Effects Of Exogenous Melatonin On Photosynthesis And Physiological Characteristics In Maize Under Drought Stress

Drought is an important abiotic stressor that has a strong negative effect on maize production in Northwest China.Enhancing the drought resistance of maize is beneficial to stabilizing grain production in this region.As a powerful antioxidant and plant growth regulator,melatonin can enhance the plant resistance to abiotic stresses.In this study,we employed pot experiment to investigate the effect of exogenous melatonin on the dry matter accumulation,chlorophyll content,gas exchange parameters,chlorophyll fluorescence,energy conversion efficiency,photosystem genes expression,photosynthetic enzymes activity and genes expression,and antioxidant system in two maize genotypes differing in drought tolerance,namely,‘Shaandan 609’（SD609）（drought-tolerant）and‘Shaandan 902’（SD902）（drought-sensitive）.Our objective was to clarify the physiological mechanism of two maize genotypes differing in drought tolerance in response to exogenous melatonin at the physiological and molecular levels and to explore the important potential of melatonin to enhance crops drought resistance in the field.Our research will provide theoretical basis for the application of melatonin in crop production.The main results obtained are as follows:1. Exogenous melatonin significantly improved photosynthetic characteristics of Shaadan 609 and Shaadan 902 under drought stress.Under drought stress,exogenous melatonin significantly increased dry matter accumulation,chlorophyll content,net photosynthetic rate（P_n）,stomatal conductance（G_s）,and transpiration rate（T_r）of Shaadan 609and Shaadan 902.Under moderate drought stress,exogenous melatonin increased intercellular carbon dioxide concentration（C_i）while reduced intercellular carbon dioxide concentration under severe drought stress,indicating that melatonin improves photosynthetic capacity of maize under drought stress by alleviating stomatal and non-stomatal limitation.2. Exogenous melatonin significantly improved the energy conversion efficiency and electron transfer rate in the light reaction of Shandan 609 and Shandan 902 under drought stress.Under moderate and severe drought stress,exogenous melatonin effectively improved the photochemical efficiency of PSII and the effective quantum yield of PSII and PSI[Y（II）and Y（I）],but it decreased the quantum yields of energy dissipation[Y（NO）and Y（NPQ）]and the donor-side and acceptor-side limitations of PSI[Y（ND）and Y（NA）]under drought stress.Additionally,application of melatonin enhanced the stability of electron transport chain and promoted photosynthetic electron transfer in maize leaves under drought stress.Moreover,exogenous melatonin upregulated expression of photosynthetic apparatus related genes,which revealed the molecular mechanism of the photosystem protected by exogenous melatonin.Compared with Shaandan 902,Shaandan 609 had higher expression levels of PSII and PSI-related genes under drought stress and exogenous melatonin.3. Exogenous melatonin significantly improved the CO₂assimilation in the dark reaction of Shaandan 609 and Shaandan 902 under drought stress.Under moderate and severe drought stress,exogenous melatonin up-regulated the activity and gene expression levels of carbonic anhydrase（CA）,phosphoenolpyruvate carboxylase（PEPC）,pyruvate phosphate dikinase（PPDK）,NADP-malate enzyme（NADP-ME）and ribulose-1,5-bisphosphate carboxylase/oxidase（Rubisco）involved in CO2 assimilation,suggesting that exogenous melatonin enhances the CO₂assimilation capacity in the dark reaction of maize seedlings.4. Melatonin treatment significantly reduced the content of superoxide anion,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）of Shaandan 609 and Shaandan 902 under drought stress,thus effectively alleviating oxidative stress induced by drought stress.Compared with Shaandan 902,melatonin has a more significant effect on reducing the accumulation of reactive oxygen species in Shandan 609.Under moderate and severe drought stress,melatonin treatment significantly increased the content of ascorbic acid（As A）and glutathione（GSH）.Application of Melatonin not noly enhanced the activity and gene expression levels of superoxide dismutase（SOD）and catalase（CAT）,peroxidase（POD）but also enhanced the activity and gene expression levels of key enzymes including ascorbate dehydrogenase（APX）,glutathione reductase（GR）,monodehydroascorbate reductase（MDHAR）,dehydroascorbate reductase（DHAR）in As A-GSH cycle.These results indicated that melatonin treatment could effectively increase the activity of antioxidant enzymes and the content of antioxidants and enhance the ability of maize seedlings’reactive oxygen species scavenge to inhibit the accumulation of reactive oxygen species and protect the membrane from membrane lipid peroxidation.Compared with Shaandan 902,melatonin-treated Shaandan 609 had more antioxidants and higher antioxidant enzyme activities under drought stress.In summary,exogenous melatonin significantly enhanced the drought tolerance of Shaandan 609 and Shaandan 902 under drought stress.Exogenous melatonin significantly improved the energy conversion efficiency in the light reaction and the CO₂ assimilation in the dark reaction of Shaadan 609 and Shaadan 902,and mitigated the inhibition of photosynthesis under drought stress.Meanwhile,exogenous melatonin effectively activated the antioxidant system and improved the antioxidant capacity of maize seedling.Therefore,exogenous melatonin enhanced the drought tolerance of maize seedlings.Compared with Shaandan 902,exogenous melatonin stimulates Shandan 609 to accumulate more antioxidants and higher activity of antioxidant enzymes.Thus Shandan 609 could more effectively alleviate reactive oxygen damage-induced by drought stress and effectively protect the structure and function of photosynthetic apparatus.It was the main reason that Shandan 609has stronger drought tolerance.