NODULATION AND NITROGEN-FIXATION IN EXTREME ENVIRONMENTS

Biological nitrogen fixation is a phenomenon occurring in all known ec osystems. Symbiotic nitrogen fixation is dependent on the host plant g enotype, the Rhizobium strain, and the interaction of these symbionts with the pedoclimatic factors and the environmental conditions. Extrem es of pH affect nodulation by reducing the colonization of soil and th e legume rhizosphere by rhizobia. Highly acidic soils (pH <4.0) freque ntly have low levels of phosphorus, calcium, and molybdenum and high c oncentrations of aluminium and manganese which are often toxic for bot h partners; nodulation is more affected than host-plant growth and nit rogen fixation. Highly alkaline soils (pH >8.0) tend to be high in sod ium chloride, bicarbonate, and berate, and are often associated with h igh salinity which reduce nitrogen fixation. Nodulation and N-fixation are observed under a wide range of temperatures with optima between 2 0-30 degrees C. Elevated temperatures may delay nodule initiation and development, and interfere with nodule structure and functioning in te mperate Iegumes, whereas in tropical legumes nitrogen fixation efficie ncy is mainly affected. Furthermore, temperature changes affect the co mpetitive ability of Rhizobium strains. Low temperatures reduce nodule formation and nitrogen fixation in temperate legumes; however, in the extreme environment of the high arctic, native legumes can nodulate a nd fix nitrogen at rates comparable to those observed with legumes in temperate climates, indicating that both the plants and their rhizobia have successfully adapted to arctic conditions. In addition to low te mperatures, arctic legumes are exposed to a short growing season, a lo ng photoperiod, low precipitation and low soil nitrogen levels. In thi s review, we present results on a number of structural and physiologic al characteristics which allow arctic legumes to function in extreme e nvironments.