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CROP PHYSIOLOGY & METABOLISM

Bradyrhizobium japonicum Mutants Allowing Improved Soybean Yield in Short Season Areas with Cool Spring Soil Temperatures

^a Dep. of Natural Resource Science, Macdonald Campus of McGill Univ., 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9
^b Dep. of Biology, Univ. of Waterloo, Waterloo, ON, Canada N2L 3G1

* Corresponding author (Dsmith{at}Macdonald.McGill.Ca)

In short-season production areas, cool soil temperature is a major factor potentially limiting soybean [Glycine max (L.) Merr.] plant growth and yield. Genistein (4',5,7-trihydroxyisoflavone) is a signal compound secreted from soybean roots and is essential to establishment of the soybean–Bradyrhizobium japonicum N-fixing symbiosis. The addition of genistein to soybean inocula has proven to be an effective means of generating increases in N fixation and yield; however, genistein is costly. We used ultraviolet (UV) mutagenesis to make 10 mutants of USDA 110 that express the nod genes without exposure to 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 or the wildtypes 532 C or USDA 110] and soybean cultivar (OAC Bayfield and Maple Glen). Inoculation with mutant strains Bj 30055 and Bj 30058 resulted in greater soybean yields than inoculation with 532 C (6.2% increase, averaged across the 2 yr) or the wildtype USDA 110 (9.9% increase, averaged across the 2 yr). These increases were largely due to increases in pod and seed number. These results showed that mutants that express nod genes in the absence of plant-to-bacteria signal compounds can help to overcome the low temperature limitation of soybean nodulation leading to improved growth and yield of soybean crops grown in areas with cool spring soils.

Abbreviations: LCO, lipo-chito-oligosaccharides • RZT, root zone temperatures • UV, ultraviolet • YEM, yeast-extract mannitol