Sellstedt A, Atkins CA
Composition of Amino-Compounds Transported in Xylem of Casuarina Sp
Journal of Experimental Botany: 1991 42:1493-1497
Seedlings (180-d-old) of Casuarina cunninghamiana L., C. equisetifolia Miq. and C. glauca Sieber inoculated with each of two different sources of Frankia, were analysed for translocated nitrogenous compounds in xylem sap. Analyses were also made on sap from nodulated and non-nodulated plants of C. glauca grown with or without a range of levels of combined nitrogen. Xylem exudates were collected from stems, roots, and individual nodules of nodulated plants and from stems and roots of non-nodulated plants. While the proportional composition of solutes varied, the same range of amino compounds was found in xylem sap from the three different symbioses. In C. glauca asparagine was the major amino acid in the root sap followed by proline, while in symbiotic C. cunninghamiana arginine accounted for more than 25% of the amino compounds. Citrulline was the major translocated product found in the stem exudate of symbiotic C. equisetifolia. Increasing concentrations of ammonium nitrate in the nutrient solution resulted in increasing levels of free ammonia and glutamine in xylem sap from stems of nodulated and non-nodulated C. glauca, but there was relatively little change in the prominent solutes, e.g. citrulline, proline, and arginine. The composition of nitrogenous solutes in stem or root exudates of C. glauca was similar to that of exudate collected from individual nodules and on this basis it was not possible to distinguish specific products of current N2 fixation in xylem. The main differences in N solute composition between the symbioses were apparently due to host plant effects rather than nodulation or the levels of combined N. Also, the data indicate that the use of the proportion of N in sap as citrulline (or indeed any other orgnaic N solute) could not be used as an index of nitrogen fixation.
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