Effects Of Tea Polyphenols On Osmotic Adjustment And AsA-GSH Cycle In Wheat Roots Under Salt Stress

At present,the increasing trends of soil salinization,as a result of natural factor and unreasonable irrigation,seriously restrict agricultural production and affect eco-environment.Salt stress increases the osmotic potential of plants and disturbs the dynamic balance of inorganic ions as well as reduces the ability of scavenging reactive oxygen species(ROS)in ascorbate-glutathione cycle,which threatens the normal growth and development of plants.Therefore,it is of great significance to study how to alleviate the damage mechanism on plants of salt stress.In this experiment,a new spring wheat(Triticum aestivum L.)cultivar cv Longchun 30 was used to research the effects of tea polyphenols(TP)on the growth,osmotic adjustment and ascorbic acid-glutathione(AsA-GSH)cycle in salt-stressed wheat roots.The main findings are as follows:1.150 mmol·L-1 NaCl induced the decrease of cell viability and the increase of malondialdehyde(MDA)content in wheat seedling roots,the growth of the seedlings was obviously suppressed.TP treatment alone significantly promoted root cell viability and declined MDA level.Compared with salinity stressed alone,the addition of 25 or 100 mg·L-1 TP enhanced cell viability and reduced MDA content in salt-treated ones,Eliminates salt stress on the growth of wheat seedlings.2.The levels of total and apoplast hydroxyl radical(OH),hydrogen peroxide(H2O2)and superoxide anion(O2·-)in wheat roots rose due to salinity stress.TP treatment alone increased total O2·-content but reduced OH content.Two TP concentrations showed different effects on the production of total H2O2,25 mg·L-1 TP stimulated the total H2O2 accumulation,while 100 mg·L-1 TP led to its reduction.In addition,the apoplast ROS contents were significantly reduced under TP treatment.The addition of 25 mg·L-1 TP did not affect the NaCl-induced total ROS,while declined apoplast ROS levels.100 mg·L-1 TP caused significant decreases in salt-induced total and apoplastic ROS contents.3.Proline,soluble sugar and soluble protein contents were significantly increased when wheat roots exposed to 150 mmol·L-1 NaCl,but TP treatment alone had no significant effect on these indexes as compared with control.Compared with 150 mmol·L-1 NaCl treatment alone,soluble sugar,soluble protein and proline contents were notably decreased by adding 100 mg·L-1 TP in salt treatment.Proline dehydrogenase(PDH)activity decreased in response to salt stress,TP or and salt + TP treatments in wheat roots,ornithine aminotransferase(OAT)activity did not change notably in salt treatment,but OAT activity in other treatments is weakened comparision with control.Glutamate kinase(GK)activity had no obvious change in each treatment.4.Salinity stress induced the increases of Na content in roots and leaves as well as K content in roots,while the decreases of Ca content in roots and leaves as well as K content in leaves.Two TP concentrations both significantly stimulated the accumulation of Na,K and Ca in wheat roots.25 mg·L-1 TP increased Na content but reduced K and Ca contents in leaves,100 mg·L-1 TP elevated the amount of Na and Ca in leaves,but did not affect the amount of element K.Compared with salt stress,the addition of TP had no significant effect on the Na and K levels in roots,while reduced Na content and promoted K content in leaves.The amount of Ca was remarkably elevated in salt-treated roots and declined in salt-treated leaves by adding 25 mg·L-1 TP.The addition of 100 mg·L-1 TP significantly increased the accumulation of Ca in roots and leaves of salt-stressed seedlings.5.Salt treatment reduced the accumulation of reduced ascorbic acid(ASA)and reduced glutathione(GSH)in wheat roots as well as increased the contents of dehydrogenase ascorbic acid(DHA)and oxidized glutathione(GSSG),and reduced the ratio of AsA/DHA and GSH/GSSG.The reduced activities of glutathione reductase(GR),dehydroascortate reduetase(DHAR)and ascorbate peroxidase(APX)as well as the enhanced activities of ascorbic acid oxidase(AAO)and monodehydroascortate reduetase(MDHAR)were observed in salt-stressed roots.Under TP treatment alone,except for the decrease of APX activity,the changes of other indicators were opposite to that under salt treatment.Different TP concentrations increased the salt-reduced AsA and GSH levels and decreased the DHA and GSSG levels in roots as well as elevated the ratio of AsA/DHA and GSH/GSSG.In addition,the addition of 25 mg·L-1 TP reduced AAO and APX activities,while enhanced the activities of DHAR and GR.High TP concentration inhibited AAO activity,while enhanced the activities of MDHAR,DHAR and GR,but had no significant effect on APX activity.The above results indicated:(1)salt stress induced a large accumulation of O2·-,H2O2 and ·OH in wheat roots,which caused serious oxidative damage,and declined cell viability,thereby inhibiting seedlings growth.TP treatment alone reduced total OH as well as apoplastic H2O2 and OH and MDA contents,and elevated cell viability,which promoted the growth of wheat seedlings.The addition of TP at different concentrations reduced salt-induced ROS generation as well as MDA levels and enhanced cell viability in roots,and thus alleviated the salt damage to plants.(2)Salt stress-induced the increases of soluble protein,soluble sugar and proline content in wheat roots enhanced permeability regulation capacity to relieve salt damage to plants.The addition of TP reversed the accumulation of these permeability regulators and the reduction of Ca content by salt stress.(3)The application of TP increased the activities of NaCl-induced GR,DHAR and other antioxidant enzymes,the levels of AsA and GSH as well as the ratios of AsA/DHA and GSH/GSSG.It indicated that TP could improve the ascorbate-glutathione cycle and elevate the ROS scavenging capacity.In summary,TP enhanced the antioxidant capacity of wheat seedlings,alleviated the salt stress damage,which might be associated with the improving the ability of ascorbate and glutathione to scavenge active oxygen in wheat roots threated with salinity.