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  1. Vol. 21 No. 6, p. 938-942
     
    Received: June 30, 1980
    Published: Nov, 1981


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doi:10.2135/cropsci1981.0011183X002100060032x

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

  1. J. R. Pearen and
  2. D. J. Hume2

Abstract

Abstract

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 (NO3), 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, NO3-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 NO3-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 NO3-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.

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