Root Carbon Enrichment Alleviates Ammonium Toxicity in Cucumber Plants

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 NH₄NO_3. 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.

Keywords:

• nitrate
• ammonium
• ¹³C
• Cucumis sativus
• glutamine synthetase
• inorganic carbon enrichment
• nitrate reductase
• ¹⁵N
• PEPCase
• toxicity

ACKNOWLEDGMENTS

Financial support from the Iranian Ministry of Science, Research and Technology is gratefully acknowledged.