Simultaneous Measurement of NH4+ Absorption and N2 Fixation by Glycine max L.

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Abstract
Ammonium absorption rates by intact nodulated and unnodulated soybean plants (G. max [L.] Merr., cv. Amsoy 71 [inoculated with Rhizobium japonicum]) 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 0.05-1.0 mM NH4Cl, and root nodules concurrently were exposed to 0.12 atm acetylene. The NH4+ absorption system of nodulated and unnodulated plants were nearly saturated at 0.3-0.5 mM NH4+. Increasing the pH from 4.0 to 6.8 stimulated NH4+ absorption rate in 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.degree. C to 18.degree. C did not significantly affect NH4+ absorption rate in flowering nodulated or unnodulated plants. Acetylene reduction rate increased as the absorption solution temperature increased. The Q10 value was 1.5 for the reaction rate. Increasing external NO3- concentration from 0.1 to 5.0 mM NO3- did not significantly influence the kinetics of NH4+ absorption or acetylene reduction rate.