密植和施氮对高产玉米品种氮素分配与转移效率的调控机制研究

Nowadays, maize (Zea mays L.) has largest planting area in China, with total production surpassing any other crops. The investment of nitrogen fertilizer in maize production was huge, but most of them were wasted. Thus, improve the nitrogen utilization efficiency (NutE) of maize is important not only for food security but also for agro ecology sustainability. An appropriate increase in planting density can increase maize yield and nitrogen uptake capacity on a certain area. However, it is largely unknown that how agronomic management practices (such as planting density and nitrogen fertilizer supply) affect on nitrogen utilization efficiency within maize plants. In this project, we study NutE with the triple high (high-density tolerant, high yield and high nitrogen efficiency) maize cultivars. Under different planting density and nitrogen supply condition, we quantified nitrogen uptake, allocation and translocation by 15N isotope labeling method. In response to high-density growth condition, stem structure and components’ changes will be measured by histology analysis. Concentration of nitrate, ammonium and amino acids in different maize organs will be measured by salicylic acid method and HPLC method, respectively. Combine with the transcriptome analysis, expression analysis and localization analysis of NutE-related candidate genes in the ear nodes; we try to reveal the molecular mechanisms for nitrogen allocation and translocation in maize, as while as the physiological and molecular mechanisms for nitrogen utilization efficiency of high-yield maize in response to planting density and nitrogen fertilizer supply.

玉米是目前中国种植面积最大、产量最高的作物。氮肥在玉米生产中投入量大、浪费严重。因此提高玉米的氮素利用效率对于我国粮食安全和农业生态可持续发展都具有重要意义。适度增加种植密度可以有效提高玉米产量和对单位面积上氮素的吸收能力，然而密植对玉米体内氮素利用过程的调控机理尚不明确。本研究以耐密高产氮高效玉米品种为研究对象。在不同种植密度和施氮处理下，采用15N标记的方法对穗形成过程中氮素的吸收、分配和转移进行定量分析。通过组织学分析研究密植群体中玉米茎部结构和组成的变化；采用水杨酸比色法和HPLC法测定硝态氮、铵态氮和氨基酸态氮在玉米体内各器官分布的变化；结合穗位茎节的转录组分析以及关键氮效率候选基因定量和定位的研究，揭示穗形成过程中调控氮素分配和转移的分子机制，进而阐明密度和施氮处理调控玉米体内氮素利用效率的生理与分子机制。

通过比较不同时期5个高产玉米品种的密度适应性，发现登海618（DH618）和郑单958（ZD958）是“耐密高产品种”。其中ZD958对密度的响应更高，于是用于开展密植后氮效率调控机制的研究。研究对玉米高产氮高效品种郑单958进行不同密度和供氮处理，发现低氮条件下增加种植密度会显著增加倒伏风险，主要原因是低氮高密条件下茎杆细且株间根夹角小。因此，充足的供氮是玉米增密增产的前提条件。在中等供氮和高供氮处理条件下对比发现：随着供氮量的增加，玉米群体产量先增加后趋于平缓，进一步增加供氮并不能增产，反而使氮素利用效率下降。通过15N对氮密互作条件下的玉米吐丝后的氮素分配与转移过程进行示踪，结果发现：吐丝至灌浆期玉米的氮素吸收量不断增加，而灌浆至成熟期的氮素吸收量不再变化，与此同时玉米籽粒中的氮含量不断增加，说明籽粒中的氮在灌浆期后主要来源于体内其它部位氮的转移。密度处理对氮素分配和转移的影响与供氮水平有关，将密度从4000株/亩增加至6000株/亩后，低氮处理使倒伏率高无法真实反映高密度对氮素利用的影响；中等供氮处理使高低密度下氮素利用变化不显著；过量供氮处理下高密度则显著降低了15N向籽粒的分配和转移比例。其原因是增加了氮向茎的分配和向根的转移。氮素转运蛋白基因表达分析结果表明，铵态氮转运蛋白（AMTs）和氨基酸转运蛋白（AAPs）分别在叶片和茎中响应氮素和密度的调控。研究筛选到3个氨基酸转运蛋白与氮素在籽粒和根茎之间的分配高度相关，这些基因表达模式可用于检测高密度条件下过量施氮的问题，可用于指导玉米密植群体中氮素施用以提高氮肥利用效率。

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