Journal of Experimental Botany, Vol. 51, No. 343, pp. 287-297,
February 2000
© 2000 Oxford University Press
Effects of symbiosis with Frankia and arbuscular mycorrhizal fungus on the natural abundance of 15N in four species of Casuarina
1 Plant Science Group,Division of Biochemistry and Molecular Biology, Bower Building, Glasgow University, Glasgow G12 8QQ, UK
2 Forest College and Research Institute, TamilNadu Agricultural University, Mettupalayam 641 301, India
3 Department of Chemistry, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
4 School of Applied Sciences, University of Glamorgan, Pontypridd, Mid-Glamorgan CF37 1DL, UK
5 Department of Cellular and Environmental Physiology, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
The effect of interactions between Casuarina species, Frankia strains and AMF on nitrogen isotope fractionation within the plant were determined under conditions where changes in source nitrogen were minimized by growing plants in mineral nitrogen-deficient conditions and without added organic N. Casuarina cunninghamiana, C. equisetifolia, C. glauca, and C. junghuniana were inoculated singly with three Frankia strains or were dual inoculated with Frankia and Glomus fasciculatum. The %N and 
15N of separated parts of plants inoculated with the three Frankia strains or with Frankia+Glomus were not significantly different within Casuarina species. However, the slow-growing C. junghuniana differed in several variables from the other three species. There was a highly significant, linear relationship between the natural logarithms of cladode N content and 15N of plants of the four Casuarina species when inoculated with Frankia or with Frankia+Glomus, showing that nitrogen supply and the correlated variable, plant growth rate, were major determinants of 15N. Provision of small quantities of (NH4)2SO4 or KNO3 increased several-fold the growth of three of the Casuarinaspecies when inoculated with Frankia alone or with Frankia+Glomus. Within species, mycorrhizal and non-mycorrhizal plants receiving supplementary soluble phosphate were of similar dry weights at harvest. 15N values for cladodes of C. cunninghamiana, C. equisetifolia and C. glauca were similar, but values for the poor growing C. junghuniana were more variable and, with the exception of plants receiving KNO3, were lower than those of the other three species. Reduced growth due to suboptimal availability of N or P had a major influence on 15N and, in these conditions where plants could not access significant amounts of organic N, outweighed any effects on cladode 15N of colonization by Glomus. 15N values of nodules were higher than other parts of Frankia or Frankia+Glomus inoculated Casuarinas, conceivably due to retention in nodules of fixed N, with 15N close to zero.
Key words: Arbuscular mycorrhizas, Casuarina, Frankia, nitrogen fixation, nodules