Detection and characterization of nitrogen circulation through the sieve tubes and xylem vessels of rice plants

Nitrogen movement through the xylem vessels and sieve tubes in rice plants was studied using xylem and phloem sap analysis in combination with stable and radioactive nitrogen isotope techniques. More than 90% of nitrogen was translocated in the sieve tubes of rice plants as amino acids. When ¹⁵N (99.6 atom%) was applied as a nitrate to the root, ¹⁵N first appeared in phloem sap of the leaf sheath within 10 minutes and increased to 37 atom% excess 5 hours after the experiment had started. In long-term experiments, 63% of nitrogen in the phloem sap of the leaf sheath and 15% in that of the uppermost internode came from nitrogen absorbed within the last 24 hours and 50 hours, respectively. To obtain information about the more rapid circulation of nitrogen in the plant, radioactive ¹³N was used as a tracer. A positron-emitting tracer imaging system was used to show that ¹³N was transferred to the leaf sheath within 8 minutes of its application to the roots. Analysis of the xylem sap of the leaf sheath showed that when the nitrate was applied to the roots, most of the nitrogen in the xylem was transported as a nitrate. These data showed that phloem and xylem sap analysis together with the stable and radioactive nitrogen techniques provide a good method for the detection of nitrogen cycles in plants.