| Chrysanthemum(Chrysanthemum morifolium Ramat.)is native to China.It is one of the four major cut flowers in the world and one of the top ten traditional famous flowers in our country.Under natural conditions,chrysanthemum usually blooms in autumn,and its seedling grows in summer with high temperature and is easily inhibited.Melatonin is a new type of plant hormone and has been widely studied as a growth regulator in recent years.Studies have shown that it has a significant alleviating effect on abiotic stresses of various horticultural crops.This article mainly studied the effect of melatonin on alleviating heat stress of chrysanthemum and its physiological and molecular mechanisms.The main findings are as follows:1.Set 0,50,100,200,400 μmol/L melatonin concentration gradient,stress treatment of chrysanthemum seedlings ?Jinba? under 40℃ high temperature conditions,determine the growth and development indexs of chrysanthemum seedlings on the 6th day after treatment,and evaluate the relative conductivity,MDA content,proline content,SOD activity and POD activity at 0,6,12,24,36,48 h after treatment.The results showed that after treatment of chrysanthemum seedlings with 100,200,and 400 μmol/L melatonin,on the 6th day of heat stress,it was observed that the plant height was significantly increased by 11.87%,10.07% and 10.07%,the dry weight was obviously increased by 19.57%,16.3%and 10.87% compared with the control treatment with water spraying,however,there was no significant difference in fresh weight.In addition,there was no significant difference between 50 μmol/L melatonin treatment and control.At the same time,at 48 h of heat stress,compared with spraying water treatment,200 μmol/L melatonin treatment reduced the relative conductivity and MDA content of chrysanthemum seedlings by 24.71% and26.71% respectively,while proline content increased by 32.14%,SOD and POD enzyme activity increased by 20.88% and 13.14% respectively.It shows that the higher concentration of melatonin alleviates the inhibition of high temperature on the growth of chrysanthemum seedlings,and the concentration of 200 μmol/L has the most obvious effect.2.Spray 200 μmol/L melatonin under 40℃ heat stress,set four different treatments:normal temperature spraying clean water,high temperature spraying clean water,high temperature spraying melatonin,and normal temperature spraying melatonin.The relative conductivity,MDA content,osmotic adjustment substance content,peroxide content,antioxidant content and antioxidant enzyme activity were measured at 0,6,12,24,and 48 h of treatments.Growth and photosynthetic indicators were measured at 0 d and 6 d determination.The results showed that the chrysanthemum leaves treated with melatonin and then subjected to heat stress for 48 h,the relative conductivity and the MDA content decreased by 44.03% and 17.36%,the content of proline,soluble sugar,and soluble protein increased by 10.74%,29.19% and 30.1%,respectively,the generation rate of superoxide anion radicals decreased by 14.1%,the ascorbic acid content increased by 16.91%,and the activities of SOD,POD,CAT and APX increased by 7.94%,17.7%,3.37% and 19.78%,respectively.On the 6th day of heat stress,spraying melatonin increased the total chlorophyll content by 38.01% compared with spraying water.At the same time,photosynthetic capacity was enhanced and plant growth was promoted.Fresh weight and dry weight increased by 30.97% and 22.12%,respectively;while spraying melatonin at normal temperature has little effect on the above physiological and growth and development indicators.3.Spray 200 μmol/L melatonin under heat stress(40℃),sampled the leaves of chrysanthemum seedlings for transcriptome sequencing(RNA-seq)at the 24 th hour of heat stress,and explored molecular mechanism on the melatonin pretreatment to improve plants resistance under high temperature conditions.RNA-seq involved the reactive oxygen species(ROS),heat shock transcription factor-heat shock protein,calcium ion-calmodulin,carbon fixation,starch and sucrose metabolism,hormone,and chlorophyll,flavonoid,and carotenoid pathway-related genes,suggesting that spraying melatonin under heat stress improved plant resistance,which was mainly regulated by the above differential expressed genes.4.Under the sub-high temperature stress condition of 35 ℃,set four different treatments: normal temperature spraying clear water,high temperature spraying clear water,high temperature spraying melatonin,and normal temperature spraying melatonin.The growth indicators were measured on the 0th,10 th,20th,30 th and 40 th days of the treatments,and the photosynthetic parameters and chlorophyll content were measured after the treatments.The results showed that under medium and long-term sub-high temperature conditions,compared with spraying with water,spraying melatonin at 40 days increased the plant height by 16.96%,the leaf area by 23.25%,the stem thickness by 21.19%,the fresh weight of the ground and underground parts increased by 14.58% and 12.94%,the dry weight of the ground and underground parts increased by 25.19% and 38.4%,the leaf spacing increased by 18.53%,the strong seedling index increased by 14.23%,the relative growth rate increased by 16.93%,the total chlorophyll content increased by 34.44%,and the photosynthetic rate increased by 58.42%. |
