大气CO2浓度升高对大豆共生生物固氮能力影响及其调控机制

Biological nitrogen fixation process in soybean is of metabolisms of energy and product. Elevated atmospheric CO2 concentration affects the photosynthetic carbon distribution and the C-N balance in plant. Although soybean nodulation in response to elevated CO2 has been addressed in many studies, the variation of symbiotic nitrogen fixation ability under such condition and the regulation mechanisms are still unknown. Different genotypes of soybean will be used in this study. Using stable isotopes of 13C and 15N labeling, we will investigate the contribution of photosynthetic carbon to symbiotic nitrogen fixation under elevated atmospheric CO2 condition. It will reveal the changes of primary (sucrose) and secondary metabolites of photosynthesis (malate and flavonoids) in response to elevated CO2, which are likely to be associated with nitrogen fixation. Thus, taking an insight into the photosynthetic carbon distribution and photosynthate composition, the mechanisms of nodulation and nitrogen fixation regulated by elevated CO2 would be understood. In addition, two key enzymes in carbon metabolisms, Rubisco and PEPC activities will be investigated under elevated CO2 as well, which are physiologically preliminary to the nitrogen fixation process. This study would improve understanding on the interaction between soybean and nodule, and broaden the knowledge of the global C-N cycle under elevated CO2. This would also provide theoretical basis on increasing the nutrient use efficiency in soybean in the future.

大豆共生生物固氮是涉及能量和物质代谢的复杂过程。大气CO2浓度升高改变了植物体内光合碳的分配和C-N平衡。研究表明大气CO2浓度升高明显影响大豆结瘤，但大气CO2浓度升高后大豆共生生物固氮能力改变与调控的机制尚不明晰。本项目以不同基因型大豆为研究对象，利用稳定同位素13C和15N双标方法，定量研究大气CO2浓度升高后光合碳在大豆-根瘤间分配的改变；揭示光合碳初级代谢产物蔗糖、次级代谢产物苹果酸和类黄酮对大气CO2浓度升高的响应与生物固氮能力变化之间的关系，从光合碳供应量和光合代谢产物方面解析大气CO2浓度升高大豆对根瘤形成和共生生物固氮的调控机制；同时研究大气CO2浓度升高对C代谢关键酶Rubisco和PEPC活力的影响，阐明光合碳对大豆共生生物固氮调控的生理基础。预期研究结果对全面认识大豆-根瘤间互作关系、丰富全球C-N循环理论具有重要科学意义，也为未来提高大豆养分利用效率提供理论依据。

大豆共生生物固氮是涉及能量和物质代谢的复杂过程。大气CO2浓度升高改变了植物体内光合碳的分配和C-N平衡。研究表明大气CO2浓度升高明显影响大豆结瘤，但大气CO2浓度升高后大豆共生生物固氮能力改变与调控的机制尚不明晰。本项目利用开顶式气室模拟大气CO2浓度升高到550 ppm时大豆光合速率、叶绿素含量、Rubp酶活性和产量对大气CO2浓度升高的响应；利用稳定同位素方法分析了不同大豆品种N素利用变化以及与大豆产量形成之间的关系；利用高通量测序技术分析了高CO2浓度下大豆根际固氮菌nifH基因丰度。结果表明：大气CO2浓度升高显著提高了大豆鼓粒始期（R5）叶片叶绿素含量、Rubp酶活性和光合速率，8个大豆品种产量平均增加40%；高CO2浓度下大豆植株体内N素积累和利用发生变化，R5-R8期生物固氮和土壤氮贡献显著提高，肥料氮贡献下降；共生生物固氮贡献的提高与大豆产量的增加显著相关。大气CO2浓度升高显著提高了部分大豆品种根际nifH基因丰度；大气CO2浓度升高会刺激大豆根际固氮细菌的生长，但是不会改变其群落结构，这与大豆品种和大豆结瘤对高CO2浓度的响应有关。大气CO2浓度升高还会显著影响大豆籽粒品质。高CO2浓度下大豆完熟期籽粒中蛋白质和游离氨基酸含量显著降低，脂肪和类黄酮含量显著升高；营养元素方面主要表现为Fe和Zn元素浓度的降低，这对未来气候变化条件下人类营养健康产生了威胁。