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This article in CS

  1. Vol. 20 No. 2, p. 239-244
    Received: Sept 4, 1979

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Root Growth and Dry Matter Distribution of Soybean as Affected by Phosphorus Stress, Nodulation, and Nitrogen Source1

  1. K. G. Cassman,
  2. A. S. Whitney and
  3. K. R. Stockinger2



In the field, plant root development is of primary importance under P deficient conditions. Two sand culture experiments were conducted to examine the effects of P stress, nodulation, and N source on the growth, dry matter distribution, and root development of ‘Clark 63’ soybean (Glycine max L. Merr.). In both experiments two levels of N (0 and 5.0 mM N) were employed: plants were either solely dependent upon symbiotic N fixation (N-fixing), or primarily dependent upon uptake of combined N from the nutrient solution (N supplied). Nodule dry weight of N-fixing plants grown at the highest P level (2.0 µ/ml) comprised 9% of total plant dry weight and 61% of root dry weight of 35-day-old soybeans. A decrease in the P supply inhibited nodule growth relatively more than either root or shoot growth. For N-supplied soybean, a decrease in the P supply affected shoot growth relatively more than either nodule or root growth. When grown at intermediate P levels (0.02 to 0.5 µ/ml), the root + nodule:total plant dry weight ratio was similar in both N-fixing and N-supplied plants. However, the rootttotal plant dry weight ratio of N-fixing plants was was less than that of N-supplied plants at all P levels. There was an inverse relationship between nodule mass and total root length although the number of first-order lateral roots on nodulated and nonnodulated plants was the same.

The data suggest that two functional equilibria operate in the N-fixing plant, namely, the partitioning of dry matter between (i) the underground portion of the plant and the shoot and (ii) the root and nodules. Phosphorus stress affected the root-nodule equilibrium relatively more than the partitioning of dry matter between below-ground and above-ground parts of the plant.

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