Studies On Symbiotic Nitrogen Fixation Under Soil Stress Cond:

Fatma Mohamed Ahmed El-hadldy

Until perhaps 10 years, studies of environmental stress and thelegume-Rhizobium or Bradyrhizobium symbiosis were restricted todefining the problem, and to using physical or chemical amendments toovercome it, i.e., mulching to reduce soil temperature or liming toameliorate soil acidity. The identification of bacterial strains and in some cases host cultivC;lfs that are tolerant to these stresses opens the way for alternate, lower cost solutions to these problems. c’ Therefore, in the present study, the ability of2S local and foreign Rhizobium and Bradyrhizobium strains represent R. meliloti (5 strains),R, leguminosarum bv. viceae (5 strains), R leg!lminosarum bv. trifolii(2 strains), Bradyrhizobium sp. nodulating peanut (5 strains) and.Bradyrhizobium . japonicum. ( 5 strains) to tolerate increasedconcentrations of NaCI, high temperature, high levels of nitrogen, high pH values and desiccation was investigated. Rhizobial strains tolerant to the above mentioned stress conditions were selected and the symbiotic performance of the obtained strains was evaluated as well. Results are summarized as follows :1- Effect of NaCI salinity on growth and survival of rhizobia:1.AU tested strains were able to tolerate NaCl concentration up to O.1°/0 However, tolerance of NaCI salinization differed according torhizobial spp. and strains. In general, R. meliloti and Kleguminosarum bv. phseoli were found to be more sensitive to NaClsalinity . 2.There was a great variation in the response of the tested strains tosodium chloride regimes. All strains were sensitive to higherconcentration of NaCI(S%). The growth of some strains wascompletely inhi~ited at 20/0 NaCI. Some ,strains were sensitive to even20g IL NaCl. 1. Two Rhizobium l~guminosa,.um bv. viceae ARC 200Fand ICARDA 441 were tolerant to 10 and 40 gIL NaCl, the two strains wereidentified as low and high tolerance to NaCl salinity respectively ..2- Growth and survival res onse of rhizobia to increasin tem erature:I 1.The most of the rhizobial strains-tested showed optimum growth at 30-350C , then higher temperature resulted in gradual decreases inrhizobial numbers. Susceptibility to higher temperature of 50 ~Ciscommon among the rhizobia! species and strains under investigation.2. The decline in, the number of surviving strains was especiallynoticeable between 40 and 45°C. This behavior depends on therhizobial sp. and ·strain.3. In general, fourteen strains were found to tolerate temperature up to 450C. such as Rhizobium meliloii (ARC 1, ARC 2, and A 2), Rhizobiumleguminosarum bv. viceae (ARC 200 F and ARC 202F), Rhizobiumle~minosarum . bv. trifolii (TAL), Rhizobium lflEU_mirlOsaromby.phaseoli tARC 302 and UMR), peanut Bradyrhizobium (619, 3339and 601) and firadyrhizobium jqponicum (138, USDA 110 and ARC4. Three strains, however, were found to be susceptible to elevated temperature more than 3S °C such as Rhizobium le.,gpminosarufIl bv.viceae (ARC 206 F and ARC 207 F) and Rhizobium legumino.,arumby. tYi/olii (ARC 103).5. Four rhizobia 1 strains tolerant to high temperature representing peanut Bradyrhizobium (3456 T 40°C and 619 T 45°C ) andBradyrhizobium jqponicIlm (3407 T 40°C and 138 T 45°C) wereselected for further competence study.3- Effect of mineral nitrogen levels on survival of rhizobia:1. All tested strains were able to grow and survive under stress of200ppm N as KNO, (20 kg N I fed) in YEM medium 2. Strains representing Rhizobium le~”,i’lO.