钾素营养优化油菜叶片光合的水-碳协调机制解析

The progression of leaf K deficiency symptoms, with chlorosis starting to evolve from the tip to the edge and finally turning into necrosis, is usually considered to be associated with putrescine accumulation. Actually, this progression is intimately connected with insufficient supply of H2O and CO2, which, ultimately, attributes to organ optical damage. Both processes have independently been the focus of a large number of studies, however, very few have focused on their coordination by ignoring the common structural and biochemical pathway for H2O and CO2 transport and their coordination in photosynthetic regulation. A series of hydroponic experiments would be conducted to initially understand the decline of H2O and CO2 transport capacity during the progression of leaf K deficiency, and its relationship with symptoms evolution by simultaneously using gas exchange, leaf hydraulic conductance technique and online carbon and oxygen isotopes fractionation. Then, the attention would be payed to (1) anatomical structure, by analyzing leaf vein density, intercellular air space and stomata characteristics to elucidate the common anatomical basis of H2O and CO2 coordinate supply; (2) biochemical properties, by revealing the effects of potassium on aquaporins expression and activity, as well as the successive roles in the regulation of H2O and CO2 transport. The results of the study could reveal the structural and physiological mechanisms of potassium-coupled H2O and CO2 transport, and deepen the understanding of the essence of potassium on leaf photosynthesis.

缺钾叶片边缘由黄化至焦枯的发展过程通常被认为与腐胺的过多积累有关，但事实上，该症状的出现还与叶片水分失衡和叶绿体内CO2供应不足两个过程密切相关，最终导致组织光损伤。现有的研究较多关注钾在调节某一生理过程上的作用，而忽略了与水分和CO2传输相关的叶片结构和生化过程及其对光合作用的协同调控。采用营养液培养的方法，结合光合气体交换、叶片水力性状测定和碳氧稳定同位素在线分析手段，首先明确持续缺钾过程中油菜叶片水和CO2供应能力的衰退规律及其与缺钾症状演变的关系；在此基础上探索（1）结构学，分析叶脉、叶肉细胞排布和气孔形貌等特征，明确钾素协调叶片水-碳运输的结构基础；（2）生化特性，分析水通道蛋白表达及活性，明晰钾对水通道蛋白的调控及其在叶片水-碳供应上的作用。最终揭示钾素耦合水-碳运输的结构生理机制，推进对钾素影响光合作用本质的进一步认识。

叶缘黄化焦枯是作物缺钾的典型症状，该症状的发生与叶片水分失衡和叶绿体内CO2供应不足降低能量消耗导致组织光损伤两个过程密切相关。然而现有的研究较多关注钾在调节某一生理过程上的作用，而忽略了与水分和CO2传输相关的叶片结构和生化过程及其对光合作用的协同调控。因此本研究利用不同钾素供应的田间和水培试验研究了持续缺钾过程中油菜叶片水和CO2供应能力的衰退规律及其与缺钾症状演变的关系，剖析了钾素协调叶片水-碳运输的结构基础，明确了钾对水通道蛋白的调控及其在叶片水-碳供应上的作用，为理解钾素提高光合效率的水-碳耦合机制提供理论基础。主要结果表明，（1）当油菜叶片K含量小于1.0%时，开始出现黄化症状，叶片水力导度（Kleaf）和净光合速率（A）同步降低；随着叶片缺钾胁迫程度（或时间）的增加，黄叶面积占比显著增加，Kleaf和A快速下降。（2）缺钾胁迫显著降低了油菜叶片细叶脉密度（改变叶片水文运输系统），从而减小Kleaf，其中木质部外导水阻力的增加贡献了总增加阻力的60%；缺钾胁迫降低了叶片叶肉导度，增加叶片内部CO2的运输阻力，从而降低叶片A；缺钾叶片CO2和H2O运输阻力的协同增加与钾对叶片结构的调整密切相关，充足的钾有利于叶片细胞排列松散，增加细胞空隙，提高叶绿体面向细胞空隙的比例和气相H2O传输，从而在促进叶面积扩张的同时协调增加叶片H2O和CO2的传输效率。（3）缺钾显著诱导水通道蛋白BnPIP1s基因的上调表达，利用AgNO3抑制水通道蛋白功能后叶片水力导度和光合速率显著降低，充足的钾有利于缓解水通道蛋白活性下调对叶片H2O和CO2运输的影响。研究成果从碳水耦合的角度剖析了作物缺钾症状发生的机制，为通过合理的钾肥管理来提升油菜光合生产力和籽粒产量提供理论支撑。