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¹⁴C-Labelled Assimilate Utilization by Soybeans Grown with Three Nitrogen Sources¹

The use of a non-destructive method of determining ¹⁴C content of soybean plants after labelling allowed the fate of this ¹⁴C, including proportions distributed to various plant parts or lost in respiration, to be studied. Plants grown in buckets in the field with Low-N, nitrate (NO^–₃), or ammonium (NH⁺₄) N sources were allowed to assimilate ¹⁴CO² in photosynthesis at five stages in ontogeny. Plants were harvested after 1 week or at maturity and ¹⁴C in plant parts was determined. In the first week after each labeling, NO^–₃-N plants lost an average of 14% of their ¹⁴C, compared to 23% in NH⁺₄-N plants and 34% in Low-N plants fixing N₂. Between labellings and maturity, average ¹⁴C losses were 40% in NH⁺₄-N plants, 45% in NO^–₃-N plants, and 52% in Low-N plants. The largest proportion of the assimilated ¹⁴C was either lost in respiration or contained in plant parts that were growing rapidly at labelling. Nodules of Low-N plants contained more than those grown on NO^–₃-N or NH⁺₄-N 1 week after labelling at stages R1 and R2.5. The percentages of assimilated at stages V4, R1, R2.5, R4 and R5.5 which were recovered in beans at maturity were 2.2, 3.7, 5.5, 13.8 and 36.5% respectively. Losses of ¹⁴C from vegetative parts between 1 week after early labelllngs and maturity were mainly a result of respiration rather than retranslocation to beans.

Key Words: Glycine max (L.) Merrill • Photosynthate distribution • Nitrate • Ammonium • N₂ fixation

² Former graduate student and professor, respectively,

Received for publication June 30, 1980.