The influence of arginine-specific reagents on nitrate uptake by corn seedlings

The effects of three arginine-specific reagents on NH3- uptake were studied using corn seedlings (Zea mays L., Golden Cross Bantam). In the presence of borate, 0.25 mM 2, 3-butanedione (BD) and 1.0 mM 1, 2-cyclohexanedione (CHD) inhibited NH3- uptake by 76% and 68%, respectively, compared to the controls. However, in the absence of borate, only 18% and 38% inhibition was observed for 0.25 mM BD and 1.0 mM CHD, respectively. Similarly, 0.5 mM phenylglyoxal (PGO) resulted in 75% inhibition. The degree of inhibition of nitrate uptake exhibited a concentration-dependence with respect to the reagents. Corn seedlings are 2- or 3-fold more sensitive to BD than to PGO and CHD, respectively, presumably due to the unfavourable steric effects of the benzal ring. Uptake of NH3- was partially restored after removal of BD, CHD, and PGO from the uptake medium. No significant differences were observed for the ATPase and plasma membrane-associated vanadate-sensitive H⁺-ATPase or K⁺-stimulated ATPase activity in homogenates and microsomal fractions prepared from corn seedlings which had been incubated for 2 h in the presence or absence of 0.5 mM BD or 1.0 mM PGO. This suggests that inhibition of nitrate uptake by the arginine-specific reagents was not caused by the indirect effect of their binding and inhibiting H⁺-ATPase. The fact that the arginine-specific reagents strongly inhibit NH3- uptake indicates that the NH3- transport system has arginine residues at or near the active site.