Nitrogen fixation by white lupin under phosphorus deficiency

Area of Science:

• Plant Physiology
• Soil Science
• Biochemistry

Background:

• White lupin exhibits remarkable adaptation to low-phosphorus (P) environments.
• Understanding its physiological and biochemical mechanisms is key to improving crop performance in nutrient-limited soils.

Purpose of the Study:

• To investigate adaptations in nodulation and nitrogen (N2) fixation in white lupin under phosphorus stress.
• To determine if these adaptations facilitate continued functioning of N2 fixation.

Main Methods:

• Plants were grown in sand with varying P concentrations (0-0.5 mM).
• Measurements included growth, nodulation, P and N concentration, N2 fixation (15N2 uptake, H2 evolution), and CO2 exchange in nodules.
• In vitro enzyme activities and transcript abundance were analyzed at 21 days after inoculation (DAI).

Main Results:

• Phosphorus deficiency increased nodule number, associated with cluster root formation, but total nodule mass remained constant.
• Nitrogen fixation rates were maintained at 21 DAI despite lower P concentrations, with nodules showing higher P concentration than other organs.
• Increased CO2 fixation in nodules, linked to higher phosphoenolpyruvate carboxylase and malate dehydrogenase activity, supported N2 fixation under P-stress.

Conclusions:

• Enhanced nodulation in cluster root zones contributes to white lupin's resilience to P-deficiency.
• Increased organic acid production potential in root nodules supports sustained N2 fixation under low-P conditions.