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

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Abstract
Plants supplied with moderate concentrations of NH4+ in solution generally grow poorly compared with plants supplied with other sources of N. 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 N acquisition. Vegetative soybeans 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 N 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 .mu.E [micro einsteins] m-2 s-1) levels were controlled to produce distinct steady-state rates of leaf, root and whole-plant growth. The source of N did not alter growth or N accumulation within either environment. Growth of whole plants and plant parts and accumulation of N remained exponential. Plants can apparently effectively utilize NH4+ as a N source as long as root-zone pH is strictly controlled and a balance is maintained between carbohydrate availability and acquisition of NH4+.

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