Studies On Influences Of Exogenous Nitric Oxide On Physiological Characteristics In Tomato Seedlings Under Salt Stress

Soil secondary salinization is becoming Worse in vegetable protected culture,which causes decline of yield and quality, brings a severe effect on sustainabledevelopment of vegetable production. Nitric oxide (NO) is an important activemolecule in biology and has various functions. NO was proved to participate in manykey physiological processes in plant such as seed germination, leaf expansion, rootgrowth, organs consenescence, programmed cell death and stress tolerance. It hasbeen reported that the addition of NO increased the salt tolerance of crops inexperiments within wheat (Triticum aestivum), maize (Zea mays), rice (Oryza sativa),lupin (Lupipus luteus), and reed (Phragmites communis). However, information is stilllacking regarding the effects of NO on salt tolerance of tomato (Lycopersiconesculentum). Therefore, the effects of sodium nitroprusside (SNP, an exogenous nitricoxide donor) on the physiological characteristics in hydroponic tomato seedlingsunder salt stress were investigated, in order to clarify the possible mechanisms ofenhancing salt-tolerance in plants. The main results were presented as follows.Firstly the effects of SNP at the concentration range of 0.05～0.8 mmol·L^-1 wereinvestigated on the growth, leaf protective enzymatic activities (superoxide dismutase(SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX)) andoxidative damage in tomato seedlings under 100 mmol·L^-1 NaCl stress condition. Theresults showed that the best effect on the alleviation of NaCl stress damage wasobserved in the treatment of 0.1 mmol·L^-1 SNP. At this concentration, the seedlinggrowth, chlorophyll content, activities of protective enzymes in leaves, and contentsof proline and soluble sugar in leaves were significantly increased, whilemalondialdehyde (MDDA) content and O₂·producing rate in leaves were significantlydecreased. The above results indicated that dosage effect of exogenous nitric oxidedonor (SNP) existed on the alleviation of NaCl stress in tomato seedlings, and the bestalleviating effect on NaCl stress damage was found at 0.1 mmol·L^-1 SNP, whichelevated the salt tolerance of plants. By the treatment of exogenous nitric oxide, leaf protective enzymatic activities(SOD, POD, CAT and APX), antioxidants contents (AsA and GSH), osmoticadjustment substances (proline, soluble sugar) were obviously increased, whilesoluble protein contents were elevated at a degree, MDA contents and O₂·producingrate were markedly decreased under 100 mmol·L^-1 NaCl stress condition. SNPtreatment alone had no significant effects on other upper indexes except that itincreased significantly APX activity, contents of AsA and GSH of salt-sensitivetomato cultivar. The activities of protective enzymes and the contents of antioxidantsand osmotic adjustment substances in salt-tolerant tomato cultivar were higher thansalt-sensitive one, MDA contents and O₂·producing rate were lower thansalt-sensitive one. The results showed that NO could alleviate the inhibition of growthof tomato seedlings, increase protective enzymatic activities, contents of osmoticadjustment substances and antioxidants under NaCl stress, which played an importantrole in enhancing the resistance to NaCl stress of tomato seedlings and alleviatingmembrane oxidative damage through keeping low ROS contents.By the treatment of exogenous nitric oxide, contents of Na⁺ and Cl^- weresignificantly decreased, contents of K⁺, Ca²⁺ and Mg²⁺, K⁺/Na⁺, Ca²⁺/Na⁺ andMg²⁺/Na⁺ values, S_{(K/Na)(absorption)} value and S_{(Ca/Na)(absorption)} value, S_{(K/Na)(transportation)} valueand S_{(Ca/Na)(transportation)} value were obviously increased in the roots, stems and leaves oftomato seedlings under 100 mmol·L^-1 NaCk stress condition. SNP treatment alone hadno significant effects on above parameters. The accumlation of Na⁺ and Cl^- in theroots, stems and leaves was higher in salt-sensitive cultivar than in salt-tolerant one,but the reverse was found for K⁺, Ca²⁺ and Mg²⁺. S_{(K/Na)(absorption)} value and S_{(Ca/Na)(absorption)} value, S_{(K/Na)(transportation)} value and S_{(Ca/Na)(transportation)} value in salt-sensitivecultivar were lower than in salt-tolerant one. The results showed that NO coulddecrease ion toxicity and improve the nutrient balance and alleviate the damage ofNaCl stress in tomato seedlings, and elevate the salt tolerance of plants.By the treatment of exogenous nitric oxide, net photosynthetic rate (Pn),transpiration rate (Tr) and stomatal conductance (Gs), Light saturation point (LSP),photosynthetic rate at light saturation point, apparent quantum yield (AQY), CO₂saturation point (CSP), carboxylation efficiency (CE), maximum RuBP regenerationrate and the chlorophyll contents were obviously enhanced. Light compensation point(LCP), CO₂ compensation point (CCP) were significantly decreased, Whileintercellular CO₂ concentration (Ci) was increased and stomatal limited value (Ls) was decreased at the early stage of stress (0～4 d), Ci was decreased and Ls wasincreased (6～8 d) in the leaves of tomato seedlings under 100 mmol·L^-1 NaCk stresscondition. SNP treatment alone had no significant effects on above parameters exceptthat it remarkably promoted maximum RuBP regeneration rate and obviouslydecreased CCP. Pn, Tr, Gs and chlorophyll contents of salt-tolerant tomato cultivarwere higher than those of salt-sensitive one. The results showed that NO couldimprove light energy conversion of tomato seedlings and reduce the photosynthesis ofseedlings under NaCl stress and elevate the salt tolerance of plants.By the treatment of exogenous nitric oxide, maximum fluorescence (Fm),photochemical efficiency of PSâ…¡(Fv/Fm), potential photochemical efficiency(Fv/Fo), the electron transport rate (ETR), actual photochemical efficiency of PSâ…¡(Yield), photochemical quenching coefficient (qP) and photochemical efficiency (P_rate) were increased, minimal fluorescence (Fo), non-photochemical quenchingcoefficient (qN), lowering the PFD limitation (L_PFD)and antenna heat dissipationrate (D_rate)were obviously in the leaves of tomato seedlings decreased under 100mmol·L^-1 NaCl stress condition. SNP treatment alone had no significant effects onupper indexes. Fm, Fv/Fm, Fv/Fo, ETR, Yield, qP, P_rate were higher in salt-tolerantcultivar than in salt-sensitive one, but the reverse was found for Fo, qN, L_PFD and D_rate. The results showed that NO could alleviate the damage of PSâ…¡caused by NaClstress and play an important role in enhancing salt resistance in tomato seedlings.By the treatment of exogenous nitric oxide, contents of abscisic acid (ABA),spermine (Spm), putrescine (Put) and Spd (spermidine), total polyamines (PA)contents and (Spd+Spm)/Put value were elevated, Put/PAs value was decreased underin the leaves of tomato seedlings 100 mmol·L^-1 NaCl stress condition. SNP treatmentalone had no significant effects on upper indexes. Upper parameters of salt-sensitivecultivar were lower than those of salt-tolerant one except that Put/PAs value washigher than that of salt-tolerant one. The results showed that NO could decrease thedamage of salt stress in tomato seedlings and elevate the salt tolerance of plants.