ABSTRACT

Nitrogen (N) fixation by rhizobia plays an important role in N assimilation of soybean (Glycine max). Nitrogenase, the enzyme responsible for N fixation in rhizobia, contains a large amount of sulfur (S) molecules and requires phosphorus (P) in adenosine triphosphate. Thus, S and P are essential for plant growth and N fixation by rhizobia. However, P in soils forms insoluble PO₄^{3 –} salts with metals. Plant roots solubilize these PO₄^3- salts by secreting organic acids and absorb solubilized PO₄^3-. In this study, the effects of S application on P solubilization by soybean roots and the involvement of rhizobia were evaluated. Soybean plants inoculated with Bradyrhizobium diazoefficiens USDA110 were grown in vermiculite, and Ca₃(PO₄)₂ was added as the sole P source. A hydroponic solution supplemented with three S concentrations (0, 20, and 1000 µM) was applied, and plants were grown for 5 weeks. Root fresh weight, the soil-plant analysis development (SPAD) values, and shoot and root S concentrations increased significantly with S application, but root nodule numbers and nitrogenase activity were not significantly affected. Organic acid secretion by roots was drastically increased by S application, and shoot P concentration and content also increased significantly. These results indicated that S application can promote P solubilization by increasing the secretion of organic acids by roots and increase in N fixation is not involved in this promotion at least in early vegetative growth period.

摘要

根瘤菌对氮 (N) 的固定在大豆 ( Glycine max ) 的N 同化中起重要作用。固氮酶是根瘤菌中负责固氮的酶，它含有大量的硫 (S) 分子，需要三磷酸腺苷中的磷 (P)。因此，S 和 P 是植物生长和根瘤菌固氮所必需的。然而，土壤中的 P与金属形成不溶性 PO ₄ ^{3 –}盐。植物根部通过分泌有机酸溶解这些PO ₄ ^3-盐并吸收溶解的PO ₄ ^3-。在这项研究中，评估了施硫对大豆根系溶磷的影响和根瘤菌的参与。用重氮缓生菌接种的大豆植物USDA110 在蛭石中生长，Ca ₃ (PO ₄ ) ₂被添加为唯一的 P 源。应用补充有三种 S 浓度（0、20 和 1000 µM）的水培溶液，植物生长 5 周。根鲜重、土壤-植物分析发展 (SPAD) 值以及地上部和根部 S 浓度随 S 施用而显着增加，但根瘤数和固氮酶活性未受到显着影响。施硫显着增加根系有机酸分泌，地上部磷浓度和含量也显着增加。这些结果表明，施硫可以通过增加根系有机酸的分泌来促进磷的溶解，而固氮的增加至少在营养生长早期不参与这种促进。