Nitrogen Fixation by White Lupin under Phosphorus Deficiency

Abstract

• Background and Aims White lupin is highly adapted to growth in a low-P environment. The objective of the present study was to evaluate whether white lupin grown under P-stress has adaptations in nodulation and N₂ fixation that facilitate continued functioning. • Methods Nodulated plants were grown in silica sand supplied with N-free nutrient solution containing 0 to 0·5 mm P. At 21 and 37 d after inoculation (DAI) growth, nodulation, P and N concentration, N₂ fixation (¹⁵N₂ uptake and H₂ evolution), root/nodule net CO₂ evolution and CO₂ fixation (¹⁴CO₂ uptake) were measured. Furthermore, at 21 DAI in-vitro activities and transcript abundance of key enzymes of the C and N metabolism in nodules were determined. Moreover, nodulation in cluster root zones was evaluated. • Key Results Treatment without P led to a lower P concentration in shoots, roots, and nodules. In both treatments, with or without P, the P concentration in nodules was greater than that in the other organs. At 21 DAI nitrogen fixation rates did not differ between treatments and the plants displayed no symptoms of P or N deficiency on their shoots. Although nodule number at 21 DAI increased in response to P-deficiency, total nodule mass remained constant. Increased nodule number in P-deficient plants was associated with cluster root formation. A higher root/nodule CO₂ fixation in the treatment without P led to a lower net CO₂ release per unit fixed N, although the total CO₂ released per unit fixed N was higher in the treatment without P. The higher CO₂ fixation was correlated with increased transcript abundance and enzyme activities of phosphoenolpyruvate carboxylase and malate dehydrogenase in nodules. Between 21 and 37 DAI, shoots of plants grown without P developed symptoms of N- and P-deficiency. By 37 DAI the P concentration had decreased in all organs of the plants treated with no P. At 37 DAI, nitrogen fixation in the treatment without P had almost ceased. • Conclusions Enhanced nodulation in cluster root zones and increased potential for organic acid production in root nodules appear to contribute to white lupin's resilience to P-deficiency.