Physiological And Molecular Mechanisms Of Leaves In Rhododendron Delavayi Seedling In Response To Waterlogging Stress

In this study,two-year-old Rhododendron delavayi seedlings were used as experimental materials.They were flooded for 0d（CK）,10d（WL10）,20d（WL20）,and 30d（WL30）,respectively.They were cultured for 10 days after flooding stress and marked WL10R,WL20R,and WL30R,respectively.Physiological index determination,transcriptome and metabolome were analyzed in the treated leaves of Rhododendron delavayi to explore the physiological and molecular mechanism in response to waterlogging stress.The resultes are important for the cultivation of Rhododendron delavayi and water management of wild Rhododendron.The main resultes are as follows:1.From the phenotypic response of Rhododendron delavayiseedlings to waterlogging,the growth state of Rhododendron delavayi did not change significantly after 10 and 20 days of waterlogging stress,and the growth state could be restored after recovery culture;but after 30 days of waterlogging stress,the leaves of Rhododendron delavayi had obvious withering,yellowing,curling and other symptoms,and the growth state could not be restored immediately after recovery culture.2.The photosynthetic parameters of Rhododendron delavayi seedling leaves were measured.The results showed that the photosynthetic rate（Pn）,stomatal conductance（Gs）and transpiration rate（Tr）showed a decreased change during waterlogging stress,of leaves but the intercellular CO₂ concentration（Ci）did not change significantly during the waterlogging stress compared with the control.In addition,the maximum light quantum transfer efficiency（Fv/Fm）,variable fluorescence（Fv）,maximum fluorescence（Fm）,and photochemical quenching coefficient（q P）in the fluorescence parameters of Rhododendron delavayi seedlings showed a downward trend,while the initial Fluorescence（Fo）and non-photochemical quenching coefficient（NPQ）showed an upward trend.3.Compared with the control,the ascorbic acid（ASA）content,hydrogen peroxide（H₂O₂）content,peroxidase（POD）,and ascorbate peroxidase（APX）activities in leaves of Rhododendron delavayi seedling did not change significantly after 10 days of waterlogging stess,but they all increased in varying degrees after 20 and 30 days of waterlogging stress.The results showed that the leaves of Rhododendron delavayi showed severe stress after 30 days of waterlogging.4.According to the previous experimental results,transcriptome and metabolome sequencing were performed on the seedling leaves of Rhododendron delavayi before the waterlogging treatment（CK）,30 days of waterlogging（WL30）,culture for 10 days after waterlogging for 30days（WL30R）.The results of transcriptome sequencing were analyzed for gene annotation,differentially expressed genes screening,pathway analysis.The results showed that many genes involved in the photosynthetic pathway were down-regulated after 30 days of waterlogging stress,and these genes were involved in encoding the photosynthetic system compared with the control.The changes in their expression may limit the photosynthesis of the treated leaves.In addition,some genes are involved in the glutathione-ascorbic acid（GSH-ASA）cyclce,such as NADPH oxidase gene（c61127.graph＿c0）,glutathione synthase synthesis gene（c52719.graph＿c0）,monodehydroascorbic acid reductase（MDHAR）gene（c58095.graph＿c0）and glutathione reductase（GR）gene（c56863.graph＿c0,c59612.graph＿c0）were significantly up-regulated after 30 days of waterlogging stress.It is speculated that these genes may be involved in the antioxidant stress system.Analysis of metabolome data showed that the content of methyl jasmonate accumulated significantly after 30 days of waterlogging stress compared with the control group.It is speculated that jasmonic acid may act as a signal to indirectly regulate the antioxidant system of Rhododendron delavayi in response to waterlogging stress.