Effects Of Melatonin On Photosynthetic Performance And Antioxidant Enzyme System Of Quercus Rubra Under Simulated Drought Stress

The research on plant stress is a hot topic in recent years,and it is one of the main problems in agricultural production,among which the research on drought stress is of great significance.Quercus rubra L.is a large number of urban greening and vegetation restoration tree species introduced and cultivated in China in recent years.At present,there are few studies on the stress resistance of Q.rubra,which limits its introduction to arid areas.At the same time,as a new plant hormone,the role of melatonin(MT)in plant drought resistance has been confirmed in some species,but its role in the North American holly drought resistance needs to be explored.In order to study the effect of melatonin on the photosynthetic performance and antioxidant enzyme system of Q.rubra under drought stress,the seedling of Q.rubra was used as the experimental material.One group was sprayed with 100 μM melatonin solution for 5 times,and the other group was sprayed with distilled water.The next day after the last spraying of melatonin or distilled water,a simulated drought stress test was carried out on Q.rubra with different concentrations of PEG(5%,15%,25%,prepared with 1/2 Hoagland nutrient solution).The photosynthetic performance and antioxidant enzyme activity of North American oak were measured at 0,14,21 and 30 days after drought stress.The experimental results show that:Drought stress reduced RWC and Chl in leaves of Q.rubra.Melatonin can effectively improve the relative water content and chlorophyll content of plant leaves under drought stress.Under the condition of drought stress,Q.rubra can enhance its adaptability to drought by improving its water use efficiency.The water use efficiency of leaves of Q.rubra pretreated with melatonin will be further improved.At the same time,the net photosynthetic rate and gas exchange parameters will be kept at a higher level,and the function of photosystem II(PSII)will also be effectively protected,so that the same plant The chemical capacity remained at a high level.At 30 days,the maximum photosynthetic efficiency(Fv/FM),the actual photosynthetic efficiency(Y(II))and the electron transfer efficiency(ETR)of the leaves of Quercus acutissima under drought stress decreased,especially in the severe drought stress group(P3).However,after melatonin pretreatment,the related parameters of PSII of Q.rubra were all improved,which indicated that spraying could improve the photosynthetic efficiency of PSII.Melatonin can effectively protect PSII from drought stress.This study showed that the activities of SOD,pod and APX in leaves of Q.rubra under drought stress increased,and the activities of these antioxidant enzymes in melatonin pretreated group increased further.The increase of the activities of antioxidant enzymes can effectively eliminate excessive active oxygen in plants and maintain the normal redox balance in plants.As a broad-spectrum antioxidant,melatonin can directly scavenge hydrogen peroxide(H2O2)in plants.It also has the function of up regulating the expression of antioxidant enzyme gene and increasing its activity under stress.Drought stress caused the increase of MDA and proline content in plants,which indicated that the degree of membrane lipid peroxidation of plant cells was deepened under drought stress,and the resistance of plants to drought was gradually strengthened.Compared with the control group without melatonin,the MDA of melatonin treated group decreased,while proline increased.The results show that melatonin can reduce the degree of membrane lipid peroxidation by increasing the antioxidant capacity and the content of osmoregulation substance.The results showed that under PEG simulated drought stress,the photosynthesis of Quercus acutissima was inhibited and the redox balance was destroyed,and the antioxidant enzyme activity was increased.Spraying 100 μ m melatonin before the beginning of drought stress can improve the ability of scavenging reactive oxygen species,maintain higher net photosynthetic rate and effectively alleviate the damage caused by drought stress on plants.