Nitrogen

Abstract

The amino acid analog p‐fluorophenyl al anine (FPA) was used to test the hypothesis that nitrate uptake is mediated by a transport protein in the plasmalemmae of root cells. Experiments were conducted with dark‐grown corn seedlings that had been detopped to allow collection of xylem exudate. Energy was supplied to the roots via intact endosperms. To maximize induction of the nitrate transport system, the roots were exposed to 1.0 mM nitrate for 10 hours. They were then pretreated in 1.0 mM nitrate ± 0.1 mM FPA for 0, 4, 8 or 12 hours, following which nitrate assimilation was measured during a 4‐hour exposure to 1.0 mM ¹⁵N‐nitrate in the presence and absence (control) of 0.1 mM FPA. In a companion experiment, shorter pretreatments (0, 1, 2 and 3 hours) were applied, and nitrate assimilation was measured during a subsequent 1‐hour exposure to ¹⁵N‐nitrate ±FPA. During the initial 1‐hour exposure period, ¹⁵N‐nitrate uptake and ¹⁵N‐protein synthesis (i.e. incorporation of ¹⁵N into the insoluble N fraction) were restricted 30% and 7%, respectively, in FPA‐treated plants and control plants. The restriction increased with time, reaching values of 58% and 69% during the final exposure period, 12 to 16 hours. The latter effect, a gradual limitation of the nitrate uptake process, is consistent with a transformation of the nitrate transport system to a less active status, as would occur when FPA replaces phenylalanine during protein synthesis. Part of this effect, however, may have been due to a general inhibition of protein synthesis by FPA. In contrast, it is proposed that the more rapid restriction of ¹⁵N‐nitrate uptake by FPA, which was apparent within one hour, may include in addition to ineffective protein synthesis, a more direct effect of FPA, perhaps in altering the driving force for influx.

Key words:

• Corn
• nitrate assimilation
• amino acid analog
• p‐f1uorophenylalanine.

Notes

Supported by National Science Foundation Grant PCM‐81–18661. Paper No. 10604 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, N.C

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