NITRATE CONTENT AND AMINO-ACID COMPOSITION OF THE XYLEM FLUID OF SPRING WHEAT THROUGHOUT THE GROWING-SEASON

The xylem exudate from the cut stem bases of spring wheat plants grown in a glasshouse were collected at periods during the growing season. The rates of xylem exudation and the concentration of nitrate and various amino acids in the exudate were measured from plants grown with four levels of N and four levels of K. Rates of exudation were linear over 2‐3 h after excision of the stems and were generally increased by N treatment. At most harvests the amount of nitrate‐N in the exudate was less than the total amino acid‐N; although both were increased by increased N nutrition, the effect on the former was greater. Glutamic and aspartic acids were the major components measured when the amino acids were analysed by gas‐liquid chromatography (g.l.c.); however, these were probably present in the exudate as their respective amides. Comparison of the amino acid composition of the xylem exudate from the cut shoot base, with the xylem contents of the final node subtending the ear and of the final seed protein suggested that the majority of the amino acids were either transported to the ear in the phloem or made there from asparagine and glutamine. Copyright © 1979 John Wiley & Sons, Ltd