Nitrate Absorption and Acetylene Reduction by Soybeans during Reproductive Development

Nodulated and unnodulated soybeans ( Glycine max [L.] Merr.) were grown in N‐free or N‐containing nutrient solutions, respectively. Starting at the initial flowering stage, and throughout reproductive growth, the NO 3 ‐ absorption capacity of roots of intact plants from both treatments was determined in short‐term uptake experiments. Acetylene reduction activity was determined for nodulated plants. Nitrate absorption rate, expressed on a root dry weight basis, was greatest at early flowering for both nodulated and unnodulated plants. At 33 days after germination, the NO 3 ‐ absorption rate of unnodulated plants was twice as great as that of nodulated plants. During the remainder of the sampling period, NO 3 ‐ absorption rates of both nodulated and unnodulated plants decreased progressively and similarly. Maximum nodule specific activity occurred 30 days after germination, or initial flowering. However, maximum total C 2 H 2 reduction activity, oner plant basis, was observed during the early stages of pod‐filling. Compared to unnodulated plants dependent on NO 3 ‐ assimilation, nodulated plants were smaller, had less N in vegetative tissues, and produced less seed per plant. We suggest that the higher NO 3 ‐ absorption rate of unnodulated soybean roots, particularly during early reproductive growth, may have reflected a more favorable supply of photosynthate translocated to the roots from larger, more vigorous, non‐N‐stressed shoots.