INFLUENCE OF SOIL-SALINITY ON THE POPULATIONS AND COMPOSITION OF FLUORESCENT PSEUDOMONADS IN PLANT RHIZOSPHERE

Effect of soil salinity associated with intensive cultivation including horticulture on microbial communities in the soil-root system was investigated in a growth chamber experiment. Spinach plants were grown on three soil samples of the same origin but with low, medium, and high levels of salinity. Microbial populations, including fluorescent pseudomonads, in three sites of the soil-root system, namely root-free soil, rhizosphere soil, and rhizoplane, of 3-week-old spinach plants were compared in relation to the levels of soil salinity. Effect of soil salinity on the microbial populations differed widely among the three sites of the soil-root system, as well as among the microbial groups, In the root-free soil, populations of total bacteria (TB) and gram-negative bacteria (GN) did not change significantly through salinity, while the populations of total fluorescent pseudomonads (FP) apparently increased. In the rhizosphere soil, however, soil salinity induced changes in the populations depending on the microbial groups; the TB and GN populations increased while the FP populations were not affected. In the rhizoplane, however, soil salinity exerted an adverse effect on the TB and GN populations, and a markedly deleterious effect on the FP populations. Accordingly, soil salinity induced a significant decrease in the R/S ratio for FP, while the ratio increased for TB and GN. Soil salinity affected differently the two groups of FP, namely the Pp-group (Pseudomonas putida-dominant) and the Pf-group (Pseudomonas fluorescens-dominant). In the soil with a low salinity level, the Pf-group predominated with values as high as 71, 60, and 57% of the total FP, in the root-free soil, rhizosphere soil, and rhizoplane, respectively, while the corresponding values for the Pp-group were as low as 14, 3, and 38%, respectively. As salinity progressed, however, the populations of the Pf-group in the root-free and the rhizosphere soils drastically decreased with corresponding increases of the Pp-group. In the soil with a high salinity level, the values for Pp-group were as high as 62 and 52% of the total FP, respectively. Trends in the rhizoplane were similar, though the extent was less appreciable. These results indicated that soil salinity markedly modified the populations of FP in the soil-root system, thereby the populations of the Pp-group became dominant in the rhizosphere and replaced those of the Pf-group which appeared to be a major group of FP in the soil-root system which was not subjected to salinity.