Simultaneous Measurement of NH(4) Absorption and N(2) Fixation by Glycine max L. : RESPONSE TO TEMPERATURE, pH, AND EXTERNAL NITROGEN CONCENTRATION.

Ammonium absorption rates by intact nodulated and unnodulated soybean plants (Glycine max [L.] Merr., Amsoy 71) were determined from the liquid phase of a mist assay chamber. From the gas phase, simultaneous measurements of acetylene reduction rates were made from nodulated plants. Ammonium absorption capacity was consistently greater in unnodulated plants.At the beginning of flowering, plant roots were sprayed with an uptake solution ranging from 0.05 to 1.0 millimolar NH(4)Cl, and root nodules concurrently were exposed to 0.12 atmosphere acetylene. The NH(4) (+) absorption system of both nodulated and unnodulated plants were nearly saturated at 0.3 to 0.5 millimolar NH(4) (+).Increasing the pH from 4.0 to 6.8 stimulated NH(4) (+) absorption rate in both flowering and preflowering nodulated and unnodulated plants. Acetylene reduction rate was not altered by short-term exposure to increased acidity. Short-term exposure to lower absorption solution temperatures from 32 degrees C to 18 degrees C did not significantly affect NH(4) (+) absorption rate in flowering nodulated or unnodulated plants. However, acetylene reduction rate increased as the absorption solution temperature increased. The Q(10) value was 1.5 for the reaction rate. Increasing external NO(3) (-) concentration from 0.1 to 5.0 millimolar NO(3) (-) did not significantly influence the kinetics of NH(4) (+) absorption or acetylene reduction rate.