Photoprotective Mechanism Of Exogenous Spd In Tomato Leaves Under Salinity-alkalinity Stress

Salinity-alkalinity stress is becoming one of the most serious abiotic stresses affecting crop growth, development and yield, and causes considerable economic losses. In recent years, due to the rapid development of protected cultivation and much fertilizer used in process of cultivation, the soil secondary salinization has become one of the most severe problems in the protected cultivation.Tomato(Solanum lycopersicum) is an economically important horticultural crop that is sensitive to salinity-alkalinity stress. The growth and development of tomato seedlings are inhibited when they are subjected to salinity. Thephotosynthesis efficiency of tomato is significantly declined by salinity-alkalinity stress, which severely affects yield and quality of tomato. PAs are low molecular weight, ubiquitous, nitrogenous compounds that function in many cellular processes in numerous organisms. PAs areinvolved in plant growth, development, cell differentiation, fruit maturation, cell ageing and stress responses.At present, the view that PAs could increase the photosynthesis under stress has been recognized. However,the regulation of exogenous spd to photoprotective mechanism in salinity-alkalinity resistance of tomato leaves is still unclear.The present study was conducted hydroponics to investigate the effects of exogenous spraying spermidine(0.25 mM Spd) on the structure and function of photosynthetic apparatus in tomato cultivar(Jinpengchaoguan) were exposed to75 mM salinity-alkalinity stress(NaCl:Na2SO4:NaHCO3:Na2CO3=1:9:9:1, pH=8.6). We discussed the interaction mechanism of adaption variation that exogenous Spd regulated the changes of the structure and function of photosynthetic apparatus, chlorophyll fluorescence, chloroplast ultrastructure, xanthophyll cycle, chlorophyll metabolism, D1 protein metabolism, yield,quality and transcriptomein tomato under salinity-alkalinity stress.The main results are as follows:1. Salinity-alkalinity stress induces photoinhibition and photodamage in tomato seedlings. Exogenous Spd alleviates the stress-induced inhibition of plant shoot growth and Pn by reducing ROS and stabilizing thylakoid membrane structure. Exogenous Spd indirectly improves total V + A + Z pool size, especially the amount of Z, which acts as an antioxidant in the lipid phase of the thylakoid membrane to block photo-oxidative stress. However, under salinity-alkalinity stress, Spd-induced increases in Z content did not increase NPQ compared with that in seedlings without Spd. This may be because Spd protects leaves from light-induced damage due to qP, which differs from heat dissipation.2. Salinity-alkalinity stress inhibits plant growth at least in part by inhibiting photosynthesis. The evidence for this suggestion is that exogenous Spd mitigated many of the negative effects of salinity-alkalinity stress through regulating chlorophyll and D1 protein metabolism. Under salinity-alkalinity stress, PBGD was down-regulated and Chlase was up-regulated; both of these effects were alleviated by exogenous Spd. Exogenous Spd increased the chlorophyll content of tomato seedlings 4 days after stress. Exogenous Spd may also increase D1 protein content during stress by activating psbA expression. The effects of exogenous Spd on chlorophyll and D1 protein content may protect the integrity of the photosynthetic apparatus and increase Pnmax and LSP. Increased photosynthesis will have clear positive effects on plant growth under stress.3. Transcriptome and cluster analysis of differential expression genes showed that CS treatment is closest to CK, followed by SS. S treatment was most different to CK. There were 3512 differential expression genes between S treatment and CK. These differential expression genes were involved in 114 metabolic pathways. RT-PCRanalysis of photosynthesis, photoprotection and polyamines metabolism related genes showed that exogenous Spd alleviated the expressions of psbC, psbD, rbcL, rbcS, PBGD, Chlase, ZEP, VDE, ODCand SAMDCunder salinity-alkalinity stress. These result showed that exogenous Spdalleviated the effects of salinity-alkalinity stress on photosynthesis, xanthophyll cycle and polyamines metabolism.4.Salinity-alkalinity stress decreases tomato single fruit weight, yield per plant,number and content of volatiles.Salinity-alkalinity stress increases the content of soluble sugar, organic acid, soluble protein,nitrate nitrogen,Vc, lycopene and 1-Penten-3-one. Salinity-alkalinity stress also increases the fruit fitness. Exogenous Spd increases the osmotic regulation ability and antioxidant ability through increasing the content of soluble protein and Vcrespectively, as compared with those without Spd. Exogenous Spd also increases the content of 6-methyl-5-Hepten-2-one and decreases the fruit fitness, thus, increases the fruit taste and flavors of tomato fruit.Therefore, exogenous Spd increases the yield and quality of tomato under salinity-alkalinity stress.