Photosynthetic adaptation of soybean due to varying effectiveness of N₂ fixation by two distinct Bradyrhizobium japonicum strains

• Published in: Environmental and experimental botany Vol. 76; pp. 1 - 6
• Main Authors: Kaschuk, Glaciela, Yin, Xinyou, Hungria, Mariangela, Leffelaar, Peter A, Giller, Ken E, Kuyper, Thomas W
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.02.2012; Elsevier
• Subjects: Biological and medical sciences; Bradyrhizobium japonicum; carbon; carbon sinks; electron transfer; elevated co2; Fundamental and applied biological sciences. Psychology; glycine-max; leaves; limitations; merrill; nitrogen; nutrition; photosynthesis; plants; ribulose-bisphosphate carboxylase; soybeans; starch; symbiosis; temperature response; Bradyrhizobium japonicum; Stimulation; Biochemistry; Soybean; Modeling; Dicotyledones; Limitation; Angiospermae; Botany; Nitrogen fixation; Bacteria; Diazotrophy; Adaptation; Legumes; Plant ecology; Plant leaf; Symbiont; Glycine max; Strain; Grain legume; Biochemical model of leaf photosynthesis; Rhizobia; Leguminosae; Sink stimulation; Spermatophyta; Rhizobiaceae; Photosynthesis; Sink limitation
• ISSN: 0098-8472; 1873-7307
• DOI: 10.1016/j.envexpbot.2011.10.002

Rhizobial N₂ fixation is a costly biochemical process, which takes 6–14% of current photosynthate (C) from legumes, without compromising grain productivity. In addition to the effects of leaf N nutrition, rhizobial symbiosis could stimulate photosynthesis due to the removal of C sink limitation by nodule activity. To test that hypothesis, we compared the photosynthetic capacity of soybean plants inoculated with two different strains of Bradyrhizobium japonicum (CPAC 390 or CPAC 7), varying in the effectiveness to fix N₂, with plants fertilized with NO₃ ⁻. Nodulated plants had 14–31% higher rates of photosynthesis and accumulated less starch in the leaves than N-fertilized plants. There was evidence that B. japonicum CPAC 390 had higher carbon costs of N₂ fixation compared with CPAC 7, but the increases in carbon costs were accompanied by higher rates of photosynthesis. By applying a biochemical model of leaf photosynthesis, including the limitations of Rubisco activity [Formula: see text] , electron transport rates (J) and triose-P utilization (TPU), we show that soybean plants adapt their photosynthetic capacity to support the stronger carbon sink created by faster rates of N₂ fixation. We observed that plants associated with CPAC 7 (of low effectiveness to fix N₂) increased their photosynthesis by removing sink limitation solely (with a constant [Formula: see text] ) whereas plants associated with CPAC 390 (of high effectiveness to fix N₂) increased their photosynthesis by sink stimulation. Based on the model, we propose that sink stimulation is governed by a positive feedback between TPU and Rubisco activation, resulting in an increased [Formula: see text] .