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Growth and Nitrogen Assimilation of Soybeans in Response to Ammonium and Nitrate Nutrition

Plants supplied with moderate concentrations of NH4 + in solution generally grow poorly compared with plants supplied with other sources of nitrogen. Experiments were conducted with a flowing solution culture system to determine whether growth restrictions could be avoided over an extended period in the presence of NH4 + if root-zone pH were strictly controlled and if plants were exposed to NH4 + during exponential growth when carbohydrate fluxes to the root are coordinated with the rate of nitrogen acquisition. Vegetative soybeans (Glycine max [L.] Merrill) initially were exposed to complete nutrient solutions containing NO3 - until the exponential growth stage and then were exposed for 4 wk to solutions in which nitrogen was supplied as either 1.0 mM NH4 +, 1 0 mM NO3 -, or 0.5 mM NH4 + plus 0.5 mM NO3 -. Acidity of the solutions was constantly maintained at pH 5 8 ± 0 1 by automated control In separate experiments, irradiance (photosynthetic photon flux density [PPFD] of 700 and 325 μE m-2 s-1) levels were controlled to produce distinct steady-state rates of leaf, root, and whole-plant growth The source of nitrogen did not alter growth or nitrogen accumulation within either environment Growth of whole plants and plant parts and accumulation of nitrogen remained exponential The results support the conclusion that plants can effectively utilize NH4 + as a nitrogen source as long as root-zone pH is strictly controlled and a balance is maintained between carbohydrate availability and acquisition of NH4 +