Study On The Physiological Response Of Different Tree Peony Cultivars To High Temperature Stress In Summer And The Relieving Effect Of Melatonin

Tree Paeonia（Paeonia suffruticosa Andr.）originated in China,is one of the top ten famous flowers in China,known as the"king of flowers"and widely used in gardens,has important ornamental,economic and social value.Tree peonies are mainly planted in temperate and cool areas in the northern hemisphere.However,with global warming,extreme high temperature in summer in the cultivation area of tree peonies usually exceeds 40℃.Long-term exposure to high temperature may lead to yellow leaves,black spots and premature wilting of the above-ground parts of tree peonies,affecting the ornamental value of green leaves in summer.In this study,six tree peony varieties widely planted in Shandong were studied,including4-5 years old’Luoyanghong’,’Feng Dan’,’Bartzella’,’High Noon’and 1-2 years old’Roufurong’and’Juangyehong’,as the research objects,were observed on July 8,July 22,August 6,August 23 and September 12 under natural high temperature conditions in summer.The photosynthetic capacity,cell membrane permeability,antioxidant enzyme activity,osmotic regulatory substances and other physiological and biochemical indexes of plants were measured to explore the physiological response of different varieties of tree peonies to high temperature stress in summer.In addition,’Roufurong’and’Juanyehong’were selected as test subjects,foliar spraying concentration respectively 0μmol·L^-1,50μmol·L^-1,100μmol·L^-1,200μmol·L^-1,400μmol·L^-1melatonin,to analyze the relieving effect of melatonin on transient high temperature stress.1.The field temperature gradually increased in July and tended to be stable after the beginning of August.Since August 6,all varieties began to have different degrees of leaf damage,and morphological observation showed that’High Noon’and’Roufurong’suffered high temperature stress earlier and the damage was more serious.2.With the continuous high temperature stress in summer,the physiological and biochemical indexes of different tree peony varieties showed different changes.The photosynthetic parameters,chlorophyll fluorescence parameters and photosynthetic pigment contents of plants all showed a downward trend,and the intercellular CO₂concentration（Ci）increased slightly after reaching the lowest on August 23.The contents of H₂O₂and MDA,as well as the relative conductivity,showed an increasing trend with the continuous high temperature stress.The corresponding changes of protective enzyme activities were different.SOD activity and T-AOC increased to their respective peaks and then decreased,while POD and CAT activity showed a downward trend in general.The content of soluble protein decreased.Some of the soluble sugar content showed a downward trend for different tree peony varieties,while some showed an initialt increase and then a decrease.Proline Pro content showed an increasing trend.3.SPSS software was used to analyze the membership function value of the high temperature tolerance of each tree peony variety.With the continuous high temperature stress,upto the end of the stress,the ranking of each tree peony variety was’Luoyanghong’>’Bartzella’>’Feng Dan’>’High Noon’,’Juanyehong’>’Roufurong’.4.Exogenous melatonin can alleviate the damage caused by transient high temperature stress on tree peony.Exogenous melatonin could increase the photosynthetic pigment content and enhance the photosynthetic capacity of plants.The contents of H₂O₂and MDA and their relative conductivity were decreased,and the activities of SOD,POD and CAT were increased,and high soluble protein contentmaintained.The results showed that 100μmol·L^-1melatonin had the best relieving effect.In conclusion,the physiological and biochemical indexes of different tree peony varieties showed different changes under field high temperature stress in summer,but the physiological response pathways were basically the same.Exogenous melatonin can alleviate the damage caused by transient high temperature stress on tree peony.