p-Fluorophenylalanine-Induced Restriction of Ion Uptake and Assimilation by Maize Roots

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Abstract
Roots of decapitated maize seedlings were exposed for 12 h to 1.0 mM KNO3 (98.5 atom per cent 15N) in the presence and absence (control) of 0.1 mM p-fluorophenylalanine (FPA), an analog of the amino acid phenylalanine. FPA decreased nitrate uptake but had little effect on K uptake. Accumulation of both ions in the xylem exudate was greatly restricted. The proportion of reduced 15N that was translocated at each time was also restricted by FPA. These observations are interpreted as indicating that synthesis of functional protein(s) is required for nitrate uptake and for transport of K, nitrate and reduced-15N from xylem parenchyma cells into xylem elements. The effect of FPA on nitrate reduction is less clear. Initially, FPA limited nitrate reduction more than nitrate uptake, but by 8 h the cumulative reduction of entering nitrate was similar (.apprx. 35%) in both control and FPA-treated roots. A relationship between nitrate uptake and nitrate reduction is implied. Nitrate influx may regulate the proportion of nitrate reductase in the active state, and thereby regulate concurrent nitrate reduction in decapitated maize seedlings.