Effects Of Melatonin On Physiological And Biochemical Characteristics Of Zanthoxylum Armatum Under Waterlogging Stress

Zanthoxylum armatum DC.,a plant belonging to the Rutaceae family,is an important characteristic economic tree species in Sichuan Province,which has excellent economic,ecological and medicinal value.However,due to its shallow root system,it is very sensitive to waterlogging stress,which,together with the more concentrated summer rainfall in southern China in recent years,has severely restricted the growth and development and yield quality of Z.armatum.Melatonin is not only a natural antioxidant in plants,but also a novel plant growth regulator that is involved in abiotic stress responses in a variety of plants.Currently,the potential mechanisms of melatonin-mediated physiological effects in Z.armatum under waterlogging stress are still not elucidated.Therefore,in this study,we investigated the effects of different waterlogging times（0 d,1d,2 d,3 d and 6 d）and melatonin concentrations(0μmol·L^-1,400μmol·L^-1and 800μmol·L^-1)on the morphological appearance,photosynthesis,respiratory metabolism,antioxidant system,cell membrane permeability and osmoregulatory substance dynamics of Z.armatum seedlings by using a two-factor experimental design.and to discuss the effects of waterlogging stress on physiological characteristics of Z.armatum seedlings.The possible mechanism of melatonin treatment to improve waterlogging tolerance of Z.armatum leaves and screen out the best melatonin concentration for waterlogging tolerance,with a view to providing a reference for the management of seedlings of Z.armatum.The main results of this experiment are as follows:（1）Under waterlogging stress,the leaves of Z.armatum seedlings gradually wilted,yellowed and curled,and the roots rotted and blackened;the chlorophyll（Chl）content,carotenoid（Car）content,net photosynthetic rate（Pn）,stomatal conductance（Gs）,maximum photochemical efficiency of PSII（Fv/Fm）,photochemical quantum yield of PSII（Y（II））,PSII（F’v/F’m）,electron transfer efficiency（ETR）and relative water content（RWC）were all decreasing;superoxide dismutase（SOD）,intercellular CO₂concentration（Ci）,photochemical burst coefficient（q P）,non-photochemical burst coefficient（NPQ）,relative electrolyte（REC）,malondialdehyde content（MDA）,proline content（PRO）,ethanol dehydrogenase（ADH）and pyruvate decarboxylase（PDC）root respiratory enzyme activities showed an increasing trend;antioxidant enzyme activities such as peroxidase（POD）,catalase（CAT）and ascorbate peroxidase（APX）showed an increasing trend followed by a decreasing trend.At the same time,the physiological effects of waterlogging stress on the seedlings of Z.armatum became more obvious with the prolongation of waterlogging time,mainly manifested as lower photosynthetic capacity,stronger inhibition of respiratory metabolism,higher activity of antioxidant enzymes and more osmoregulatory substances,which eventually led to the deterioration of the growth condition of Z.armatum seedlings.（2）Melatonin treatments at different concentrations reduced the inhibitory effect of waterlogging on Z.armatum seedlings.With prolonged waterlogging,melatonin not only alleviated the wilting,yellowing and curling of seedlings by reducing the photosynthetic pigment content and photosynthetic efficiency caused by waterlogging,but also promoted the normal growth and development of Z.armatum seedlings under waterlogging stress.At the same time,the high level of antioxidant enzyme activity of melatonin-treated seedlings could effectively reduce the accumulation of reactive oxygen species（ROS）,thus reducing the level of lipid membrane peroxidation in seedlings to protect photosynthesis;the increased activity of ADH and PDC enzymes could inhibit the activity of anaerobic respiration-related enzymes and maintain normal respiratory metabolism;in addition,melatonin alleviated the degree of root damage,resulting in a reduction in the relative water content and proline content of seedling leaves.In addition,melatonin alleviated the degree of root damage,which led to a significant increase in the relative water content and proline content of seedling leaves,and a significant decrease in leaf electrolyte and MDA content,thus regulating the osmotic potential and maintaining cell membrane stability to prevent excessive water loss from seedlings under waterlogging stress.（3）Using mathematical analyses such as principal component and affiliation functions,SOD,RWC,ADH,REC,MDA,PRO,APX,PDC,CAT and POD were used as the main indicators of waterlogging tolerance in Z.armatum seedlings to comprehensively evaluate the waterlogging tolerance of three different melatonin concentrations under waterlogging stress.The combined melatonin tolerance values were 400μmol·L^-1>800μmol·L^-1>0μmol·L^-1.400μmol·L^-1melatonin was the most effective treatment,significantly improving chlorophyll content,photosynthetic rate,root respiratory metabolism and antioxidant enzyme activity of Z.armatum seedlings.In conclusion,waterlogging stress had a significant negative impact on the growth and development of Z.armatum seedlings,while 400μmol·L^-1melatonin treatment could reduce the damage caused by waterlogging stress and improve the waterlogging tolerance of seedlings.Therefore,melatonin can be applied appropriately to alleviate waterlogging stress in the subsequent field management of Z.armatum.