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SURVIVAL AND MOVEMENT OF RHIZOBIUM IN DRY SOIL

التبويبات الأساسية

Salaheddine  I.  ISSA

Univ.

Reading

Spec.

Soil  Science

Deg.

Year

#Pages

Ph.D.

1990

241

 

Double‑labeled antibiotic‑ resistant mutants were developed to study the survival and movement of Rhizobium in soil. Different techniques were used to study the effect of water potential and soil texture on survival and movement of rhizobia in soils maintained at constant water potential or exposed to different drying regimes.

Soil water potential and soil texture were both generally found to have a profound effect on survival of rhizobia. The multiplication and survival rates during drying were related to the initial water potential of soil and correlated negatively to the amount of water lost during drying. The soils that showed significant multiplication in general favored survival of rhizobia. The benefit of fine texture soil in protecting rhizobia was not observed in soils maintained at constant water potential but it was significant when the soils were exposed to rapid drying.

The microscopic observations showed that not all rhizobium strains are motile and flagellated. The swarming diameter observed on swarm plates of soft agar was positively correlated with the presence and number of flagella and the motility level of cells when observed under the microscope. The main factors affecting motility and chemotaxis in vitro were rhizoblum type, solidity of swarm plates, attractant type and concentration.

Differences between the active movement of strains were also observed in soil. The movement aided by flagella decreased in soils drier than ‑0.1 MPa. The non‑motile strain moved actively (by some unknown mechanism) in soils of ‑1.5 and ‑5 MPa more than the motile strains. The active movement of rhizobia was increased by maintaining the soil at constant water potential, supplying nutrients, but was reduced when the soils were exposed to drying. The active movement of rhizobia was faster in light‑textured soils than in clay soils. Flowing water transported rhizobia through soil columns. The passive movement of a non‑motile strain through different soils by percolating water was the same in different soils whereas the motile strains passed differently in different soil types. Strains 9C and Ch 191 passed through Rowland soil faster than Katumani soil and Acid Washed Sand. The time of the first appearance for Cl- was faster than for rhizobia. The retention of rhizobia cells by adsorption or by mechanical sieving was suggested. The retention of cells varied with strains and with soil type .