The Effects of Sodium Chloride on Growth and Nitrogen Fixation in Casuarina Obesa Miq.

S ummary In the field, nodulation of Casuarina obesa Miq. was found at sites with salinity levels up to 28 mg Cl − g −1 soil. A subsequent glasshouse study examined effects of step‐wise additions of NaCl on plant growth and symbiotic nitrogen fixation in C. obesa seedlings nodulated by Frankia from two sources. Irrespective of Frankia source, plant growth declined above 0.15 mg NaCl g −1 soil, but some growth still occurred at 15 mg NaCl g −1 soil. Nitrogen fixation also decreased above 0.15 mg NaCl g −1 soil, with total nodule weight being reduced at NaCl levels greater than 0.15 mg NaCl g −1 soil for both Frankia sources. However, increasing NaCl markedly reduced the N fixed mg −1 nodule weight for only one Frankia source: N fixed mg −1 nodule was reduced by 60% at 1.5 mg NaCl g −1 soil and inhibited at 15 mg NaCl g −1 soil. Electron micrographs of the ‘effective’ nodules from the high‐salt treatment showed Frankia infected cells to have low (putative) Cl − levels when compared with adjacent uninfected cells. At levels of 1.5 and 15 mg NaCl g −1 soil both Na + and Cl − accumulated in shoots. Electron micrographs of shoots from the 15 mg NaCl g −1 soil treatment showed Cl − to be concentrated in the intercellular spaces and in the hypodermal cells. The success of plantings of salt tolerant Casuarinaceae such as C. obesa requires the selection of highly salt tolerant plant and Frankia genotypes.