Root-induced acidification and excess cation uptake by N2-fixing Lupinus albus grown in phosphorus-deficient soil

Abstract

White lupin plants (Lupinus albus L. cv. Kiev) were grown in soil columns under controlled conditions at 20/12 °C (12/12 h) for 76 d to investigate the effect of phosphorus (P) deficiency on root-induced acidification and excess cation uptake by N2-fixing plants. Phosphorus was added in each column as FePO4 at a level of 10 (limited P) or 200 μg P g−1 (adequate P). Supply of 10 μg P g−1 restricted plant growth from 58 d after sowing (DAS) and decreased P concentrations significantly in shoots from 49 DAS and in roots from 40 DAS compared with plants supplied with 200 μg P g−1. Phosphorus concentrations in shoots of plants receiving 10 μg P g−1 decreased steadily from 2.1 to 1.1 mg P g−1 dry weight from 40 to 76 DAS, but P concentrations in roots were constant with time. Total P uptake increased with time irrespective of P supply, and the P uptake by plants at 10 μg P g−1 was only 35–75% of that at 200 μg P g−1. Plants fed with 10 μg P g−1 had higher Ca and Mg concentrations but lower S concentration in shoots than the plants fed with 200 μg P g−1. The concentrations of excess cations in plants were higher at 10 μg P g−1 than 200 μg P g−1 after 49 DAS. Phosphorus deficiency decreased the pH of root exudate solution due to the enhanced release of protons (H+) from roots. The pH of root exudate solution decreased rapidly with time and dropped to the lowest (4.28) at 58 DAS in the 10 μg P g−1 treatment. The decreased pH of root exudate solution was correlated with the increased concentrations of excess cations in plants. The pH of root exudate solution showed a different pattern of change with time compared with citrate exudation, suggesting that exudation of citrate anions contributes only a part of total acidification, but excess cation uptake dominantly contributes net proton release from roots of plants grown in P-deficient soil. Plant tissue had a significant accumulation of citrate in the treatment of 10 μg P g−1 compared with 200 μg P g−1 after 67 DAS. The results suggest that P deficiency enhances the excess cation uptake and concomitant proton release, and non-synchronous processes are involved in tissue accumulation and root exudation of organic anions under P deficiency.