Physiological And Biochemical Effects Of Melatonin On Carya Cathayensis Sarg.under Drought Stress

Chinese hickory(Carya cathayensis Sarg.)is an important nut yielding plant of Jiangsu and Zhejiang provinces.In the current study,one year old seedlings of Chinese hickory were used to access melatonin mediated regulation of PEG induced drought stress.This experiment was comprised of eight treatment group.Control group,no treatment of drought or melatonin(CK),mild drought P1(5%PEG),moderate drought P2(15%PEG),severe drought P3(25%PEG),spraying of melatonin without drought(M),spraying melatonin with mild drought,(M-P1),spraying of melatonin with moderate drought(M-P2),spraying of melatonin with severe drought(M-P3).The effect of melatonin on the growth,physiological and biochemical characteristics of Chinese hickory plants under drought stress was investigated.The experimental results showed that:1.Drought stress seriously affected the phenotype of Chinese hickory leaves and reduced the relative water content(RWC)along with reduction in the total chlorophyll content(CHl).Melatonin effectively increased the RWC and CHl content in the leaves under drought stress.2.Under drought stress,the net photosynthetic rate and gas exchange parameters of Chinese hickory plants pretreated with melatonin were maintained at a higher level.Melatonin effectively protected the function of photosystem Ⅱ(PSII),thus maintaining the assimilation capacity of the leaves.Under drought stress,Chinese hickory showed little adaptability by improving its own water utilization rate,and further we found that the water utilization rate of leaves pretreated with melatonin was improved.Chinese hickory under severe drought stress condition(30 days after treatment)showed sharp decline in the maximum photosynthetic efficiency(Fv/Fm),actual photosynthetic efficiency Y(Ⅱ)and electron transfer rate(ETR).However,we found that melatonin pretreatment resulted in recovery of all above mentioned parameters which were earlier negatively affected by drought stress.3.Melatonin also functions like an antioxidant molecule and has the efficiency of scavenging reactive oxygen species(ROS)like hydrogen peroxide in plant cells.Moreover,melatonin also up-regulated the expression of antioxidant enzyme genes and improving their activities under drought stress.In the current study,we also observed that the activities of SOD,POD and APX were increased under drought stress,and further enhancement was noticed in their activities after melatonin treatment.This activation of antioxidant system can be attributed to the effective removal of excessive ROS and finally maintaining the normal REDOX balance in plants4.The content of malondialdehyde(MDA)in plants was increased under drought stress,which indicated that the degree of membrane lipid peroxidation was increased due to drought.To induce some resistance,content of proline(an osmolyte)got enhanced under drought stress which could be helpful in maintaining water balance in plant cells along with triggering the scavenging of ROS and reduction of lipid peroxidation.After treatment with melatonin,MDA content was decreased,while Proline was increasedConclusion:The current study established that drought stress significantly reduced the photosynthetic performance of Chinese hickory,and long-term drought damaged the plant’s photosystem-Ⅱ(PSII).Spraying melatonin improved the activity and gene expression of antioxidant enzymes in plants under drought stress,maintaining the redox balance,hence protecting the normal physiological functions of chloroplasts.Enhancement in the content of osmotic regulatory substances in plants further maintained the plant water balance in Chinese hickory under drought stress.