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Effects of Hypoxia on 13NH4+ Fluxes in Rice Roots1
Kinetics and Compartmental Analysis

Herbert J. Kronzucker, Guy J.D. Kirk, M. Yaeesh Siddiqi, and Anthony D.M. Glass*

International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines (H.J.K., G.J.D.K.); and Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 (H.J.K., M.Y.S., A.D.M.G.)

Techniques of compartmental (efflux) and kinetic influx analyses with the radiotracer 13NH4+ were used to examine the adaptation to hypoxia (15, 35, and 50% O2 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 NH4+ influx into rice roots after onset of hypoxia (15% O2) 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 NH4+ uptake. Half-lives for NH4+ exchange with subcellular compartments, cytoplasmic NH4+ concentrations, and efflux (as percentage of influx) 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% O2 saturation showed no significant change in the Km value for NH4+ uptake with varying O2 supply. However, Vmax was 42% higher than controls at 50% O2 saturation, unchanged at 35%, and 10% lower than controls at 15% O2. The significance of these flux adaptations is discussed.

1   This work was supported by funds from the New Frontier project grant to the International Rice Research Institute and by a National Sciences and Engineering Research Council of Canada grant to A.D.M.G.
*   Corresponding author; e-mail aglass{at}unixg.ubc.ca; fax 1-604-822-6089.

Plant Physiol. (1998) 116: 581-587
Copyright Clearance Center:   0032-0889/98/116/0581/07
© 1998 American Society of Plant Physiologists




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