Study On The Secondary Flowering Characteristics Of Rhododendron Simsii

China is the country with the most abundant Rhododendron plant resources in the world,which has made a great contribution to the world’s Rhododendron breeding.Rhododendron simsii,a native species of the genus Ericaceae,is a wild flower resource with high ornamental value.The general flowering period is from March to April.Field investigation has found that there is a wild mangrove that can stable second flowering in the broad-leaved forest in the Qincun Reservoir area of Xinchang County,Shaoxing City,Zhejiang Province.The local observation results for many years show that after the spring flowering,this species can have a second flowering in the autumn of the current year,the second flowering character is stable,and the flowering time lasts from the middle of October to February of the next year.It has a large amount of flowers and good ornamental traits,which is of great significance to the study of the regulation of safflower stage in Rhododendron simsii,and can be used as an important material to study the secondary flowering traits of woody plants.In order to explore the causes of the secondary flowering phenomenon,this study took the secondary flowering red wine as the research object,and selected the primary flowering red wine as the control.Through phenology observation,flower bud development morphology observation,and flower trait evaluation,the flowering process and flower traits of the secondary flowering red wine were clarified.The potential reasons for the secondary flowering traits of Rhododendron simsii were explored from the aspects of endogenous hormones,flower bud nutrients and flowering molecular regulation mechanism.Key findings were as follows:（1）By using the improved BBCH coding system,the phenology of the second flowering Rhododendron in one year was recorded in detail.It was found that the second flowering Rhododendron was earlier than the first flowering Rhododendron in bud development,leaf development,shoot development and bud development,and the leaf fall and dormancy time were shorter than the first flowering Rhododendron.The results showed that the bud fullness of secondary flowering was higher than that of primary flowering.Phenotypic differences were observed in the middle and late September of secondary flowering.Some of the buds developed rapidly and expanded significantly,blooming in autumn and flowering in spring of the following year.The evaluation of flower characters found that the flower character index values of secondary flowering were lower in autumn than in spring,but the difference was not significant,and it had obvious advantages in flowering period,and the flowering lasted for a long time.（2）The contents of main plant hormones in the flower buds of secondary flowering Rhododendron simsii were measured.It was found that the contents of gibberellin,auxin and brassinolide were higher in the flower buds with obvious expansion in the late stage of development than in the normal buds,abscisic acid was lower than in the normal buds,and the level of zeatin nucleoside remained low.It was speculated that gibberellin,auxin and brassinolide positively regulated the bud development and flowering of secondary flowering,and low levels of abscisic acid and zeatin nucleoside were beneficial to the bud development and flowering.The ratio analysis of different hormones showed that the decrease of ABA/IAA,ABA/GA₃,ZR/GA₃,ABA/BR ratio was beneficial to flower bud development and autumn flowering.（3）The nutrients in the leaves surrounding the flower buds were measured during the bud development of secondary flowering.It was found that a large amount of nutrients were accumulated in the leaves at the early stage of bud development.The contents of soluble sugar,soluble starch,and soluble protein in the leaves around the flower bud were all lower than those in the ordinary flower bud,suggesting that a large amount of these nutrients were transferred from the leaves to the flower bud,and the flower bud with more nutrients accumulated developed faster and directly blossomed after maturity without dormancy.（4）Transcriptome sequencing was performed on the secondary flowering of the flower buds at the late stage of flower bud development,and differentially expressed genes were screened for the flower buds with phenotypic differences.Functional enrichment of differentially expressed genes revealed that plant hormone signal transduction pathways,metabolic pathways of starch and sucrose were closely related to the phenotypic differences in flower bud development.42 genes related to plant hormone synthesis and signal transduction pathways were screened.There were 39 genes related to synthesis and metabolic pathways of starch and sucrose,including ABF2,ABF3,PYL11,MPK5 and SCRM genes involved in the absidic acid pathway,GAI1,GA20OX3,GA20OX4,GASA7 and GID2 genes involved in the gibberellin pathway.IAA33,ARF6,ARF8,ARF1 and PHL7 genes in auxin pathway,AHP1,AHK2and AHK4 genes in cytokinin pathway,BAK1 and BRL2 genes in brassinolide pathway;The expression levels of ISA1,SBE,PU1 and LECRK genes in starch biosynthesis pathway and SUS5,SUS2,AGPS1,AGPS3 and AGP23 genes in sucrose biosynthesis pathway changed significantly in the differential flower buds.In conclusion,the metabolism of plant hormones was active in the late stage of bud development,and a large number of nutrients were transferred to flower buds in leaves.The joint regulation of plant hormones and nutrients promoted the rapid development of flower buds and early flowering.This study clarified the specific process of red second flowering in Rhododendron simsii,and initially revealed that plant hormone regulation and nutrient accumulation may be the main reasons for red second flowering in Rhododendron simsii.