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Articles

Nitrate Uptake by Dark-grown Corn Seedlings

Some Characteristics of Apparent Induction ¹

Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27607, Department of Agronomy, University of Illinois, Urbana, Illinois 61801

Five-or six-day old seedlings of corn (Zea mays L.) were exposed to 0.25 mM Ca(NO₃)₂, 1.0 mM sodium 2-[N-morpholino]-ethanesulfonate, 5 µg Mo per liter and 50 µg of chloramphenicol per ml at pH 6. Nitrate uptake was determined from depletion of the ambient solution. The pattern of nitrate uptake was characterized, after the first 20 minutes, by a low rate which increased steadily to a maximal rate by 3 to 4 hours. Transfer of nitrate to the xylem did not totally account for the increase. Development of the maximal accelerated rate did not occur at 3 C with excised roots nor with seedlings whose endosperm had been removed. Use of CaCl₂ rather than Ca(NO₃)₂ resulted in a linear rate of chloride uptake during the first 4 hours, and chloride uptake was not as restricted by endosperm removal as was nitrate uptake.

Nitrite pretreatments or the addition of cycloheximide (2 µg ml^–1), puromycin (400 µg ml^–1) and 6-methylpurine (0.5 mM) restricted maximal development of the accelerated nitrate uptake rate. Actinomycin D (20 µg ml^–1) inhibited the rate only after about three hours exposure. The RNA and protein synthesis inhibitors also restricted nitrate reductase induction in the apical segments of the root tissue. The data suggest that development of the maximal accelerated rate of nitrate uptake depended upon continuous protein synthesis, and the hypothesis that synthesis of a specific nitrate transport protein must occur is advanced. But the alternative hypothesis, i.e., that induction of nitrate reductase (and/or a consequence of the act of nitrate reduction) provided the required stimulus, remains tenable.

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