Certain environmental factors affecting rhizobia and symbiotic systems.

The interrelation between rhizobia and certain fungi, bacteria, actinomycetes, nematodes, and seed-coat diffusates of Phaseolus vulgaris were investigated. The effect of pesticides, i.e. fungicides, herbicides, and nematocides on growth of rhizobia, and the symbiotic systems between rhizobia and their respective host is reported. Degradation of certain herbicides and insecticides is shown. The movement of rhizobia in soil as affected by water tension, tolerance of salts, and soil temperatures are discussed. Environmental factors may affect the successful establishment of an effective symbiosis between rhizobia and their hosts at any or all the three stages. They may 1) affect occurrence, growth, and survival of root nodule bacteria, 2) modify nodule formation, or 3) affect the function of the formed nodules (VINCENT 1962). The environmental aspect considered here include the antagonistic factors against rhizobia, the pesticides, and some ecological aspects of rhizobia in soil, e.g., the movement and salts and heat tolerance. These aspects were investigated by Egyptian workers over the period 1948-1972. Comprehensive reviews on the effect of environmental factors on rhizobia were reported by VINCENT (1962) and NUTMAN (1972).