A physiological dissection of nitrogen fluxes and pools in higher plant tissues

Compartmental analysis by efflux (CAE) was used with the high-energy positron-emitting nitrogen (N) tracer ¹³N to study: 1) fluxes and compartmentation of ammonium (NH₄⁺) in leaf slices from rice (Oryza sativa L. cv. IR-72), wheat (Triticum aestivum L. cv. 'Max Red'), and tomato (Lycopersicon esculentum L. cv. Trust') seedlings; and 2) fluxes and compartmentation of ammonium and nitrate(NO₃⁻) in roots of barley [Hordeum vulgare L. cv. CM-72, cv. 'Klondike') and rice seedlings. An equation set based on CAE kinetic data was developed to model underestimates in tracer influx measurements resulting from concurrent tracer efflux. Leaf-slice experiments showed that cellular fluxes and compartmentation of NH₄⁺ in leaves were similar to those in roots. NH₄⁺ efflux:influx ratios varied with plant species, in the order rice > wheat > tomato, corresponding to a declining gradient in NH₄⁺ tolerance. Fluxes in barley (ammonium-intolerant) and rice (ammonium-tolerant) roots also showed pronounced interspecific differences, particularly that at high (10 mM) external ammonium concentrations ([NH₄⁺]0), NH₄⁺ efflux, influx, and efflux:influx across the plasma membrane were 2.7,1.9, and 1.4 times higher, respectively, in barley than in rice. Much greater futile cycling of NH₄⁺ in barley was thus indicated under this condition. A Nernstian analysis of electrophysiological and compartmentation data indicated that the elevated efflux in barley was active, i.e., energy-dependent. While in barley this flux was associated with 41% higher oxygen consumption by intact barley roots at 10 mM[NH₄⁺]₀ , compared to 0.1 mM, no such difference was found in rice. Results are discussed in the context of NH₄⁺ toxicity. Half-lives (t½) of cytosolic ¹³N efflux from a wide variety of plants were independent of external [N] for a given nitrogen ion and a given plant species, and were restored within 5 min following ten-fold changes in [NH₄⁺]₀. The evident steady-state constancy in cytosolic NH₄⁺ and NO₃⁻ turnovers suggests a condition in which cytosolic [N] and influx are linearly related, in contrast to cytosolic potassium (K⁺) concentrations, which are held constant as external [K⁺] and K⁺ influx change. These observations suggest the presence of fundamentally different control mechanisms for cytosolic pools of the two elements.Science, Faculty ofBotany, Department ofGraduat