Physical Environment and Symbiotic Nitrogen Fixation II. Root Temperature Effects on the Relative Nitrogen Assimilation Rate

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Abstract
Nitrogen fixation by six varieties of Trifolium subterraneum L.. each inoculated with a number of strains of Rhizobium trifolii, was examined over a range of root temperatures. Significant differences in the rate of nodule establishment and early nitrogen fixation were found between varieties, and between bacterial strains. In order to minimize the effect of such differences, relative nitrogen assimilation rates (RN) and relative growth rates (RW) were used to compare the different legume-bacteria associations. RN was a more satisfactory parameter than RW for comparing the varietal and temperature treatments. This was particularly true for the higher root temperatures (25-30%) which severely reduced nitrogen fixation by three of the eight strains tested, but did not seriously affect the RW of the plants inoculated with these strains. The highest RN values were found with the variety Yarloop grown with a root temperature of 25% (0.142 mg N/mg N/day), and Tallarook at 20C (0-137 mg N/mg N/day). At 8% root temperature, the highest RN values were found with Tallarook (0-098 mg N/mg N/day); the values for most of the symbiotic combinations were considerably lower than this. Despite these generalizations, the presence of strong statistical interactions in the analyses of the data suggest that the nitrogen-fixing ability of the varieties depends on both the root temperature and the strain of nodule bacteria. That this is a general characteristic of T. subterraneum is indicated by its common occurrence on the reasonably wide range of material examined.

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