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Symbiotic Nitrogen Fixation in Soybeans. II. Interrelationship Between Carbon and Nitrogen Assimilation¹

Reciprocal intervarietal grafts were made in all root: shoot combinations using seedlings of eight soybean (Glycine max (L.) Merr.) genotypes, grown in sandfilled pots in the greenhouse. After grafting, the plants were inoculated with Rhizobium japonicum (USDA strain #138) and watered with a low-nitrogen nutrient solution. Two replicate plants of each root:shoot combination were sampled at each of four times, 28, 38, 50, and 58 days after grafting, for photosynthetic rate of leaves; leaf area, shoot and root dry weight; and number, fresh weight, and acetylene reduction activity of nodules.

Significant shoot genotype, sampling date, and shoot genotype x sampling date effects occurred for all traits measured. Root genotype effects, independent of the shoot genotype, occurred for all root traits measured, except nodule fresh weight. No overall root effects occurred for any of the shoot traits. Shoot genotype effects were small at the first two sample dates relative to root genotype effects, but were large at the last two sampling dates due to differences in stage of physiological development. Shoot genotype effects on total nodule activity (TNA) per plant were strongly and positively associated with shoot effects on nodule fresh weight, but showed no association with shoot effects on specific nodule activity (SNA). Shoot genotype effects on nodule fresh weight per plant were, in turn, strongly and positively associated with shoot effects on photosynthesis per plant and shoot dry weight. Root genotype effects on TNA per plant were due to inherent differences in SNA.

Key Words: Glycine max (L.) Merr. • Grafting • Acetylene reduction

² Formerly graduate research assistant, Department of Agronomy and Plant Genetics, (now Research Scientist, Division of Tropical Agronomy, CSIRO, Brisbane, Australia); Formerly research specialist, Department of Agronomy and Plant Genetics (now Research Fellow, CIMMYT, Mexico); and Associate Professor, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55101.

Received for publication July 14, 1973.