Exudation of Organic Anions by White Lupin (Lupinus albus L.) and Their Role in Phosphate Mobilization from Soil

Abstract

Using an anion exchange resin, a method has been developed which allows the quantification of organic anions exuded from individual proteoid roots and root tips of white lupin (Lupinus albus L.) plants grown in nutrient solution and in soil. In both culture systems succinate was the dominant organic anion exuded followed by citrate and malate. In nutrient solution mean citrate exudation-rate was 0.57 pmol cm−1 s−1 and was highly dependent on the citrate content and on the age of the proteoid roots. In the rhizosphere of soil-grown plants, citrate was the predominant organic anion amounting to 91% of the total organic anions. Mobilization of soil P by plants was estimated from P depletion in the rhizosphere compared to the bulk soil. As an overall average 26% of the total inorganic P (Pi) depletion could be attributed to mobilization of labile P fractions (resin-P, NaHCO3-Pi, and NaOH-Pi) and 29% and 44% to the less mobile HCl- and residual-P fractions, respectively. This suggests that complexation of Fe and Al by organic anions was involved in solubilization of soil and fertilizer P.