Ameliorative Effect And Physiological Mechanism Of Exogenous Melatonin On Drought Stress Injury Of Tea Chrysanthemum

In this study,we used the common cultivation of tea chrysanthemum Chrysanthemum morifolium‘Hangbaiju’and Chrysanthemum morifolium‘Jinsiju’as test materials,the method of spraying different concentrations of exogenous melatonin by foliar application were used,We performed the determination of the growth indexes and physiological indicators which included photosynthetic parameters,chlorophyll content,fluorescence parameters,osmotic regulation material content and protective enzyme activity of the two tea-applied chrysanthemum under the natural drought stress.And the mitigation of drought stress in two kinds of tea chrysanthemum was investigated in order to providescientific basis for alleviating the production practice of drought stress injury of tea-applied chrysanthemum and exploring the mechanism of drought tolerance in chrysanthemum.The main findings were as follows:1.Exogenous melatonin treatment with appropriate concentration can alleviate the growth inhibition and plasma membrane injury of the two tea chrysanthemum cultivar‘Hangbaiju’and‘Jinsiju’under drought stress and the decrease rate of relative water content（RWC）was effectively slowed down in leaves.We found that 50μmol·L^-1200μmol·L^-1melatonin could maintain higher dry weight and root dry weight of the tea-applied chrysanthemum varieties to varying degrees.The malondialdehyde content and the relative permeability of the plasma membrane were reduced and the rate of decline of the relative water content（RWC）of the two tea-applied chrysanthemum varieties was effectively alleviated.Comprehensive evaluation and analysis showed that the treatment with 100μmol·L^-11 exogenous MT and 150μmol·L^-11 exogenous MT had the best effect on the two tea chrysanthemum varieties,and effectively alleviated the damage to the two tea-applied chrysanthemum varieties under drought stress.2.Under drought stress,exogenous MT can reduce the accumulation of excessive reactive oxygen species in the plants through the growth of antioxidant enzyme activities of chrysanthemum cultivar‘Hangbaiju’and‘Jinsiju’.Under drought stress,the levels of reactive oxygen species in the two varieties increased significantly.Exogenous application of MT further increased the activities of SOD,POD and CAT in the two tea varieties under drought stress,and the activities of SOD,POD and CAT showed a significant increase and then had a decrease significantly.The synergistic action of SOD,POD and CAT in the antioxidant enzyme system in the leaves alleviated the damage of reactive oxygen species.Regardless of whether or not melatonin was sprayed,there was no significant difference of activities of SOD,POD and CAT after rehydration in comparison to under stress.It was indicated that under drought stress,exogenous melatonin can eliminate excessive reactive oxygen species in the plant by increasing the activity of antioxidant enzymes,thereby alleviating the drought stress.3.Exogenous MT was beneficial to improve the osmotic regulation function of Chrysanthemum morifolium‘Hangbaiju’and Chrysanthemum morifolium‘Jinsiju’under drought stress.Under drought stress conditions,the contents of proline and soluble sugar in the two varieties of Chrysanthemum morifolium‘Hangbaiju’and Chrysanthemum morifolium‘Jinsiju’were significantly increased,and the content of soluble protein increased first and then decreased slowly.After spraying exogenous melatonin,the contents of proline,soluble sugar and soluble protein were significantly higher than those of pure drought stress.After rehydration,the content of free proline and soluble sugar in the two chrysanthemum varieties decreased under several treatments,which was close to the pre-stress level.Under drought stress,the soluble protein content of melatonin-treated Casuarina seedlings was significantly lower than that under stress.Seedlings under melatonin may regulate the osmotic pressure of the plants by regulating the organic osmotic adjustment substances in the two tea chrysanthemum planting plants,thereby improving the drought resistance of the two tea chrysanthemum varieties.4.Under drought stress,exogenous MT can improve the photosynthetic characteristics of Chrysanthemum morifolium‘Hangbaiju’and Chrysanthemum morifolium‘Jinsiju’and maintain strong photosynthetic ability.Under drought stress,chlorophyll degradation and stomatal conductivity decreased in two chrysanthemum cultivars,which affected the net photosynthetic rate,photochemical quenching coefficient（qP）and PSII maximum potential photochemical efficiency（Fv/Fm）and PSII actual photochemical efficiency were also reduced.After exogenous spraying of MT,the net photosynthetic rate,transpiration rate and stomatal conductivity of chrysanthemum leaves were increased by 44.11%,62.16%and79.61%compared with pure drought stress,and the concentration of intercellular CO₂ was22.33%lower than that of pure drought stress,and the difference was significant（P<0.05）.The net photosynthetic rate,transpiration rate,water utilization rate and stomatal conductivity of‘jinsi’increased by 38.19%,40.06%,41.33%and 68.03%respectively compared with pure drought stress.The concentration of intercellular CO₂ was 12.21%lower than that of pure drought stress,and the difference was significant（P<0.05）.After 2d of rehydration,the gas exchange parameters of two chrysanthemum cultivars were close to the control level.The Fv/Fm,qP andφPSII values of two chrysanthemum cultivars were all improved.The results showed that exogenous MT can relieve the decrease of net photosynthetic rate which caused by drought stress,increase the activity of PSII and slow down the decrease of chlorophyll content,thereby enhancing the drought tolerance of two chrysanthemum cultivars.