HISTOLOGICAL, HORMONAL AND METABOLIC RESPONSE TRIGGERED BY N-1-NAPHTHYLPHTHALAMIC ACID-INDUCED STEM SWELLING IN Solidago canadensis L.

Abstract

The effect of N-1-naphthylphthalamic acid (NPA, 5.0%, w/w in lanolin) on the growth of Solidago canadensis (Canadian goldenrod) stem was studied, focusing on histological analyses, comprehensive analyses of phytohormones and polar metabolites. NPA substantially induced stem swelling at and above the application site and stimulated vascular cambium activity around the area of its application. The cambial zone in the swelling part of the stem was twice as wide as that treated with lanolin only (control). The proliferation of cambial cells increased xylem production and, consequently, vascular bundle thickness. A significant enlargement of parenchymatous pith cells and an increased diameter of the pith were also observed. Comprehensive phytohormone analyses revealed that NPA increased the content of indole-3-propionic acid, indole-3-acetic acid, and indole-3-acetyl-aspartic acid in the swelling part of the stem, as well as trans-zeatin riboside. These results suggest that NPA-induced stem swelling depends on the dynamics of changes in aux-in and cytokinin metabolites. Furthermore, the contents of monosaccharides (glucose, fructose and galactose) as well as malic, succinic, fumaric acids, cyclitols and quinic acid derivatives increased markedly in the swelling stem. This may indicate that the site of NPA-induced stem swelling is a physiological sink for polar metabolites needed for the growth of this tissue. Thus, it seems that auxins, in interaction with cytokinins, regulate the strength of the sink, controlling the transport of polar metabolites into the swelling part of S. canadensis stem.