Exogenous Application Of Melatonin Improves Drought Tolerance In Coffea Arabica Through Regulating Antioxidant Enzymes Activity And Photosynthesis

Coffee is the world's most widely consumed beverage and the second most traded commodity after oil.It belongs in the Rubiaceae family with approximately 124 species,of which three main coffee species are cultivated throughout the world,C.arabica,C,canephora and C.liberica.With the global population anticipated to largely increase in the near future,there is need for increased and sustainable food production to feed the ever-growing population.However,food production is faced with numerous challenges ranging from limited land for cultivation,drought,salinity,extreme temperatures as well as biotic stresses.Thus,crops that are able to tolerate these adverse growth conditions and maintain high yields are required.Drought stress is one of the key abiotic stresses threatening agricultural production and is considered to be the major environmental constraint affecting coffee production leading to an approximately 80%decrease of yields.Thus,it is important to develop strategies that enhance drought resistance in coffee plants in order to maintain its productivity.The protective role of melatonin in regulating physiological,morphological and biochemical functions of plants during abiotic and biotic stress has been widely studied,but little information is available on its mitigation effect on Coffea arabica plants.In this study,the effect of exogenous application of 100?M melatonin to Coffea arabica leaves was investigated.We expect to provide better insights in the regulatory role and effective application methods of melatonin in improving drought tolerance in coffee.The major findings obtained were as follows:1.Drought stress tolerance was greatly enhanced by both foliar leaf spraying and direct application of 100?M melatonin to the soil where C.arabica trees grew.2.Drought stress elevated levels of chlorophyll degradation,electrolyte leakage,lipid peroxidation(MDA),and ROS generation,which could be mitigated by melatonin application.3.Exogenous application of melatonin enhanced the activities of various antioxidant enzymes,such as catalase(CAT),guaiacol peroxidase(POD)and superoxide dismutase(SOD).4.Melatonin application suppressed the upregulation of chlorophyll degradation gene PAO encoding pheophorbide a oxygenase,and upregulated the expression of photosynthetic gene RBCS2 encoding ribulose-1,5-bisphosphate oxygenase(Rubisco)protein and a drought-related gene AREB encoding abscisic acid-responsive element binding protein.5.The photosynthetic efficiency of photosystem ?(PS?)under dark adaptation was also improved upon melatonin application.