Elevated CO2 concentration around alfalfa nodules increases N2 fixation

Area of Science:

• Plant Biology
• Biochemistry
• Agricultural Science

Background:

• Nodule CO2 fixation, primarily via phosphoenolpyruvate carboxylase (PEPC), is crucial for supplying malate and oxaloacetate, essential for efficient nitrogen fixation.
• Nodule CO2 fixation rates are known to be influenced by external CO2 availability.

Purpose of the Study:

• To investigate the hypothesis that elevated CO2 concentrations enhance nitrogen fixation in alfalfa plants.
• To determine the impact of direct CO2 supply to nodules on nitrogen fixation efficiency and plant growth.

Main Methods:

• Alfalfa plants were grown in a hydroponic system with separate aeration for the root/nodule compartment.
• Nodules were exposed to either elevated CO2 (2500 µL L⁻¹) or ambient/zero CO2 conditions.
• Measurements included nodule CO2 fixation, nitrogen fixation rates, plant growth, and biochemical analysis of nodules and xylem sap.

Main Results:

• Elevated CO2 significantly increased nodule CO2 fixation, nitrogen fixation, and overall plant growth over a 3-week period.
• Higher CO2 levels led to increased concentrations of amino acids, organic acids, and asparagine in nodules and xylem sap.
• Plants with elevated CO2 showed a tendency towards higher nodule %N concentration and individual nodule activity.
• No significant effect of elevated CO2 was observed in plants with inefficient nodules (fix⁻).

Conclusions:

• Nodule CO2 fixation is pivotal for efficient nitrogen fixation in alfalfa.
• Strategies enhancing nodule CO2 fixation can improve nitrogen fixation and nodule formation.
• Adequate CO2 supply to roots and nodules is essential for optimal growth and nitrogen fixation in hydroponic and aeroponic systems.