Journal of Experimental Botany, Vol. 51, No. 343, pp. 227-237,
February 2000
© 2000 Oxford University Press
Rapid effects of nitrogen form on leaf morphogenesis in tobacco
1 Institut für Pflanzenernährung, Universität Hohenheim, D-70593 Stuttgart, Germany
2 Institut für Obst-, Gemüse- und Weinbau, Universität Hohenheim, D-70593 Stuttgart, Germany
3 Abteilung Agrarökologie, Fachgruppe Geowissenschaften der Universität Bayreuth, D-95440 Bayreuth, Germany
Ammonium (NH+4) instead of nitrate (NO-3) as the nitrogen (N) source for tobacco (Nicotiana tabacum L.) cultivated in a pH-buffered nutrient solution resulted in decreased shoot and root biomass. Reduction of shoot fresh weight was mainly related to inhibition of leaf growth, which was already detectable after short-term NH+4 treatments of 24 h, and even at a moderate concentration level of 2 mM. Microscopic analysis of the epidermis of fully expanded leaves revealed a decrease in cell number (50%) and in cell size (30%) indicating that both cell division and cell elongation were affected by NH+4 application. Changes in various physiological parameters known to be associated with NH+4-induced growth depression were examined both in long-term and short-term experiments: the concentrations of total N, soluble sugars and starch as well as the osmotic potential, the apparent hydraulic conductivity and the rate of water uptake were not reduced by NH+4 treatments (duration 1–12 d), suggesting that leaf growth was neither limited by the availability of N and carbohydrates, nor by a lack of osmotica or water supply. Although the concentration of K+ in leaf press sap declined in expanding leaves by approximately 15% in response to NH+4 nutrition, limitation of mineral nutrients seems to be unlikely in view of the fast response of leaf growth at 24 h after the start of the NH+4 treatment. No inhibitory effects were observed when NH+4 and NO-3 were applied simultaneously (each 1 mM) resulting in a NO-3/NH+4 net uptake ratio of 6 : 4. These findings suggest that the rapid inhibition of leaf growth was not primarily related to NH+4 toxicity, but to the lack of NO-3-supply. Growth inhibition of plants fed solely with NH+4 was associated with a 60% reduction of the zeatine+zeatine riboside (Z+ZR) cytokinin fraction in the xylem sap after 24 h. Furthermore Z+ZR levels declined to almost zero within the next 4 d after start of the NH+4 treatment. In contrast, the concentrations of the putative Z+ZR precursors isopentenyl-adenine and isopentenyl-adenosine (i-Ade+i-Ado) were not affected by NH+4 application. Since cytokinins are involved in the regulation of both cell division and cell elongation, it seems likely that the presence of NO-3 is required to maintain biosynthesis and/or root to shoot transfer of cytokinins at a level that is sufficient to mediate normal leaf morphogenesis.
Key words: Ammonium, nitrate, leaf growth, cytokinin.
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