Effect of Cd concentration in growth media on Secale cereale roots and Cd interaction with rhizosphere microorganisms originating from different parts of the grain

Abstract

Roots of Secale cereale grown for 3 weeks in nutrient solution with Cd added (1 or 10 μg ml−1) were colonized by microbes originating only from the interior of the grain (PSSG) or from the interior and the surface of the grain (PNSG). The number of bacteria in the PNSG growth medium without Cd added was approximately 15-fold lower than in PSSG, and the number of bacteria on PNSG root surface was 3 times higher than on PSSG roots. The effects were studied of PSSG and PNSG communities on plant growth, export of organics (siderophores, phenols, citric acid, proteins, and total Fe(III)-chelators) to the growth medium, total amount of immobilized Cd, and the ratio between its operational fractions extracted sequentially from roots. The dry and fresh weight of PSSG in the absence of Cd was only 36% of the roots and 68% of the shoots of PNSG. The amount of Cd immobilized by PSSG roots at 10 μg ml−1 of growth medium was twice as large as the amount immobilized by PNSG. Results of sequential extraction suggest that the metal immobilized by PNSG roots at 1 μg Cd ml−1 was more stable because the ion exchange fraction extracted from PNSG was 20% of that extracted from PSSG and 8-times smaller than that accumulated inside roots.