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SOYBEAN

Low Temperature–Tolerant Bradyrhizobium japonicum Strains Allowing Improved Soybean Yield in Short-Season Areas

^a Plant Sci. Dep., Macdonald Campus of McGill Univ., 21111 Lakeshore Rd., Ste-Anne-de-Bellevue, QC, Canada H9X 3V9
^b Dep. of Nat. Resour. Sci., Macdonald Campus of McGill Univ., 21111 Lakeshore Rd., Ste-Anne-de-Bellevue, QC, Canada H9X 3V9
^c Dep. of Biol., Univ. of Waterloo, Waterloo, ON, Canada N2L 3G1

* Corresponding author (dsmith{at}macdonald.mcgill.ca)

Received for publication May 21, 2001. In short-season soybean [Glycine max (L.) Merr.] production areas, low soil temperature is potentially an important factor limiting soybean growth and yield. Some strains originating from cooler areas can cause more nodulation and nitrogenase activity under low-temperature conditions. We have attempted to find Bradyrhizobium japonicum strains that can fix more N than strain 532 C under low-temperature conditions. We selected 40 B. japonicum strains from the USDA collection based on their isolation from soils of northern locations. These 40 strains were tested for their ability to grow at a low (15°C) temperature, and the best two (USDA 30 and USDA 31) were selected for evaluation under field conditions. Inoculation with USDA 30 and USDA 31 resulted in greater soybean yields (an 8% increase, averaged over the 2 yr) than inoculation with 532 C. The increased yield was due to the formation of more pods per plant, and more seeds per plant, but not due to an increase in 100-seed weight. This indicated that the benefit caused by the superior strains occurred early in plant development, probably due to increased N fixation early in the growing season. This possibility was supported by the observations that leaf areas, grain protein production, and total protein levels for plants inoculated with USDA 30 and USDA 31 were greater than those inoculated with 532 C. These findings clearly demonstrate that inoculant strains likely to perform best in a given geographical area are those selected for the conditions prevalent in the area.

Abbreviations: OD₆₂₀, optical density at 620 nm • YEM, yeast extract mannitol