical basis and support for improving the quality of R.erythroxylon Pall.leaves by UV induction.The results are as follows:1.The responses of the secondary metabolism of R.erythroxylon Pall.leaves to UV-A,UV-B and UV-C radiation were significantly different.With the increase of UV-B and UV-C radiation time,the phenolic substances including flavonoids in the R.erythroxylon Pall.leaves showed a trend of rising first and then falling.The flavonoids reached a higher level when the leaves were induced for 2 or 4 h by UV-B and UV-C.The accumulation of total phenols in the R.erythroxylon Pall.leaves induced with UV-B for 4 h was more significant,increasing by26.77%.The activities of the key enzymes PAL,4CL and C4H in upstream of the flavonoid biosynthesis pathway,were also markedly enhanced when the leaves were induced with UV-B and UV-C for 2 or 4 h.UV-A radiation had relatively little effect on the secondary metabolism of the R.erythroxylon Pall.leaves.Short-term UV-B and UV-C radiation significantly increased the activity of key enzymes in secondary metabolism,promoting the accumulation of phenolic substances（including flavonoids）in R.erythroxylon Pall.leaves,especially UV-B.At 12 h after UV-B radiation（4 h）,the content of flavonoids and total phenols in the leaves increased by about 32%and 37%,respectively.2.The flavonoids and total phenols also changed significantly in R.erythroxylon Pall.leaves after being irradiated with UV-B,and generally reached a higher level at 12 h after radiation.The activities of the key enzymes PAL,4CL and C4H,in upstream of biosynthesis pathway,decreased sharply after UV-B radiation.UV-B induction had a certain residual effect on the accumulation of phenolic substances（including flavonoids）in R.erythroxylon Pall.leaves.3.UV-B radiation for 2h caused a sharp increase of H₂O₂level,and a decrease in the enzyme activities of SOD and POD in R.erythroxylon Pall.leaves.With he prolongation of radiation time（4 and 6 h）,the enzyme activities of SOD and POD increased significantly,and the level of H₂O₂decreased significantly.These results indicated that UV-B radiation caused oxidative stress,promoted the enhancement of antioxidant enzyme activity,scavenging excessive ROS.During UV-B radiation,DPPH free radical scavenging power markedly increased and then decreased,and this change trend was very similar to that of total phenolics.These results indicated that the oxidative stress caused by UV-B radiation,also promoted the increase of antioxidant substances,maintaining the oxidative balance in the cell.In summary,moderate UV-B radiation can induce the accumulation of phenolic substances（including flavonoids）in R.erythroxylon Pall.leaves,and enhance its antioxidant activity,improving its quality;UV-B radiation caused oxidative stress on R.erythroxylon Pall.leaves,then stimulated the enhancement of antioxidant enzyme activity in the cell,meanwhile,stimulated the enhancement of the activity of key enzymes in the synthesis of flavonoids,caused the increase of flavonoids to absorb the energy of UV-B radiation and eliminate ROS,thereby protecting against UV-B radiation.