Effects of 2,4-epibrassinolide and Arbuscular Mycorrhizal Fungi on Photosynthetic Characteristics and Hormone Metabolism of Rhododendron hybridum under High Temperature Stress

GUO Jia; XIONG Dingwei; YANG Lijuan; CHENG Jianjun

doi:10.15933/j.cnki.1004-3268.2025.10.012

Abstract：To explore the mechanism of heat tolerance in Rhododendron hybridum under high temperature stress by foliar application of 2,4-electrobrassinolide(EBR),inoculation with arbuscular mycorrhizal fungi(AMF)Diversifiospora epigaea,as well as their combined treatment,using Rhododendron hybridum'Yunqi Yanxia'cuttings as experimental materials,indoor pot experiments were conducted to investigate the effects of control(CK,normal temperature,without adding AMF or EBR treatment),separate high temperature treatment(HS),HS+AMF,HS+EBR,HS+EBR+AMF treatment on the photosynthetic pigment content,gas exchange parameters,chlorophyll fluorescence parameters,and hormone content of R.hybridum.Under high temperature stress,the growth of R.hybridum was inhibited,and the appearance phenotype was characterized by yellowing and reduced leaf size,with most leaves dying.Spraying EBR on leaves could promote the increase of AMF infection rate,branch attachment rate,and vesicles,but had no significant effect on AMF number of entry point.Compared with high temperature treatment alone(HS),the content of chlorophyll a,chlorophyll b,total chlorophyll,and carotenoids in R.hybridum increased by 84.3％,182.3％,105.8％,and 150.6％,respectively,under HS+EBR+AMF treatment.The net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(Gs),and instantaneous water use efficiency(WUE)increased by 129.5％,78.9％,32.7％,and 28.1％,while the intercellular CO2 concentration(Ci)decreased by 11.7％.At the same time,HS+EBR+AMF treatment increased the maximum photochemical quantum efficiency(Fv/Fm),PS Ⅱ potential activity(Fv/Fo),and photochemical quenching coefficient(qP)of PS Ⅱ in the leaves of R.hybridum,while reducing the non photochemical quenching coefficient(NPQ).It promoted the energy used for electron transfer per unit reaction center(EETo/RC),absorbed light energy per unit area(EABS/CSm),captured light energy(ETRo/CSm),and quantum yield of electron transfer(EETo/CSm)in R.hybridum leaves.Under high temperature stress,the contents of zeaxanthin(ZT),gibberellin(GA3),and indole-3-acetic acid(IAA)in the leaves of R.hybridum decreased,while the content of abscisic acid(ABA)increased.After HS+EBR+AMF treatment,ZT,GA3,and IAA contents increased by 54.2％,43.2％,and 29.5％,respectively,while the content of ABA decreased by 26.5％.EBR can promote the infection and development of AMF,and compared with single AMF or EBR treatment,the combination of the two treatments has the best effect on enhancing the heat tolerance of R.hybridum.