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Soybean Plant Growth, Nodulation, and Nitrogen Fixation as Affected by Root Temperature¹

ABSTRACT

The objective of this study was to investigate the influence of root temperature, Rhizobium japonicum strain, host variety, and plant age on soybean growth, nodulation, and N₂ fixation.

Soybean plants were grown in sand-filled Leonard jars and watered with N-free nutrient solution. Soybean root temperatures were held constant at 15, 20, 25, and 30°C in a growth chamber environment. At harvest, plant height and weight, nodule number and weight, and acetylene reduction activity were measured.

Interactions between root temperature, R. japonicum strain, and soybean variety were complex and changed with plant age. Temperature by strain interactions were more common than temperature by variety, variety by strain, or temperature by variety by strain interactions. Plant height was solely dependent on root temperature, but nodulation and acetylene reduction activity parameters depended on temperature, strain, and variety. Such interactions were more complex at later growth stages.

Maximum plant dry weight and nodule number occurred at 25°C. Nodule weight and acetylene reduction activity were greater at 25°C than at the other root temperatures early in plant growth (3–4 weeks after planting.) However, 5–6 weeks after planting, these parameters were greater at 20° than at 25°C. Apparently more nodules were formed at 25°C, but eventual nodule development, size and acetylene reduction activity was greater at 20°C. Nodules also degenerated sooner at 25°C. Nitrogen fixation significantly increased plant growth over that of the uninoculated controls except at 15°C. The confined root environment of the Leonard jar may have restricted further nodulation after 5 weeks from planting and may have influenced nodule weight and acetylene reduction activities as well as nodule numbers.

¹ Contribution from the Dept. of Soil Science, University of Minnesota, St. Paul, MN 55108. Paper no. 10676, Scientific Journal Series, Minnesota Agric. Exp. Stn., Univ. of Minnesota.

² Former Graduate Assistant, now Assistant Professor of Agronomy, New Mexico State University; and Professor of Soil Science, University of Minnesota.

Received for publication December 4, 1978. Accepted for publication July 6, 1979.

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