Biological nitrogen fixation is a phenomenon occurring in all known ecosystems. Symbiotic nitrogen fixation is dependent on the host plant genotype, the Rhizobium strain, and the interaction of these symbionts with the pedoclimatic factors and the environmental conditions. Extremes of pH affect nodulation by reducing the colonization of soil and the legume rhizosphere by rhizobia. Highly acidic soils (pH <4.0) frequently have low levels of phosphorus, calcium, and molybdenum and high concentrations of aluminium and manganese which are often toxic for both partners; nodulation is more affected than host-plant growth and nitrogen fixation. Highly alkaline soils (pH >8.0) tend to be high in sodium chloride, bicarbonate, and borate, and are often associated with high salinity which reduce nitrogen fixation. Nodulation and N -fixation are observed under a wide range of temperatures with optima between 20-30°C. Elevated temperatures may delay nodule initiation and development, and interfere with nodule structure and functioning in temperate legumes, whereas in tropical legumes nitrogen fixation efficiency is mainly affected. Furthermore, temperature changes affect the competitive 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 and 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 temperatures, arctic legumes are exposed to a short growing season, a long photoperiod, low precipitation and low soil nitrogen levels. In this review, we present results on a number of structural and physiological characteristics which allow arctic legumes to function in extreme environments.

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Short-term (10 minutes) measurements of plasmalemma NO3-influx ( into roots of intact barley plants were obtained using 13NO3-. In plants grown for 4 days at various NO3-levels (0.1, 0.2, 0.5 millimolar), ,0. was found to be independent of the level of NO3-pretreatment. Similarly, pretreatment with Cl-had no effect upon plasmalemma 13NO3-influx. Plants grown in the complete absence of 13N03-(in CaSO4 solutions) subsequently revealed influx values which were more than 50% lower than for plants grown in NO3-. Based upon the documented effects of NO3-or Cl-pretreatments on net uptake of NO3-, these observations suggest that negative feedback from vacuolar NO3-and/or Cl-acts at the tonoplast but not at the plasmalemma. When included in the influx medium, 0.5 millimolar Cl-was without effect upon 13N03-influx, but N1H4 caused approximately 50% reduction of influx at this concentration.was based upon a form of 'pump and leak' system. Previous studies, particularly those of Jackson and his associates (13,15,16), have established that net NO3-efflux can be considerable under appropriate conditions. Through the use of '3NO3-, it is possible to test the above hypothesis directly and to investigate the effect of NO3-or Claccumulation upon plasmalemma NO3-influx. Using barley plants pretreated for 4 d at various NO3-levels and at a single Cl-level, plasmalemma 13NO3-influx values were found to be independent of pretreatment. While preliminary, the results of these experiments confirm the original conclusions regarding the insensitivity of influx to vacuolar [NO3 1. Likewise, it is evident that 13N03-influx is insensitive to prior loading with Cl-, a condition which has been clearly demonstrated to reduce net 7,19). These results are discussed in the context of current perceptions of the regulation of NO3-uptake in roots.

Sainfoin (Onobrychis viciifolia), a temperate perennial forage legume, can be nodulated by rhizobia isolated from 3 arctic legume species: Astragalus alpinus, Oxytropis maydelliana and Oxytropis arctobia. Arctic rhizobia, which are adapted to growth at low temperatures, may be useful in improving symbiotic nitrogen fixation during cold phases of the growing season, if they are effective on a temperate legume. In this study, we report on the symbiotic effectiveness of arctic rhizobia on sainfoin, as appraised by the total shoot dry matter yield obtained from 2 harvests. Under N-free conditions, 5 arctic strains at the first harvest and 8 at the second harvest were as effective as temperate standard strains. In the presence of 30mgl-NO3-N, 7 arctic strains gave significantly higher yields than temperate strains at the second harvest. These results indicate that effective arctic rhizobia have a potential for use as inoculants on sainfoin.