Tricalcium-phosphate solubilizing efficiency of rhizosphere bacteria depending on the P-nutritional status of the host plant

Annette Deubel, Andreas Gransee and Wolfgang Merbach

Martin Luther University Halle-Wittenberg, Institute of Soil Science and Plant Nutrition,

Adam-Kuckhoff-Str. 17b, D-0618 Halle / Saale, Germany, e-mail: deubel@landw.uni-halle.de

Root deposits of higher plants contain ecological relevant C amounts (11-20 % of net CO₂ assimilation e.g. 13-32 % of C incorporated into the plant). About 75 % of the organic root deposits are water soluble. Sugars, amino acids and carboxylic acids are main components, with sugars usually representing the largest part. P-mobilizing effects of carboxylic acid exudation are well documented, but less is known about the influence of sugar exudates on phosphate availability.

This study assesses the influence of saccharides in the rhizodeposition on the phosphate solubilizing ability of rhizosphere bacteria. Water-soluble rhizodeposits were analysed of C labelled pea plants (Pisum sativum, cv. ‘Grapis’) which were grown at two different levels of P-nutrition (1). The sugars produced were fed in vitro either as single compounds or as synthetic mixtures to three bacterial strains and the ability of the bacteria to mobilize Ca₃(PO₄)₂ was measured (2).

The relative glucose proportion of pea exudates decreased under P-deficiency while the content of galactose, ribose, xylose and fucose increased. In vitro feeding of single sugars and sugar mixtures showed, that the ability of Pseudomonas fluorescens (PsIA12) to dissolve tertiary calcium phosphate was lower with pentoses and the mixed sugars of the P- deficient plants than with glucose. On the other hand, that shift in the sugar pattern observed under P-deficiency increased the P-mobilizing ability of Pantoea agglomerans (D 5/23) and Azospirillum sp. (CC 322) considerably.

This observation can only partly be explained by the acidification of the nutrient medium. Bacteria also produced different carboxylic anions depending on sugar supply. In addition to low-molecular mono-, di-, and tricarboxylic acids which are known as P-solubilizing substances, sugar acids also played an important role in cultures D 5/23 and CC 322.

References:

1. Gransee, A. and Wittenmayer, L. (2000) J. Plant Nutr. Soil Sci. 163, 381-385

2. Deubel, A., Gransee, A. and Merbach, W. (2000) J. Plant Nutr. Soil Sci. 163, 387-392