Studies on the rhizosphere microflora of Citrus trees: II. Qualitative distribution of the bacterial flora

Abstract

Studies were made on the relative incidence of various morphological, physiological, and nutritional groups of bacteria in the rhizosphere regions of growing and non-growing roots of three species of Citrus as compared to those in the soil. Gram-negative non-sporing rods were more abundant in the rhizospheres of both growing and non-growing roots than in the soil, whereas Gram-positive rods, cocci, and sporeformers were more numerous in the soil than in the rhizospheres. Nitrate reducers, gelatin liquefiers, starch hydrolyzers, and methylene blue reducers occurred in greater numbers in the rhizospheres than in soil. Growing roots seemed to stimulate preferentially nitrate reducers and gelatin liquefiers, whereas cellulose decomposers and pectin reducers were in higher proportions in non-growing roots than in the growing roots as well as in soil. There was no significant difference between populations of azotobacter occurring in the rhizospheres and soil, but sulphur-oxidizing bacteria were absent in the rhizospheres of both growing and non-growing roots. Bacteria that require amino acids for growth were more abundant in the rhizosphere of growing than non-growing roots, whereas those requiring growth factors in yeast extract and soil extract were less abundant in the growing roots than in soil. The numbers associated with non-growing roots appeared to be intermediate between the growing roots and soil.