Root carbon enrichment alleviates ammonium toxicity in cucumber plants

The addition of carbonates to a nutrient solution to alleviate ammonium (NH4+) toxicity in hydroponically-grown cucumber (Cucumis sativus L.) plants was investigated. Stable isotopes [nitrogen (N-15) and carbon (C-13)] were used to assess the uptake of nitrogen [NH4+ or nitrate (NO3-)] as well as carbon [bicarbonate (HCO3-)/carbonate (CO32-)] by the roots. Ammonium as the sole N source at 5 mM decreased plant fresh weights compared to NO3-. However, at lower concentrations of NH4+ (25% of 5 mM total N), growth was increased compared to NO3- alone. Inorganic C enrichment [calcium carbonate (CaCO3)] of the nutrient solution increased the fresh weight of NH4+ grown plants with up to 150% relative to control plants receiving calcium hydroxide [Ca(OH)(2)] for pH regulation. Root N-15 enrichment was lower in (NH4+)-N-15 supplied plants compared to (NO3-)-N-15, while the C-13 enrichment in leaves was increased by NH4+ nutrition compared to NO3- or NH4NO3. The enhanced C capture was associated with high PEPCase activity in the roots. It is concluded that inorganic carbon enrichment of the root medium may alleviate NH4+ toxicity via increased synthesis of C skeletons and, accordingly, increased capacity for NH4+ assimilation and N export to the shoots.