Genotypic Difference In The Influence Of Cobalt On Barley Growth And Physiological Traits And Its Mechanisms

Cobalt(Co2+)is not an essential element in higher plants although it is usually beneficial for legume plants.However,the excessive Co2+ in plant tissues causes irreversible damage to cells,resulting in reduced growth,biomass,enzyme inactivation and even plant death.Given the increased Co2+ contamination in soils,different approaches have been sought to reduce Co2+ content in human diets.One of the best-cost effective and efficient approaches is to develop Co2+-resistant crop or cultivars.Accumulation and toxicity of Co2+ in plants differ greatly among plant species and genotypes within a species.Accordingly,the current study aimed at(1)investigating the Co2+ resistance level among 92 cultivated barley genotypes;(2)evaluating the physiological mechanisms of Co2+ resistance;(3)understanding the genotypic differences in accumulation of Co2+ on subcellular level.The main results are summarized as follows:(1)The hydroponic experiment was carried out to identify the genotypic difference in response to cobalt(Co2+)stress using 92 barley genotypes.The result showed a significant decrease in plant growth as well as high Co2+ accumulation for plants exposed to Co2+ stress relative to normal condition.However,there were significant differences among genotypes in terms of growth and Co2+ accumulation,indicating a broad genetic diversity among the tested genotypes,with Yan66(resistant)and Ea52(sensitive)showing the least and greatest affection,respectively.(2)Based on the previous finding that there was a distinct difference among barley genotypes in the effect of Co2+ stress on seedling growth,this experiment was carried out to investigate the effects of different Co2+ toxic levels(25,50,75 and 100 μM)on plant growth,photosynthetic parameters,oxidative stress and,Co2+ and some nutrient contents using three barley genotypes(Yan66,Humai4 and Ea52)differing in Co2+tolerance.The results showed that Co2+ stress inhibited plant growth,reduced chlorophyll content and photosynthetic rate,enhanced oxidative stress.However the effect of Co2+ stress on these growth and physiological parameters differed among genotypes,with Yan66 and Ea52 being most and least affected,respectively.In addition,under Co2+ stress Mn and P contents in both roots and shoots showed the significant reduction and increase,respectively,and Fe content had little changed in shoots and dramatic reduction in roots.Similarly Yan66 showed the relatively smaller change than Ea52 in these nutrient contents as affected by Co2+ stress.The results indicated that the genotypic difference in Co2+ stress resistance as reflected by plant growth and photosynthetic rate is mainly attributed to the difference of Co2+ uptake and accumulation in plant tissues.(3)The subcellular distribution and chemical forms of cobalt(Co2+)were investigated using the three barley genotypes mentioned above under two Co2+ levels(25 and 100μM).Higher Co2+ level in cultural solution significantly increased Co2+ accumulation in all subcellular fractions,with vacuole and cell wall having higher concentration.In comparison with 25 μM Co2+,100 μM Co2+ treatment caused significant increase of Co2+ content in the forms of F-NaCl(extracted with 1M NaCl),F-Ac(extracted with 2%HAc),F-HCI(extracted by 0.6M HCI)and F-residue(residue forms)in both shoots and roots.There was a significant difference among genotypes in Co2+ subcellular distribution and chemical forms,with Ea52 accumulating more Co2+ in organelles and Yan66 having more Co2+ accumulation in vacuole and cell wall.Moreover,the inorganic form of Co2+ extracted with 80%ethanol(F-ethanol)and water-soluble form(F-H2O)were significantly increased in Ea52,while Yan66 accumulated more Co2+ in the forms of low-bioavailable molecules(F-NaCl,F-HAc,F-HCl).The results suggest that the vacuolar sequestration and cell wall deposition of Co2+ is a key resistant mechanism for genotype Yan66.