The uptake and flow of C, N and ions between roots and shoots inRicinus communisL. III. Long-distance transport of abscisic acid depending on nitrogen nutrition and salt stress

Seedlings of Ricinus communis L. were cultivated in quartz sand and supplied with media which contained either different concentrations of nitrate or ammonium nitrogen and were treated with a low salt stress. The concentration of ABA was determined in tissues and in xylem and phloem saps. Between 41 and 51 day after sowing, abscisic acid (ABA) flows between roots and shoots were modelled. Long-distance transport of ABA was not stimulated under conditions of nitrate deficiency (0.2 mol m−3). However, when ammonium was given as the only N source (1.0 mol m−3), ABA transport in both xylem and phloem was increased significantly. Mild salt stress (40 mol m−3 NaCl) increased ABA transport in nitrate-fed plants, but not in ammonium-fed plants. The leaf conductance was lowered by salt treatment with both nitrogen sources, but it was always lower in ammonium-fed compared to nitrate-fed plants. A negative correlation of leaf conductance to ABA levels in leaves or flow in xylem was found only in comparison of ammonium-fed to nitrate-fed plants.