Effect of enriched rhizosphere carbon dioxide on nitrate and ammonium uptake in hydroponically grown tomato plants

Previous reports have indicated positive effects of enriched rhizosphere dissolved inorganic carbon on the growth of salinity-stressed tomato (Lycopersicon esculentum L. Mill. cv. F144) plants. In the present work we tested whether a supply of CO₂ enriched air to the roots of hydroponically grown tomato plants had an effect on nitrogen uptake in these plants. Uptake was followed over periods of 6 to 12 hours and measured as the depletion of nitrogen from the nutrient solution aerated with either ambient or CO₂ enriched air. Enriched rhizosphere CO₂ treatments (5000 µmol mol⁻¹) increased $NO_3^ - $ uptake up to 30% at pH 5.8 in hydroponically grown tomato plants compared to control treatments aerated with ambient CO₂ (360 µmol mol⁻¹). Enriched rhizosphere CO₂ treatments had no effect on $NO_3^ - $ uptake. Acetazolamide, an inhibitor of apoplastic carbonic anhydrase, increased $NO_3^ - $ uptake in ambient rhizosphere CO₂ treatments, but had no effect on $NO_3^ - $ uptake in enriched rhizosphere CO₂ treatments. Ethoxyzolamide, an inhibitor of both cytoplasmic and extracellular carbonic anhydrase, decreased $NO_3^ - $ uptake in ambient rhizosphere CO₂ treatments. In contrast, a CO₂ enriched rhizosphere increased $NO_3^ - $ uptake with ethoxyzolamide, although not to the same extent as in plants without ethoxyzolamide. It is suggested that a supply of enriched CO₂ to the rhizosphere influenced $NO_3^ - $ uptake through the formation of increased amounts of $HCO_3^ - $ in the cytosol.