| Abstract
| - Leaf growth of many plant species shows rapid changes in response to alterations of the form and the level of N supply. In hydroponically-grown tomato (Lycopersicon esculentum L.), leaf growth was rapidly stimulated by \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} application to \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{{+}}\) end{document} precultured plants, while \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{{+}}\) end{document} supply or complete N deprivation to \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} precultured plants resulted in a rapid inhibition of leaf growth. Just 10 μM \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} supply was sufficient to stimulate leaf growth to the same extent as 2 mM. Furthermore, continuous \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} supply induced an oscillation of leaf growth rate with a 48 h interval. Since changes in \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} levels in the xylem exudate and leaves did not correlate with \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document}-induced alterations of leaf growth rate, additional signals such as phytohormones may be involved. Levels of a known inhibitor of leaf growth, abscisic acid (ABA), did not consistently correspond to leaf growth rates in wild-type plants. Moreover, leaf growth of the ABA-deficient tomato mutant flacca was inhibited by \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{{+}}\) end{document} without an increase in ABA concentration and was stimulated by \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} despite its excessive ethylene production. These findings suggest that neither ABA nor ethylene are directly involved in the effects of N form on leaf growth. However, under all experimental conditions, stimulation of leaf growth by \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} was consistently associated with increased concentration of the physiologically active forms of cytokinins, zeatin and zeatin riboside, in the xylem exudate. This indicates a major role for cytokinins as long-distance signals mediating the shoot response to \batchmode \documentclass[fleqn,10pt,legalpaper]{article} usepackage{amssymb} usepackage{amsfonts} usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{{-}}\) end{document} perception in roots.
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