Nitrate/ammonium ratio effects on mineral element uptake by sorghum1

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Abstract
Experiments were conducted using different NO3-/NH4+ ratios to determine the effects of these sources of N on mineral element uptake by sorghum [Sorghum bicolor (L.) Moench] plants grown in nutrient solution. The NO3-/NH4+ ratios in nutrient solution were 200/0, 195/5, 190/10, and 160/40 mg N/l. Nutrient solutions were sampled daily and plants harvested every other day during the 12-day treatment period. Moderately severe Fe deficiencies were observed on leaves of plants grown with 200/0 NO3-/NH4+ solutions, but not on the leaves of plants grown with the other NO3-/NH4+ ratios. As plants aged, less Fe, Mn, and Cu were translocated from the roots to leaves and leaf/root ratios of these elements decreased dramatically in plants grown with 200/0 NO3-/NH4+ solutions. Extensive amounts of Fe, Mn and Cu accumulated in or on the roots of plants grown with 200/0 NO3-/NH4+ solutions. Mn and Cu may have interacted strongly with Fe to inhibit Fe translocation to leaves and to induce Fe deficiency. As the proportion of NH4+ in solution increased, K, Ca, Mg, Mn and Zn concentrations decreased in the leaves, and Ca, Mg, Mn and Cu concentration decreased in roots. K and Zn tended to increase in roots as NH4+ in solution increased.