Cytokinins in the perennial herb Urtica dioica L. as influenced by its nitrogen status

Abstract

The effect of nitrogen on the cytokinin relations of Urtica dioica, the stinging nettle, has been investigated. The plants were grown in quartz sand and nutrient solutions providing levels of nitrate ranging from 1 to 22 mM. Nitrogen supply did not affect biomass production within the range of 3–15 mM NO 3 - . However, the shoot: root ratio of biomass was significantly higher at 15 mM (standard plants) than at 3 mM (low-nitrogen plants) nitrate supply. The cytokinin patterns of the roots, stems and adult, as well as meristematic leaves of plants grown at these two levels of nitrate supply, were determined by means of high-performance liquid chromatography (HPLC) and immunoassays. Enzyme-linked immunosorbent assays (ELISAs) for zeatin riboside, dihydrozeatin riboside, isopentenyladenosine, benzyladenosine and o-hydroxybenzyladenosine enabled the quantification of 17 cytokinins, 13 of which were found in the various tissues of Urtica. trans-Zeatin and its conjugates were the predominant cytokinins in all examined samples. While the free base trans-zeatin and its O-glucoside were the major cytokinins in adult leaves, trans-zeatin riboside was prominent in the other tissues of at least the standard plants. Glucosides of the trans-zeatin type cytokinins were present only in lower amounts. However, considerable amounts of a compound, tentatively identified as cis-zeatin riboside-O-glucoside, were found, particularly in roots and meristematic leaves. Comparatively high amounts of trans-zeatin nucleotide as well as isopentenyladenosine phosphate were also demonstrated in these tissues. Analysis of the root-pressure exudates similarly showed trans-zeatin riboside and, at a lower concentration, trans-zeatin to be the only substantial components. In the low-nitrogen plants, shortage of nitrogen was manifest only in the roots; the nitrogen contents of the shoots did not respond to the nitrogen supply. Likewise, the total content of cytokinins in the shoots of the low-nitrogen plants equaled that of the standard-plant shoots, while it was lower by about 25% in the roots of the low-nitrogen plants. In the latter, the amounts of cytokinins exuded via the root-pressure fluid were also approximately 25% lower. Since the levels of only the trans-zeatin cytokinins in the roots showed a linear correlation with the shoot-to-root ratios, these cytokinins may play an important role in biomass partitioning in Urtica dioica.