Alleviative Effect And The Regulation Mechanism Of The Red To Far-red Light Ratio At 0.8 On Tomato Seedlings Under Salt Stress

Tomato（Solanum lycopersicum L.）is one of the main vegetables produced in greenhouse cultivation.However,salt stress caused by soil secondary salinization has become one of the prominent problems limiting the yield and quality of facility tomato.Our previous studies have proved that the red to far-red light ratio of 0.8（R:FR=0.8）can improve salt tolerance of tomato,but the regulation mechanism is still unclear.In this study,the tomato seedlings“Jinpeng Chaoguan”were used as the experiment materials and randomly treated with different light and salt stress conditions.There were four different treatments:（1）CK（normal hydroponics+white LED）;（2）L（normal hydroponics+R:FR=0.8）;（3）S（salt stress+white LED）;and（4）SL（salt stress+R:FR=0.8）.The alleviative effect of R:FR=0.8 on tomato seedlings under salt stress and the underlying regulation mechanism were investigated mainly from the following aspects:plant morphological formation,synthesis and metabolism of endogenous substances,photosynthetic performance,and the transcriptome response of tomato leaves.The main findings are listed as follows:1.R:FR=0.8 showed significant alleviating effect on tomato seedling growth under salt stress of 150 m M Na Cl,which was selected as the optimal salt concentration for the subsequent studies.R:FR=0.8 alleviated the growth inhibition of tomato seedlings at all concentrations of Na Cl.It increased the plant height,stem diameter,leaf area,leaf number,and biomass weight of tomato seedlings,and the increase was negatively correlated to the salt concentration.No significant symptoms of salt injury were observed when tomato seedlings were grown under 90 or 120 m M Na Cl stress conditions,whilst tomato seedlings could not survive under 180 m M Na Cl.Under 150 m M Na Cl,the tomato seedlings could survive,but showed obvious symptoms of salt injury.Therefore,150 m M Na Cl was screened and used as the salt stress condition for further studies.2.R:FR=0.8 significantly promoted the growth and development of root and shoot of tomato seedlings under salt stress,and increased the dry and fresh weight of roots,stems,and leaves,thereby improving the salt tolerance.R:FR=0.8 enhanced the activity of antioxidant enzymes such as superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,and ascorbate peroxidase（APX）in roots,reduced the contents of malondialdehyde（MDA）and H₂O₂,increased the contents of indole-3-acetic acid（IAA）and gibberellin A₃（GA₃）but reduced the content of abscisic acid（ABA）,and also improved the root activity（by 6.73-24.34%）,thereby facilitating the growth and development of the roots.Moreover,it facilitated the shoot growth,that is,significantly increased the plant height,stem diameter,leaf number,and leaf area with a 38.27-55.36%increase of the sound seedling index.For example,the biomass of roots,stems,and leaves were increased by 43.95%,98.77%,and 47.47%in fresh weight,and by 49.21%,122.22%,and 53.01%in dry weight,respectively,at day 12.3.R:FR=0.8 significantly improved the photosynthetic performance of tomato seedlings under salt stress.R:FR=0.8 increased the content of photosynthetic pigments in tomato leaves and improved the capture of light;it also increased the maximum quantum yield of PSII photochemistry（Fv/Fm）and thus improved the leaf health.Moreover,it increased the actual quantum yields of PSII[Y（II）]and PSI[Y（I）],decreased the quantum yield of non-regulatory energy dissipation of PSII[Y（NO）]and the non-photochemical quantum yield due to PSI acceptor side limitation[Y（NA）],thereby increasing the activity of PSII and PSI.It also increased the apparent quantum efficiency（AQE）,apparent carboxylation efficiency（ACE）,maximum electron transfer rate(J_max),maximum carboxylation efficiency(V_cmax),and triose phosphate utilization（TPU）of tomato leaves,thus enhancing the ability to utilize weak light and low concentration of CO₂,the regeneration ability of Ru BP,the carboxylation ability of Rubisco,and the utilization rate of triose phosphate,which are all conducive to the organic matter synthesis.Furthermore,it increased the relative water content（RWC）,ratio of palisade tissue thickness to spongy tissue thickness（P/S）,and tightness of leaf palisade tissue structure（CTR）,resulting in higher water content in tomato leaves.In addition,the stability of chloroplast morphology and structure were enhanced,which could sustain the chloroplast function.Reduced stomatal density and increased stomatal opening of tomato leaves were also observed,which helped decrease water loss and improve the CO₂ uptake.Meanwhile,it increased the activity of Rubisco and RCA and also up-regulated the expression of the related genes,which could lead to higher CO₂ fixation to improve organic matter synthesis.Therefore,R:FR=0.8 could significantly improve the photosynthetic performance of tomato seedlings under salt stress.4.R:FR=0.8 dramatically improved the osmotic regulation and antioxidant capacity of tomato leaves.R:FR=0.8 increased the activity of antioxidant enzymes（e.g.,SOD,POD,CAT,APX,DHAR and GR）and up-regulated the expression of the relevant genes;it also increased the contents of ascorbic acid（ASA）and glutathione（GSH）,decreased the contents of dehydroascorbic acid（DHA）and glutathione disulfide（GSSG）.The activated enzymatic and non-enzymatic antioxidant systems enhanced the scavenging ability to reactive oxygen species（ROS）,thereby reducing the accumulation of O₂.^-and H₂O₂.R:FR=0.8 also maintained the integrity of cells as showed by the significantly reduced MDA content（11.30-36.13%decrease）.In addition,the contents of proline,soluble sugar,and soluble protein increased by2.05-21.27%,25.99-103.95%,and 14.12-17.64%,respectively,leading to lower osmotic pressure of cells to alleviate osmotic stress.5.R:FR=0.8 significantly reduced Na⁺/K⁺and improved ion homeostasis and nutrient homeostasis.Salt stress broke the ion balance and nutrient balance,resulting in ion stress and nutrient deficit.R:FR=0.8 significantly promoted the gene expression related to ion transporters（including NHXs,SOS1,and HKTs）in old leaves,and mediated the efflux or compartmentalization of Na⁺into vacuoles,thus leading to decreases of the Na content and Na⁺/K⁺ratio by 8.14%and 19.60%,respectively.In addition,R:FR=0.8 significantly increased the contents of total nitrogen,total phosphorus,Ca,Mg,Fe,and Mn in tomato seedling,with an increase of 8.14%,6.73%,7.78%,18.84%,16.93%,and 47.43%,respectively,at day 8.This helped alleviate the adverse effects of nutrient deficiency on tomato growth and development.6.Transcriptome analysis verified that R:FR=0.8 improved the salt tolerance of tomato seedlings by modulating photosynthesis and antioxidant system.Transcriptome analysis revealed that the highest number of differentially expressed genes was observed between the SL treatment and S treatment（4282 DEGs,including 762 up-regulated genes and3520 down-regulated genes）.R:FR=0.8 promoted the de novo synthesis of D1 protein by triggering the psb A expression,which improved the activity of PSII reaction center.Meanwhile,multiple antioxidant enzyme-related gene transcripts were upregulated,which accelerated the ROS scavenging,and in turn improved the salt tolerance of tomato.Taken together,R:FR=0.8 treatment significantly reduced the damage of osmotic stress,oxidative stress,and ion stress on tomato seedlings.It enhanced the stability of chloroplast morphology and structure,improved the root activity and photosynthetic performance of leaves,thereby increasing the biomass accumulation and alleviating the inhibitory effect of salt stress on the growth and development of tomato seedlings.