Abstract
Legume-rhizobia symbiosis has received higher attention asit enhances soil nutrition through the biological nitrogen fixation.However, stress factors such as excess salts, drought, acidity, alkalinity,and temperature etc. suppress the growth and symbiotic characteristicsof rhizobia. Identification of stress tolerant rhizobial strains and theirgenetic diversity which influence the efficiency of nitrogen fixation in Gliricidia sepium is poorly studied. This study mainlyfocused on the characterization and identification of stress-tolerant rhizobial strains that were isolated from five rootnodules of G. sepium from seven selected locations (Anamaduwa, Chilaw, Vanathawilluwa, Saliyawewa, Etiyawala,Palaviya, Kurinchanpitiya) of Puttalam district in Sri Lanka. Isolates were separately subjected to four different stressconditions, namely, pH (3.0 - 9.0), temperature (25°C - 45°C), salinity induced by NaCl concentrations ranging from 0.1%to 3.0%, and drought induced by polyethylene glycol 8000 (PEG-8000) concentrations ranging from 0.1% to 0.4%.Rhizobial strains isolated from Anamaduwa, Chilaw and Kurinchanpitiya such as An-3, An-4, An-5, Ch-1, Ch-4, Ch-5, Ku-2 and Ku-5 showed tolerance for alkaline pH (8.0 and 9.0) and extreme drought conditions (3.0% and 4.0% of PEG-8000).But their growth was adversely affected by acidic pH conditions (pH 3.0 and 4.0). Most of the rhizobial strains exceptstrains in Etiyawala were moderately tolerant to extreme salt concentrations (2.5% - 3.0% of NaCl) and extremetemperature levels (40- 45ºC). In the graphical interpretation, no growth pattern was observed with respect to fourphysiological conditions. Thirteen strains that were selected from the statistical analysis dis-played higher survivalcapacity when the combination of different stress conditions was applied. As 13 rhizobial strains showed differentbanding patterns in the Enterobacterial Repetitive Intergenic Consensus (ERIC) profile, they belonged to 09 clusters atthe 66.67% similarity level. Furthermore, these stress-tolerant rhizobial strains can be used to further studies on cross-inoculation of crop legumes as an alternative substitution for the vast usage of chemical ni-trogen fertilizers.
Highlights
Nitrogen, which is sometimes called as the most common factor that limits the growth and productivity of plants (Wagner, 2011) is most abundant in dinitrogen (N2) form in the atmosphere
Due to several negative impacts such as water eutrophication, soil acidification, and nitrogen oxide emission related to this process (Bohlool et al, 1992), effective use of Biological Nitrogen Fixation (BNF) in agriculture, has become crucial (Ishizuka, 1992)
Collection of root nodules from G. sepium Based on the geography in the Puttalum district, root nodule samples were collected from randomly selected seven locations (Anamaduwa, Chilaw, Vanathawilluwa, Saliyawewa, Etiyawala, Palaviya, Kurinchanpitiya)
Summary
Nitrogen, which is sometimes called as the most common factor that limits the growth and productivity of plants (Wagner, 2011) is most abundant in dinitrogen (N2) form in the atmosphere. It has been estimated that some 3 x 109 tons of atmospheric N2 is transformed per year globally (Mabrouk and Belhadj, 2012) This amount is not solely biological as 10 % of the total global fixed nitrogen is coming from lightning and chemically fixed nitrogen from industries (Vojvodic et al, 2014). The amount of nitrogen fixed through biological fixation is estimated to be 17.2 x 107 tons per year, which is three times higher than that of industrially fixed. These data demonstrate the importance of biological nitrogen fixation in the agricultural field and for the consistency of the natural N cycle (Wani et al, 1995)
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