Effects of melatonin foliar spray on growth and physiology regulation of pumpkin seedlings under NaCl stress

【Objective】To explore the effects of different concentrations of melatonin on the growth and physiology characteristics of pumpkin seedlings under sodium chloride （NaCl） stress,which could provide theoretical basis for using melatonin to improve the salt tolerance of pumpkin seedlings.【Method】The pumpkin hybrid variety Silver Chestnut was used as the experimental material,and pot experiment was carried out.When the pumpkin seedlings grew to 3 leaves and1 bud,the salt stress environment was simulated by watering with 150 mmol/L NaCl.Five melatonin spraying concentrations were set as 10.0 μmol/L （T1）,20.0 μmol/L （T2）,50.0 μmol/L （T3）,100.0 μmol/L （T4） and 150.0 μmol/L （T5）,another 2 groups were set as control,no melatonin treatment after NaCl stress （CK） and water irrigation treatment without NaCl stress （CK0）.On the day of salt stress,melatonin solution was sprayed at night during the period of no light,and then sprayed once every 1 d.The growth indexes of pumpkin seedlings were measured on 0,6,12 and 18 d after treatment,the photosynthetic indexes,chlorophyll content,root length,root activity and antioxidant system indexes were measured 18 d after treatment.【Result】NaCl stress affected the normal growth and development of pumpkin seedlings.After melatonin treatment,the growth effect of pumpkin seedlings was improved to varying degrees.Among them,18 d after treatment,T3 treated pumpkin seedlings showed the best improvement in plant height,stem diameter,total chlorophyll content,root activity and catalase （CAT） activity.Compared with CK,they were significantly increased by 41.71%,41.08%,83.39%,46.77%and 160.00%respectively （P＜0.05）;fresh weight,dry weight,dry-fresh ratio,superoxide dismutase （SOD） activity were also increased,and malondialdehyde （MDA） content was decreased.Compared with CK,low concentration melatonin treatments （T1 and T2） had an improved effect on photosynthetic indexes,while high concentration melatonin treatments （T3,T4 and T5） had lower photosynthetic rate （P_n） than CK.The results of principal component analysis showed that the ranking of principal component scores of different treatments was CK0＞T3＞T4＞T2＞T1＞T5＞CK.【Conclusion】Foliar spray of melatonin with appropriate concentration can effectively alleviate the damage of NaCl stress on pumpkin seedlings and improve the salt tolerance of pumpkin seedlings.Based on the growth and physiology indexes of pumpkin seedlings,spraying 50 μmol/L melatonin has the best effects on alleviating the injury of pumpkin seedlings under NaCl stress.