EFFECTS OF HYPOXIA ON (NH4-N-13 FLUXES IN RICE ROOTS - KINETICS AND COMPARTMENTAL ANALYSIS())

Techniques of compartmental (efflux) and kinetic influx analyses with the radiotracer (NH4+)-N-13 were used to examine the adaptation to hyp oxia (15, 35, and 50% O-2 saturation) of root N uptake and metabolism in 3-week-old hydroponically grown rice (Oryza sativa L., cv IR72) see dlings. A time-dependence study of NH4+ influx into rice roots after o nset of hypoxia (15% O-2) revealed an initial increase in the first 1 to 2.5 h after treatment imposition, followed by a decline to less tha n 50% of influx in control plants by 4 d. Efflux analyses conducted 0, 1, 3, and 5 d after the treatment confirmed this adaptation pattern o f NH4+ uptake. Half-lives for NH4+ exchange with subcellular compartme nts, cytoplasmic NH4+ concentrations, and efflux (as percentage of inf lux) were unaffected by hypoxia. However, significant differences were observed in the relative amounts of N allocated to NH4+ assimilation and the vacuole versus translocation to the shoot. Kinetic experiments conducted at 100, 50, 35, and 15% O-2 saturation showed no significan t change in the K-m value for NH4+ uptake with varying O-2 supply. How ever, V-max was 42% higher than controls at 50% O-2 saturation, unchan ged at 35%, and 10% lower than controls at 15% O-2. The significance o f these flux adaptations is discussed.