Low Ratio Red/Far Red Light Induced H₂O₂ Signal To Regulate Salt Tolerance In Tomato Seedlings

In recent years,the protected horticulture industry in China has developed rapidly.However,secondary soil salinization is the main limiting factor for facility cultivation and a serious obstacle to the sustainable development of protected agricultural production.As the main source of photosynthesis energy and environmental signals,light is one of the core conditions for the growth,development,yield,and quality of protected vegetables.Previous studies have found that low ratio of red/far red light（R/FR）can alleviate the harm of calcium nitrate stress to tomatoes,but the mechanism has not been clarified.Hydrogen peroxide（H₂O₂）and abscisic acid（ABA）,as typical signaling molecules,are involved in regulating plant growth and development and responding to various stress conditions.Studies have shown that there is an interaction between H₂O₂ and ABA in the regulation of certain physiological processes.Therefore,in this study,the relationship between signal molecules H₂O₂ and ABA and low ratio red/far red light to improve tomato salt tolerance was investigated,and their roles were analyzed to enrich the physiological and molecular mechanism of different light quality to improve plant salt tolerance.The LED light source used in the test has a series of advantages such as high efficiency,energy saving,small size,strong stability,fixed wavelength,wide application range,low heat generation,and no pollution.The R/FR ratio can be adjusted by adding far-red LED light source.In the experiment,tomato’Jinpeng No.1’was used as the experimental material.Five treatments were set up as follows:CK=normal nutrient solution cultivation;S=Ca（NO₃）₂ stress;SL=Low R/FR+Ca（NO₃）₂;L=Low R/FR;SLD=Low R/FR+Ca（NO₃）₂+DPI pretreatment.By measuring morphological,physiological,and photosynthetic characteristics of tomato seedlings at seedling stage,the effects of low R/FR values on morphological,physiological,and photosynthetic characteristics of tomato seedlings were explored.Inhibitor treatment was used to further explore the role of H₂O₂ and related genes in improving salt tolerance of tomato plants with low R/FR ratio.The main findings are as follows:1.Low R/FR ratio improved salt tolerance of tomato seedlings.Under calcium nitrate stress,the growth,photosynthetic gas exchange parameters,and chlorophyll fluorescence parameters of tomato seedlings significantly decreased.However,when the ratio of red light to far red light in the environment was low,the plant height,stem diameter,root length,dry weight,photosynthetic gas exchange parameters,ФPSII and Fv’/Fm’increased significantly,while NPQ decreased.Treatment with inhibition of H₂O₂ synthesis resulted in a significant decrease in plant height,root length,fresh weight,dry weight,photosynthetic rate and intercellular carbon dioxide concentration of tomato plants.2.Low ratio R/FR enhances salt tolerance of tomato seedlings by regulating stomatal movement.Through dynamic observation of stomata,it was found that adding low ratio R/FR treatment（SL treatment）to leaves under stress could quickly close stomata and reduce transpiration.At the same time,SL treatment can better balance the stomatal opening and closing,and the stomata of plants under stress treatment can open earlier to facilitate normal gas exchange.The treatment of inhibiting H₂O₂ synthesis caused the stomata of tomato plants to be unable to close in the short term,and the stomata were gradually closed after long-term treatment.In the non-stress treatment,low ratio R/FR（L treatment）also caused leaf stomata to close in a short period of time.3.Hydrogen peroxide as a signal molecule participates in the stomatal closure induced by low ratio red light and far-red light under calcium nitrate stress.The plant roots and leaves were pretreated with H₂O₂ inhibitor,and the photosynthetic gas exchange parameters,chlorophyll fluorescence parameters,ROS histochemical staining,H₂O₂content and the expression of the source gene family RBOHs were detected in the leaves of the materials supplemented with low R/FR treatment under calcium nitrate stress.The study found that low ratio R/FR could increase the H₂O₂ content and gene expression of RBOHs in tomato seedling leaves in the short term（24h）.After long-term treatment（120h）,the H₂O₂ content and gene expression of RBOH1,RBOHE,RBOHF and RBOHH significantly decreased.Pretreatment of plant roots and leaves with H₂O₂ inhibitors significantly reduced H₂O₂ accumulation in tomato leaves,and disrupted stomatal closure induced by low ratio R/FR,indicating that H₂O₂ participated in the rapid stomatal closure process of tomato leaves induced by low ratio R/FR.4.Abscisic acid plays a role in improving salt tolerance of tomato with low R/FR ratio.Salt stress can activate the expression of ABA biosynthesis gene NCED2 and increase ABA content at both 24h and 120h.Adding low ratio R/FR under calcium nitrate stress can still increase the content of ABA,and promote the expression of its synthesis and transport genes.After removal of H₂O₂,the synthesis and transport of ABA were significantly inhibited,and the content of ABA was significantly reduced,which suggested that ABA might be located in the downstream of H₂O₂ and participate in the mechanism of regulating salt tolerance of tomato with low R/FR ratio.