Physiological And Biochemical Mechanisms Of NaCl Toxicity And Detoxification In Lycopersicon Esculentum Mill. In Green House

High concentrations of salts in soils account for large decreases in the yield of a wide variety of crops.Because of the limitation of greenhouse and bad management, crops are routinely subjected to a combination of different abiotic stresses in China. Tomato(Lycopersicon esculentum Mill) is widely planted in greenhouse and easily affected by low temperature,low light intensity and salinity.Therefore,it is very important to study the physiological mechanisms of salt toxicity in tomato and to take measures to decrease the toxicity.In this study,hydroponic culture experiments were conducted to investigate the physiological mechanisms of salt toxicity to tomato and the effects of low temperature,light intensity,grafting and salicylic acid.The main results obtained were presented as follows:1.Effects of salt stress on plant growth,photosynthesis,nutrient element uptake in tomato plants under different root zone temperature.Salt stress and low root zone temperature inhibited the growth of shoots and roots, more significantly at combination of salt and low root zone temperature.Salt stress increased Na content in leaves and roots,decreased Ca content in roots,K and Mg content in leaves and roots,especially at low root temperature.Salt stress decreased chlorophyll content,photochemical quenching(qP),the efficiency of excitation energy capture by open PSII reaction center(Fv'/Fm'),electron transport rate(ETR) and the quantum efficiency of non-cyclic electron transport of PSII(ΦPSII),more significantly at low root zone temperature.As a result,the net photosynthesis rate was decreased.The promotion of photochemical quenching(NPQ) due to the increase of de-epoxidation extent of xanthophylls cycle and an elevation of water use efficiency (WUE) were observed in tomato plant under salt stress.2.Combined effects of salinity and low root zone temperature on oxidative stress in tomato plants.O₂^·- producing rate,H₂O₂ and MDA content were increased in salt-stressed plants, especially under low root zone temperature.Salt stress led to an elevation in activities of Mn-SOD and Cu,Zn-SOD,while low root zone temperature increased activities of Fe-SOD and Mn-SOD,which play its important role in removing O₂·^-.CAT were sensitive to salinity,and its activity significantly decreased under salt stress and low root temperature,while activities of GPX,APX and GR increased under salt stress and low root temperature,which indicated their important roles in scavenging reactive oxygen species in tomato tolerance to low root zone temperature and salinity.3.Effects of salinity on photosynthesis characteristics in tomato plants under different light intensities.Salinity inhibited plant growth and the inhibition in plants under low light intensity was less significantly compared to those under high intensity.Salt stress decreased net photosynthetic rate(Pn),stomatal conductance(Gs) and intracellular CO₂(Ci).Therefore,it could be concluded that stomatal limitation might be a dominant factor on Pn decrease under salt stress.Salt stress increased dark respiration rate(Rd) and light consumption point(LCP),decreased light saturated point(LSP). Rd,LCP and LSP decreased in salt-stressed plants under low light intensity compared with salt stressed plants under high light intensity.100 mmol·L^-1 NaCl had no influence on Fv/Fm,decreased qP,Fv'/Fm' andΦPSII under high light intensity.The PSII activity under low light intensity was higher than that of high light intensity. ETR and Pn were much lower in low light intensity,thus the growth rate of salt stressed plant under low light intensity was much slower.4.Effects of salinity on nutrient contents,ATPase activity in plasma membrane and ATPase,PPase activity in tonoplast of tomato under different light intensitiesSalt stress increased Na content,decreased K content in tomato plants under both light intensities.Low light intensity decreased the inhibitory effects for absorption of P,Ca,Mg,Fe and Zn under salt stress.Salt stress promoted the activities of H⁺-ATPase and Ca²⁺-ATPase in the plasma membrane of tomato roots,more significantly under high light intensity.Salt stress increased the activities of H⁺-ATPase,Ca²⁺-ATPase and H⁺-PPase in the tonoplast of tomato root,more significantly under high light intensity.5.Effects of iso-osmotic Ca(NO₃)₂ and NaCl stress on respiration and metabolism of reactive oxygen in roots of tomato seedlingsThe salt type in protected greenhouse soil is different from that in coast saline soil.The main cation of accumulated salt is Ca²⁺,while the main anion is NO₃^-.Salt stress decreased the total respiration rate due to the decrease of cytochrome respiration,particularly under NaCl stress.Salt stress increased the KCN-resistant respiration rate,however,this effect was more significantly under NaCl stress.The producing rate of O₂·^- and H₂O₂ content were increased by salt stress, especially under NaCl stress.The activities of SOD,CAT and GPX were significantly increased by NaCl treatment,however,the activities decreased after 8 d of treatment.The activities of these enzymes increased all the time under Ca(NO₃)₂ treatment.From the results,we could conclude. that the increase of alternative pathway under NaCl stress might be contributed to the removal of ROS.6.The physiological mechanism of salt toxicity in tomato mitigated by graftingThe inhibition of tomato growth increased with increasing NaCl concentration. Self-grafted plants shared the same responding with non-grafted plants under salt stress,while stock-grafted plants were more resistant to salt stress,which contributed to higher photosynthesis ability and antioxidant capacity.100 mmol·L^-1 or 150 mmol·L^-1 NaCl decreased qP,ETR,Fv'/Fm' andΦPSII in non-grafted plants and self-grafted plants,while not in stock-grafted plants.Thus,Pn in stock-grafted plants was higher than that of non-grafted and self-grafted plants.The MDA content increased in salt-stressed plants,more dramatically in non-grafted and self-grafted plants. Salt stress decreased CAT and DHAR activity in non-grafted and self-grafted plants,while increased in stock-grafted plants.Salt stress increased SOD and GPX activity,more significantly in non-grafted and self-grafted plants.GR activity was much higher in stock-grafted plants under different salt concentrations than that of non-grafted and self-grafted plants.Salt stress increased Na content,decreased K content in non-grafted,self-grafted and stock-grafted plants.7.Effect of Salicylic acid on photosynthesis and the activity of ROS scavenging system under salt stress1.0 mmol·L^-1 SA foliar spray mitigated the inhibition of tomato plant growth caused by sat stress.One aspect was attributed to the elevation of PSII activity and enhancement of Pn under salt stress.On the other hand,SA foliar spray increased activities of antioxidant enzymes(SOD,CAT,GPX and DHAR) under salinity and increased ASC and GSH contents in tomato leaves,which could efficiently scavenging reactive oxygen species,and reduced the O₂·^- productivity rate and H₂O₂ content,and this was indicated by lower MDA content and electrolyte leakage.8.Effects of SA on nutrient contents and plasma membrane activities of tomato roots under salt stressSA foliar spray increased absorption of K,Ca and Mg under salt stress,decreased Na content,and thus increased K/Na ratio in tomato plants.The activity of of H⁺-ATPase and Ca²⁺-ATPase in plasma membrane and H⁺-ATPase and H⁺-PPase in tonoplast were increased by salt stress and promoted further by SA foliar spray.Salt stress increased Ca²⁺-ATPase activity in tonoplast, while there were no difference between SA foliar and not.SA foliar spray decreased NADH oxidation rate,NADPH oxidation rate,Fe(CN)₆³⁺ reduction rate and EDTA-Fe³⁺ reduction rate under salt stress,which was close to the control.