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

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.

Key Words: N fixation • P deficiency • Acetylene reduction • Root:shoot ratio • Glycine max L.

² Research assistant, professor of agronomy, and senior agronomist, respectively, NifTAL Project, Dep. of Agronomy and Soil Science, Univ. of Hawaii, Honolulu, HI 96822.

Received for publication September 4, 1979.