Physiological Mechanism Of Root Senescence In Maize Induced By Potassium Deficiency Stress

Maize is an important food crop in Liaoning Province,and the acreage is more than 2million hectares.Maize has a large demand for potash fertilizer in process of growth and development.Production in middle and northern regions of Liaoning province is seriously inhibited due to potassium?K?deficiency in soil,characterized by premature senescence.K plays an important in process of enzyme activation,osmoregulation,control of turgor pressure,protein synthesis and leaf movement in plant.The root system is the major organ for potassium uptake in maize,but little information in root senescence related to K deficiency was understood between different genotype in maize.Therefore,the aims of study were to analyze the adjustment of root configuration,photosynthetic characteristics,osmotic regulation,endogenous hormone,nucleolysis and reactive oxygen,to explore the different response to K deficiency stress and relationship with root senescence,and to revealed the physiological mechanism of root senescence induced by K deficiency stress.The main results were as follow:1.The growth of 90-21-3 and D937 was inhibited by K deficiency,decrease of dry weight,root number,root length,root surface and volume,and more reduction in D937 than in 90-21-3.90-21-3 could maintain a undamaged root structure,while in D937 was seriously damaged involved necrotic cell and irregular vessel.Compared with K-sensitive inbred lines,D937,90-21-3 had reasonable root morphology,root activity and conducting tissue,and enhanced the capability of absorption and transportation for nutrient,which put off root senescence under K deficiency stress.2.Under K deficiency stress,the K-tolerant maize,90-21-3 and 099,maintained more chlorophyll content,photosynthase activity,stable morphology and chloroplast structure in comparison with D937 and 835.In contrast,the reaction centers of PSII and electron transfer in D937 and 835 were significantly inhibited under K deficiency stress,which resulted in severe membrane lipid peroxidation due to accumulation of O₂^-and H₂O₂.Compared with K-sensitive maize,antioxidant enzyme activity in K-tolerant maize was increased,which promoted scavenging of ROS under K deficiency stress and alleviated oxidative stress,allowing normal photosynthesis and root growth.3.Ethylene synthesis genes in root of 90-21-3 were enhanced the up-regulated expression to promote the synthesis of ethylene and,including ZmACS2,ZmACS7,ZmACO20and ZmACO35,which maybe improve the root system tolerance at the seedling stage.Compared with 90-21-3,nucleic acid hydrolysis enzyme activity in D937 was largerly increased under K deficiency stress,resulting in more reduction of DNA and RNA content than in 90-21-3.Meanwhile,the senescence of root in D937 was promoted due to the significantly decreased IAA and increased ABA.But,the IAA and ABA in 90-21-3 had little change,which could effectively delay the process of senescence of root root system under K deficiency stress.4.The K content in root systems was decreased in 90-21-3 and D937,and Na⁺,Ca²⁺,free amino acids and proline in root were promoted under K deficiency.Additionally,the accumulation of proline,soluble sugar and free amino acid in 90-21-3 maintained the osmotic potential to promote root growth under K deficiency stress,but seriously reduction of soluble protein in D937 than in 90-21-3 at the seedling stage.However,larger Na⁺and Ca²⁺in D937was largerly accumulated after tasseling and flowering stages under K deficiency which may cause antagonism of ions and reduce absorption for K⁺.Conversely,90-21-3 could alleviate the osmotic stress and maintain the growth of root by increase of free amino acids and soluble sugar content in root,which delayed the process of root senescence under K deficiency.5.The content of O₂^-and H₂O₂ in root of 90-21-3 and D937 was increased,which damaged membrane lipid structure and resulted in increased membrane permeability and membrane lipid peroxide.Compared with D937,the activity of antioxidant enzymes involved SOD,POX,CAT,GR,PPO,and non-enzymatic antioxidants involved GSH,AsA and polyphenols in 90-21-3 were promoted to enhance the capacity of reactive oxygen scavenging and reduced membrane lipid,which could delay the process of root senescence under K deficiency stress.On the contrary,antioxidant enzymes activity?SOD,CAT and PPO?and non-enzymatic antioxidants contents?GSH and polyphenols?in D937 were differently inhibited,resulting in accumulation of ROS and MDA,which accelerated the process of root senescence.6.The synthesis and accumulation of photosynthetic products in 90-21-3 and D937 were hindered due to the decreased photosynthesis.Compared with D937,the normal photosynthesis was maintained in 90-21-3 under K deficiency stress,which transporting more photosynthetic products into the roots and ears from the stems and leaves,and increasing the distribution proportion of potassium and sodium in root.In addition,the accumulation and distribution proportion of Ca²⁺in root and leaf of 90-21-3 was increased under K deficiency,promoting the absorption and accumulation of potassium and improving the utilization efficiency.These adjustments in 90-21-3 reduced the adverse effect on root growth and yield,which delayed the process of root senescence under K deficiency stress.