AMMONIUM UPTAKE BY RICE ROOTS .1. FLUXES AND SUBCELLULAR-DISTRIBUTIONOF NH4-N-13()

The time course of (NH4+)-N-13 uptake and the distribution of (NH4+)-N -13 among plant parts and subcellular compartments was determined for 3-week-old rice (Oryza sativa L. cv M202) plants grown hydroponically in modified Johnson's nutrient solution containing 2, 100, or 1000 mum NH4+ (referred to hereafter as G2, G100, or G1000 plants, respectivel y). At steady state, the influx of (NH4+)-N-13 was determined to be 1. 31, 5.78, and 10.11 mumol g-1 fresh weight h-1, respectively, for G2, G100, and G1000 plants; efflux was 11, 20, and 29%, respectively, of i nflux. The NH4+ flux to the vacuole was calculated to be between 1 and 1.4 mumol g-1 fresh weight h-1. By means of (NH4+)-N-13 efflux analys is, three kinetically distinct phases (superficial, cell wall, and cyt oplasm) were identified, with t1/2 for (NH4+)-N-13 exchange of approxi mately 3 s and 1 and 8 min, respectively. Cytoplasmic [NH4+] was estim ated to be 3.72, 20.55, and 38.08 mm for G2, G100, and G1000 plants, r espectively. These concentrations were higher than vacuolar [NH4+], ye t 72 to 92% of total root NH4+ was located in the vacuole. Distributio ns of newly absorbed (NH4+)-N-13 between plant parts and among the com partments were also examined. During a 30-min period G100 plants metab olized 19% of the influxed (NH4+)-N-13. The remainder (81%) was partit ioned among the vacuole (20%), cytoplasm (41%), and efflux (20%). Of t he metabolized N-13, roughly one-half was translocated to the shoots.