Some regulatory aspects of [(14)C]methylamine influx intoPisum sativum L. cv. Feltham First seedlings.
[(14)C]Methylamine influx intoPisum sativum L. cv. Feltham First seedlings showed Michaelis-Menten-type kinetics with apparentV max=49.2 μmol·g(-1) FW·h(-1) and apparentK m=0.51 mM. The competitive interactions between ammonium and methylamine were most obvious when biphasic kinetics were assumed with saturation of the first phase at ∼0.05 mM. The inhibitor constant for ammonium (K i)=0.027 mM. When [(14)C]methylamine was used in trace amounts with ammonium added as substrate, the influx of tracer showed Michaelis-Menten-type kinetics with apparentV max=3.46 μmol·g(-1) FW·h(-1) and apparentK m=0.15 mM. The initial rate of net ammonium uptake corresponded with that found when [(14)C]methylamine was used to 'trace' ammonium influx. The latter was also stimulated by high pHo and inhibited by nitrate. Ammonium pretreatment±methionine sulphoximine or glutamine pretreatment of the seedlings inhibited subsequent [(14)C]methylamine influx, while methylamine or asparagine pretreatment stimulated [(14)C]methylamine influx. There was also a stimulatory effect of prior inoculation withRhizobium. The results are discussed in terms of current models for the regulation of ammonium uptake in plants.
Botany Department, Science Laboratories, Durham University, DH1 3LE, Durham, UK.
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Deane-Drummond