Effects of Nitrogen Deficiency on the Absorption of Nitrate and Ammonium by Barley Plants

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Abstract
When young barley plants which had been supplied with nitrate were deprived of this source of N, an enhanced capacity for absorption of either nitrate or ammonium ions developed, reaching a maximum in about 3 d under the particular experimental conditions used. The net uptake rate of either nutrient was then approximately three times that in plants which had received nitrate throughout. Likewise, withholding external N from plants previously growing with ammonium caused a 2.4-fold increase in their subsequent capacity to absorb that ion, compared with control plants grown with an uninterrupted ammonium supply. Accelerated nitrate uptake in N-starved plants was not accompanied by additional phosphate or sulphate absorption, but the plants had the capacity to absorb more potassium, whether or not ammonium was also present in the solution. Indirect evidence from analyses of root tissue suggests that these responses to mild N-stress may depend on some property of an N fraction which does not include nitrate or ammonium.