INFLUENCE OF CHEMICAL FORM AND CONCENTRATION OF NITROGEN ON APOPLASTIC pH OF LEAVES

The objective of this study was to investigate the effects of various forms of nitrogen (NO− 3, NH+ 4) supplied to the roots via a nutrient solution on the apoplastic pH in intact leaves determined by fluorescence ratio imaging. In contrast to NH+ 4, higher apoplastic pH values in leaves of Phaseolus vulgaris and Helianthus annuus were measured with NO− 3 nutrition. In this context no significant differences were found in leaves of Vicia faba and Zea mays supplied with the various forms of N. Comparative studies on apoplastic pH in leaves of Vicia faba, Zea mays and Helianthus annuus demonstrated that NO− 3 reductase activity in roots was responsible for the differences in NO− 3 concentration and pH in the leaf apoplast. Light-induced pH changes in the leaf apoplast also occur and may overlap the effects of various forms of N. Increasing concentrations of NO− 3 supply to the roots did not significantly affect apoplastic pH in leaves of Helianthus annuus. Depletion of NO− 3 in the nutrient solution led to lower apoplastic pH in leaves of Zea mays. Leaf fertilization with NH+ 4 led to a decline in apoplastic pH of leaves whereas NH3 gas exposure caused a biphasic response in apoplastic pH.