Bradyrhizobium japonicum mutants allowing improved nodulation and nitrogen fixation of field-grown soybean in a short season area

Abstract

In short-season soybean production areas, low soil temperature is potentially a major factor limiting plant growth and yield. Although Bradyrhizobium japonicum strain 532 C is widely used in inoculants in Canadian soybean production, and USDA 110 is widely used in American soybean production, they are both inhibited by low temperatures. Genistein is an important plant-to-bacteria signal compound secreted by soybean roots. The addition of genistein has proven to be an effective means of generating increases in nitrogen fixation and yield but genistein is expensive. We used UV mutagenesis to make 10 mutants from USDA 110 that express nod genes without the addition of plant-to-bacteria signal compounds such as genistein. A field experiment was conducted at the Lods Agronomy Research Centre in southwestern Quebec in 1998 and 1999. The treatments consisted of factorial combinations of inoculant type (no inoculant (control) and inoculants containing the mutants Bj 30050, Bj 30051, Bj 30052, Bj 30053, Bj 30054, Bj 30055, Bj 30056, Bj 30057, Bj 30058, Bj 30059 and the wild type USDA110 or 532 C) and soybean cultivar (Bayfield and Maple Glen). The experiment was structured following a randomized complete block design with four blocks. Data were collected on nodule number, nodule dry weight, shoot nitrogen yield and total nitrogen fixation at five development stages. Averaged over the 2 years of the study, when pods were 2 cm long at one of the four uppermost nodes on the main stem (R4), inoculation with Bj 30055 and Bj 30058 resulted in greater nodule number (23 and 14%, respectively), nodule dry weight (16 and 13%, respectively), shoot nitrogen yield (19 and 21%, respectively) and total nitrogen fixation (10·9 and 12·7%, respectively) than 532 C, which is currently used in Canadian inoculants. The cultivar Bayfield produced more nodules, and higher nodule weight than Maple Glen, but there were no differences between the cultivars for shoot nitrogen yield and total nitrogen fixation.