Effects of Hypoxia on (13)NH(4)(+) Fluxes in Rice Roots

Techniques of compartmental (efflux) and kinetic influx analyses with the radiotracer (13)NH(4)(+) were used to examine the adaptation to hypoxia (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) seedlings. A time-dependence study of NH(4)(+) influx into rice roots after onset 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 than 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 of NH(4)(+) uptake. Half-lives for NH(4)(+) exchange with subcellular compartments, cytoplasmic NH(4)(+) concentrations, and efflux (as percentage of influx) were unaffected by hypoxia. However, significant differences were observed in the relative amounts of N allocated to NH(4)(+) assimilation and the vacuole versus translocation to the shoot. Kinetic experiments conducted at 100, 50, 35, and 15% O(2) saturation showed no significant change in the K(m) value for NH(4)(+) uptake with varying O(2) supply. However, V(max) was 42% higher than controls at 50% O(2) saturation, unchanged at 35%, and 10% lower than controls at 15% O(2). The significance of these flux adaptations is discussed.