ABSTRACT
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 (15N) and carbon (13C)] were used to assess the uptake of nitrogen [NH4 + or nitrate (NO3 −)] as well as carbon [bicarbonate (HCO3 −)/carbonate (CO3 2−)] 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 15N enrichment was lower in 15NH4 + supplied plants compared to 15NO3 −, while the 13C 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.
ACKNOWLEDGMENTS
Financial support from the Iranian Ministry of Science, Research and Technology is gratefully acknowledged.