干旱胁迫下紫花苜蓿叶片衰老过程中叶绿素降解机制研究

Abstract：Drought stress can cause leaf senescence of alfalfa and the chlorophyll degradation is the sign of plant senescence, it is very important to study the mechanisms of chlorophyll degradation in the leaves of alfalfa under drought stress to delay the leaf senescence, prolong photosynthesis and improve the biomass of alfalfa. In our preliminary studies, we found when subjected to drought stress, the net photosynthetic rate and chlorophyll content in the leaves of high drought-resistance alfalfa decreased more slowly and chloroplast shape and grana structure were less affected. In this research we will explore the mechanisms of chlorophyll degradation in the leaves of alfalfa under drought stress.Alfalfas varying in their tolerance to drought will be used to study the feature of leaf senescence，the activity of chlorophyllase，pheidea oxygenase（PAO） and mg-dechelatase （MDCase），pheophytin pheophorbide hydrolase（PPH）,the expression of three genes(CHL-1,PAO,PPH)under drought stress and rewater. The oxidative damage and the responses of enzymatic and non-encymatic anatioxidants of chloroplast ultrastructure will be studied under drought stress too. We will understand the mechanisms of chlorophyll degradation from two ways which are enzymatic degradation and antioxidant degradation in order to provide basis of genetic improvement on yield and quality of alfalfa under drought stress .

摘要：叶绿素降解是植物衰老的重要标志，研究干旱胁迫下苜蓿叶片叶绿素降解机制，对于延缓干旱胁迫下苜蓿叶片衰老，延长苜蓿光合作用，提高苜蓿产量具有十分重要的意义。本课题前期研究成果表明，干旱胁迫下抗性强的苜蓿叶片叶绿素含量降低幅度小，叶绿体的外形及基粒结构受到的影响小，净光合速率下降慢。本研究将进一步探讨干旱胁迫下苜蓿叶绿素降解机制。本课题以抗旱性强弱差异大的紫花苜蓿品种为材料，研究干旱胁迫及复水条件下苜蓿叶片衰老特征，四种叶绿素降解的关键酶（叶绿素酶、脱镁螯合酶、脱镁叶绿酸a加氧酶、脱镁叶绿素脱镁叶绿酸水解酶）活性变化及三种关键酶（叶绿素酶、脱镁叶绿酸a加氧酶、脱镁叶绿素脱镁叶绿酸水解酶）基因的表达。同时研究在干旱胁迫下叶绿体的活性氧伤害以及抗氧化系统对活性氧的清除。从叶绿素酶促降解和活性氧伤害降解两方面探讨干旱胁迫下苜蓿叶片叶绿素降解机制，为干旱条件下苜蓿产量和品质性状的遗传改良提供依据。

紫花苜蓿（Medicago sativa L）是优良的豆科牧草，也是我国种植面积最大的人工牧草。干旱是影响紫花苜蓿生长的重要环境因子之一，干旱胁迫会造成紫花苜蓿叶绿素降解，加速叶片衰老，降低紫花苜蓿的光合作用，严重影响紫花苜蓿的产量和品质。.本课题以抗旱性差异大的两种紫花苜蓿陇东（耐旱性强）和WL-712（耐旱性差）为试验材料，研究干旱胁迫及复水条件下紫花苜蓿叶片衰老特征，叶绿素降解关键酶活性变化及叶绿素降解关键酶基因的表达。同时研究干旱胁迫下紫花苜蓿叶绿体活性氧伤害以及抗氧化系统对活性氧的清除机制。从叶绿素酶促降解和活性氧伤害降解两方面探讨干旱胁迫下紫花苜蓿叶片衰老过程中叶绿素降解机制，为干旱条件下苜蓿产量和品质性状的遗传改良提供理论依据。 . 本课题研究结果表明，干旱胁迫下，紫花苜蓿叶片叶绿体超微结构被破坏，叶绿素含量降低，叶片衰老率增加，净光合速率下降。干旱胁迫下，叶绿素降解关键酶（叶绿素酶、脱镁螯合酶）活性增加。叶绿素降解关键酶（叶绿素酶、脱镁叶绿酸a加氧酶、脱镁叶绿素脱镁叶绿酸水解酶）相对基因表达量增大。干旱胁迫对紫花苜蓿叶绿体造成活性氧伤害，活性氧产生速率增大，丙二醛含量增加。而叶绿体抗氧化系统中，超氧化物歧化酶（SOD）活性、过氧化物酶（POD）活性及抗坏血酸过氧化物酶(APX)活性升高。非酶抗氧化物质谷胱甘肽（GSH）和抗坏血酸（ASA）含量增加。相对于耐旱性差的WL-712，耐旱性强的陇东苜蓿在干旱胁迫下叶绿体超微结构破坏程度低，叶绿素降解关键酶活性增加幅度小，叶绿素降解关键酶基因相对表达量低，叶绿素含量降低幅度小，叶片衰老率低。同时耐旱性强的紫花苜蓿叶绿体抗氧化系统的协调能力强，有助于消除活性氧伤害，降低叶片衰老率。复水后，耐旱性强的紫花苜蓿各项指标恢复程度高。