The Role Of Exogenous Melatonin On Regulation Of Photosynthesis In Lettuce And Tomato Seedlings Under Salt Stress

Recently,the area of protected horticultural field has been expanded rapidly.Due to the semi-closed conditions of the facilities and the lack of rain leaching,as well as the irrational irrigation and over-fertilization,salt has been accumulated in the soil.As a result,secondary salinization of soil was aggravated,causing in decline of production and quality of vegetables.Thus,secondary salinization of soil has become a key obstacle in the development of the protected horticultural industry.The soil with secondary salinization contains different kinds of salts,of which anionic NO₃-and cationic Ca2+ content were considered the main parts.There are a lot of research on the NaCl stress,while the study of soil with secondary salinization is far behind.Studies have shown that melatonin can effectively improve photosynthesis in plants under stress conditions and increase salt adaptability of the plants,but the specific mechanism remains unclear.Therefore,it is of great significance to study the mechanism of melatonin in photosynthesis under salt stress,which benefits in scientific research and practical application.At first,lettuce is one of the main leafy vegetables in protected horticulture.Due the characteristic of enrichment of nitrate in leaves,the yield and quality of lettuce were affected by the secondary salinization of soil seriously.In this paper,the effects of exogenous melatonin on the photosynthesis of lettuce under Ca?NO₃?₂ stress were studied,including the antioxidant system,activities of photosynthetic enzymes and related genes' expression.The yield and quality of lettuce were also tested.Then,in order to explain the mechanism of regulation in the repair of photosynthetic apparatus by the melatonin,NaCl stress was used as treatment for the photoinhition on tomato seedlings.At present,the photoinhibition induced by NaCl stress is a mature model for studying the mechanism of plant photosynthetic apparatus under the salt stress.In this part,photosynthetic electron transport,key protein synthesis in photosystem II and gene expression of redox regulatory proteins were tested.The main results are as follows:1.Melatonin can improve the activity of PSII under Ca?NO₃?₂ stress,enhancing the quantum efficiency of photosynthetic electron transport.Melatonin also reduced the nitratecontent under Ca?NO₃?₂ stress and promotes the equilibrium mechanism of carbon and nitrogen assimilation.At the same time,melatonin reduced the stomatal limitation,increased the intracellular carbon dioxide concentration under stress,thus increased the photosynthetic rate and the growth of lettuce under Ca?NO₃?₂ stress.2.Melatonin enhanced the antioxidant capacity of lettuce leaves under Ca?NO₃?₂ stress by the increase in the activity of superoxide dismutase?SOD?and peroxidase?POD?,and the content of reduced ascorbic acid?As A?,reduced glutathione?GSH?and the ratio of GSH / GSSG,which decreased the level of reactive oxygen species?ROS?and increased the redox capacity.Melatonin reduces the oxidative damage of chloroplast membrane,internal basement and thylakoid membrane by ROS under Ca?NO₃?₂ stress,thus partially restored the morphology of chloroplast structure and maintained the morphology of thylakoid granule.Melatonin also improved the distribution of light energy,with more proportion for the photochemisty.The results indicated that exogenous melatonin can improve the antioxidant capacity,protect the photosynthetic structure,thus promoting the progress of photosynthetic electron transport under Ca?NO₃?₂ stress.3.Melatonin increased the gene expression of Rubisco small subunit?RBCS?in lettuce leaves under Ca?NO₃?₂ stress,and also increased the activities of Rubisco enzyme and Rubisco activase.The possible reason is that melatonin improved the GSH / GSSG,thereby increasing the oxidative reduction of enzyme activity.Melatonin increased the content of sucrose,glucose and fructose in lettuce leaves under Ca?NO₃?₂ stress,and also increased the activity of sucrose phosphate synthase?SPS?and acid invertase?AI?compared with Ca?NO₃?₂ stress alone,maintaining the dynamic balance of sucrose synthesis and metabolism.These results showed that melatonin could improve the ability of carbon assimilation and the metabolism of carbohydrate under Ca?NO₃?₂ stress,and promoting the synthesis of more photosynthetic products for energy and growth.4.Exogenous melatonin couldrelieve the decrease of net photosynthetic rate by NaCl stress.Melatonin regulated the non-stomatal limitation caused by long NaCl stress,which improved the activity of PSII and promoted the denovo synthesis of D1 protein in photoreaction center.Melatonin may protect the D1 protein translation process from oxidative damage by NaCl stress.The enhancement of the the antioxidant capacity by melatonin could be contributed.5.Melatonin induced the gene expression of the TRXf and the the abundances of TRXf and TRXm gene products,thus improving the ability of redox regulation.The synthesis of D1 protein relying on redox may be regulated by melatonin-induced TRXs,and improved the ability of repair of PSII under the NaCl stress.In this paper,the effects of exogenous melatonin on the photosynthesis of lettuce under Ca?NO₃?₂ stress were related to the promotion of antioxidant system,photosynthetic electron transport,carbon fixation and metabolism.In addition,The induced photoinhibition by NaCl stress were relieved by melatonin.Melatonin could promote the synthesis of photosynthetic related protein?D1 protein?,which was related to the expression of melatonin-induced TRXf and TRXm.Melatonin could improved the ability of repair of PSII to promote photosynthesis growt under the NaCl stress,which is the first study on the role of melatonin in improvement of repair of PSII under the salt stress.