| Nitrogen(N)is an essential macronutrient and limiting factor for plant growth and development.N was preferred to be used in the form of ammonium(NH4+)because its direct absorption and assimilation by plants,which increasing energy usage,decreasing water and soil pollution caused by nitrate leach.However,excessive concentration of NH4+is harmful for up-land crop growth.In wheat,seed germination and post-germinative growth were easily suffered elevated NH4+nutrition(EAN)condition by over application of basal N fertilizer and NH4+-release fertilizers in special application methods.Thus,to elucidate the physiological responses of wheat seed germination and seedling growth to EAN is basic information for explorer the strategy of reducing the adverse effects of EAN on crop growth,and for discovering a potential management in high nitrogen use efficiency.Optimal concentration of high NH4+ nutrition and two different EAN tolerant wheat cultivars Yumai49(EAN-tolerant)and Lumai15(EAN-sensitive)were screened from different NH4+concentrations and 24 wheat cultivars to explorer physiological mechanism of wheat seed germination and seedling growth to EAN,which involved reserve mobilization,hormones changes,redox state,NH4+ assimilation and root proteome.The main results were shown as follows:1.Genotypic differences of NH4+ stress tolerance in wheat seed germination.Seed germination rate,length of coleoptiles and radicle,number of radicle and plant dry weight in 24 different wheat cultivars were significantly inhibited when NH4+concentration is 5.0 mmol L-1.Under optimal EAN concentration,24 wheat cultivars were partitioned three types by cluster analysis basing on length of coleoptiles and radicle,dry weight and ratio of root to shoot:EAN sensitive,mean and tolerant type.Yumai49 and Lumai15 were typical genotypes of EAN tolerant and sensitive types,respectively.2.Physiological responses of seed germination to elevated NH4+ nutrition in wheat.EAN inhibited seed germination rate,length of radicle and coleoptiles,and plant dry weight in both wheat cultivars,and the decreases in Yumai49 were lower than those in Lumai15.When compared to control(CON),activity of α-amylase,content of free amino acids and soluble sugar were decreased in NH4+-fed germinating seeds of both wheat cultivars,and the decreases in Yumai49 were less than that in Lumai15,indicating that reserve mobilization was suppressed by EAN.Furthermore,EAN decreased ATP content and activity of SOD and POD in germinating seeds of both cultivars,while increased MDA content.,which means that EAN weakened anti-oxidative active and energy supplement.Content of GA3 in the NH4+-fed germinating seeds of Lumai 15 were less than CON,while content of ABA and ABA/GA3 in the NH4+-fed germinating seeds of both cultivars were greater than their respective CON.In addition,comparing to CON,content of free amino acids and soluble sugar in radicle of both cultivars were increased under EAN,while content of soluble sugar and radicle/coleoptiles sugar was decreased,and the content of free amino acids and soluble sugar was greater in Yumai49 than those in Lumai15.Activity of PK,PEPc and GS were decreased in NH4+-fed coleoptiles of both cultivars,and the decrease in Yumai49 was less than that in Lumai15;while activity of PK,PEPc and GS increased in NH4+-fed radicle,and the increase in Yumai49 was greater than that in Lumai15,which indicating that EAN enhanced the capability of sugar utilization and NH4+assimilation,and Yumai49 performed better than Lumai15.3.Physiological responses of seedling root growth to elevated NH4+ nutrition in wheat.There was a reduction in shoot height,root length and plant dry weight following the EAN treatment in both cultivars,and Yumai49 perfomed better growth than Lumai15.When compared to CON,the EAN enhanced activity of anti-oxidative enzymes,O2·-production rate,content of H2O2,MDA and expression of protein lipoxygenase in the both of NH4+-fed cultivars,which indicating that the EAN increased the capability of anti-oxidative protection.Furthermore,the EAN increased activity of APX,MDHAR and DHAR,and the ratio of DHA to ASA,while decreased content of ASA,GMPase activity and expression of protein disulfide isomerase family,proliferating cell nuclear antigen and actin depolymerizing factor in the NH4+-fed roots of both cultivars indicating that redox state imbalance was induced by the decreased biosynthesis capability in ASA,leading to decreases in cell viability and cell proliferation,and protein folding,and inhibition of root growth finally.The content of IAA,CTK and IAA/CTK in the roots of both cultivars were decreased by EAN to negatively affect root growth.4.Regulation mechanism of additional nitrate on seedling growth in wheat.Additional nitrate increased plant height,root length and plant dry weight in the both of NH4+-fed wheat cultivars,and the increases in Yumai49 were greater than that in Lumai15.Under the EAN treatment,content of IAA,CTK and IAA/CTK in the shoot and root was stimulated by additional nitrate,and the changes in Yumai49 were greater than that in Lumai15 which indicating that the additional nitrate enhanced seedling growth by affecting biosynthesis and transport of IAA and CTK.When compared to CON,content of O2·-,H2O2 and MDA in the both wheat cultivars,and activity of SOD and POD in the root of Yumai49 were increased by additional nitrate under short time EAN,while there is no significant difference between long time EAN and additional nitrate treatments.In summary,EAN adversely afftected ABA/GA3 balance in germinating seeds,decreased amylase activity,leading to delayed seed reserve mobilization and decreased energy supplement to inhibit wheat seed germination.Moreover,EAN enhanced ROS accumulation with increased capability of anti-oxidative protection,which negatively affected seedling root redox balance,resulting in inhibition of cell viability,proliferation and protein folding,and finally suppressing root growth.The NH4+ assimilation was stimulated by enhanced activity of PK,PEPc and GS,and content of soluble sugar,which supply more carbon skeletons to continuously support NH4+ assimilation.The EAN tolerant cultivar obtains stronger capability of reserve mobilication,energy supplement to establish a better growth performance during seed germination,and has increased activity of antioxidant enzymes and content of DHA/ASA to maintain higher capability of anti-oxidative protection in seedling growth stage.Furthermore,the higher IAA/CTK indicates that stronger root growth resulted from lower changes in plant growth regulator in EAN tolerant cultivar.Additional nitrate alleviates bad effects of EAN on wheat seedling growth by affecting IAA/CTK balance and improving antioxidant scavenging capability. |
