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With urea as sole nitrogen source, the addition of 5×10(-5) M nickel sulfate to axenic cultures of Lemna paucicostata 6746 approximately doubles the rate of vegetative growth. Under a standard light-dark schedule, Ni(2+) changes the daily pattern of respiratory CO2 output on urea from one having a single daily peak to one with two daily peaks which resembles that on ammonium or nitrate as sole nitrogen source. It also increases CO2 output by as much as 3-fold on a fresh-weight basis. These data represent the first confirmation in an intact higher plant of proposals, based on enzymology and tissue culture responses, for a role of Ni(2+) in urea metabolism. Further, they indicate the possible existence of two distinct pathways of urea utilization.

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