Study Of Herbicide And Chilling-induced Oxidative Stress In Seedlings Of Two Rice Cultivars With Different Chilling Tolerance

Abiotic stresses including herbicide treatment usually create oxidative stress in plant. Therefore,stress resistance of plants is closely correlated with their capacity of antioxidant activities,and can affect herbicide selectivity to a certain extent.In the present study, oxidative stress was studied in seedlings of two rice cultivars with different chilling tolerance after treatments of paraquat,pyribambenz-propyl and chilling.Major conclusions were as follows:1.After 6 h of 4℃under light,5-d old seedlings of ShengDao 13(SD13)and IR50 were wilting.Electrolyte leakages(EL)of the two rice cultivars were raised to～2 and～5-fold those of controls respectively.The EL of SD13 returned to normal level after 24h recovery growth under normal temperature regime while that of IR50 remained as high as～5-fold of control even after 48h recovery growth.Similar changes were observed for malondialdehyde (MDA)levels,which increased significantly in the two rice cultivars after chilling treatment. 2-day recovery growth restored MDA level in SD13 but the MDA content of IR50 continued to rise within 4-day recovery growth period resulting in leaf death.It was thus shown that SD13 was a chilling tolerant cultivar and IR50 a chilling susceptible cultivar.2.Chilling caused increase in ascorbic acid(AsA)content but decrease in glutathione (GSH)content in the two rice cultivars,with the GSH contents essentially recovered in SD13 after 4-day recovery growth but still lower than control in IR50.The ratio of dehydroascorbate(DHA)in total ascorbate was not much changed in SD13 but significantly increased in IR50.Since AsA regeneration from DHA required GSH,which was declining in IR50,the reactive oxygen species(ROS)scavenging capacity in this cultivar was compromised.Chilling significantly reduced activities of catalase(CAT)but enhanced activities of peroxidase(POD)and caused little changes of activities of glutathione S-transferase(GST)in the two rice cultivars,implicating no apparent correlations between chilling-tolerance of rice seedlings and activities of CAT,POD and GST(CDNB). Nevertheless,activities of ascorbate peroxidase(APX)were enhanced significantly in SD13 but declined in IR50 after chilling,indicating involvement of APX in the chilling tolerance of SD13.3.50nM of paraquat significantly inhibited the growth of IR50 seedlings but had negligible effect on the growth of SD13.MDA contents in the two rice cultivars rose significantly after paraquat treatment,in line with the ROS herbicidal mechanism of the herbicide.Both of GSH and AsA contents were increased in SD13,whereas in IR50 only the GSH content rose slightly with AsA content dropped slightly.Meanwhile,the magnitude of increase of GST activities was much greater in SD13 than in IR50,probably reflecting more GST isoforms with glutathione peroxidase activity were expressed in SD13 strengthening its resistance against ROS.Moreover,after paraquat treatment,activities of CAT were declined in IR50 but little changed in SD13,activities of POD were raised in SD13 but little changed in IRS0.Therefore,partial resistance of SD13 against paraquat was likely related to its ascending responses of GSH and AsA contents and activities of GST,POD,CAT or otherwise maintaining higher levels of these antioxidants and enzyme activities.4.100nM of pyribambenz-propyl inhibited the seedling growth of both rice cultivars with significant increase of MDA and GSH contents in SD13 but without effect on MDA and GSH levels in IR50,showing oxidative damage present in SD13 but not in IR50.After the herbicide treatment,GST activities were increased by 30～40%in SD13 and～15%in IR50; CAT activities declined by 20～30%in the two rice cultivars;APX activities not significantly changed in SD13 but increased by～40%in IR50;POD activities were increased by 15～20% in both cultivars.5.H₂O₂ levels were decreased in SD13 but increased by～25%in IR50 by hydrogen peroxide treatment,suggesting the likelihood of H₂O₂ pretreatment to enhance stress tolerance in IR50.However,H₂O₂ treatment had little influence on GSH content in the two rice cultivars and caused decline of AsA content in IR50 and little effect on AsA content in SD13, accompanied with large increase of DHA level in the ascorbate pool in both rice cultivars. These would suggest that H₂O₂ pretreatment could not improve the levels of antioxidants in IR50.On the other hand,H₂O₂ treatment in IRS0 caused a～30%increase in GST activity but declines in CAT and APX activities.Therefore,H₂O₂ pretreatment probably could enhance the stress tolerance of IR50 by upregulating GST activities.