Phosphorus deficiency enhances plasma membrane H⁺-ATPase activity and citrate exudation in greater purple lupin (Lupinus pilosus)

The response of greater purple lupin (Lupinus pilosus L.) to a combination of phosphorus (P) deficiency and aluminium (Al) toxicity is unknown, and the mechanisms involved in the exudation of organic anions from greater purple lupin have not been reported. Therefore, plants grown with (+P) or without (-P) 250 μm were exposed to 0 or 50 μm AlCl₃ and the amount of organic anions exuded and the activities of plasma membrane H⁺-ATPase (E.C 3.6.3.6) and H⁺-pumps were investigated. Twenty days of P deficiency resulted in significantly reduced shoot growth and increased proteoid root formation. Exposure to 50 μm AlCl₃ did not induce citrate exudation but did induce some malate exudation in -P plants. In contrast, P deficiency did induce exudation of citrate. Enhanced citrate exudation was attributed to the large increase in the activity of plasma membrane H ⁺-ATPase and associated H⁺ transport. This was shown by the inhibitory effect of vanadate on plasma membrane H⁺-ATPase activity in vitro and on citrate exudation in vivo. However, vanadate did not suppress the exudation of malate. During 9 h of Al exposure, exudation of citrate showed a continuing increase for both -P and +P plants, while malate exudation increased only during the first 3 h, after which it rapidly declined. The total amount of organic anion exudation was significantly higher for -P plants. In the presence of 50 μm anion channel blockers [anthracene-9- carboxylic acid (A-9-C), niflumic acid (NIF) and phenylglyoxal (PG)], the exudation of citrate and malate was reduced by 25-40%. It was concluded that P deficiency induces citrate exudation by enhancing the activity of plasma membrane H⁺-ATPase and H⁺ export. In L. pilosus, exudation of organic anions occurs primarily in response to P deficiency but not Al toxicity. This contrasts with previous results obtained in Brassica napus L.