Regulation of nitrogen uptake by Fagus sylvatica on a whole plant level -: interactions between cytokinins and soluble N compounds

Pedospheric nitrate uptake is a strongly regulated process adapted to the N demand of the whole plant. Pre-requisites for an integrating regulatory system are signal substances communicating the N demand of the shoot to the roots. In the current study it was shown that an additional atmospheric N source results in activation of cytokinins in the shoot as indicated by an increase in the trans - hydroxylation ratio, and in increased shoot-to-root transport of cytokinins in the phloem. Phloem mobility and basipetal transport of cytokinins were also demonstrated by feeding zeatin riboside into the phloem. The resulting enrichment of cytokinins in the roots caused an increased expression of a high-affinity nitrate transporter, the enrichment of amino compounds (Glu, Val, Phe, Lys) in the fine roots and a significant decrease in nitrate net uptake. Significant enrichment in amino acids - accompanied by decreased nitrate net uptake but not by increased expression of the high-affinity nitrate transporter - was also observed when Gln the major long-distance transport form of nitrogen in beech was fed into the phloem. These results provide experimental evidence that, in addition to amino compounds, cytokinins that are known to cycle within the plant are also communicating changes in N metabolism from the shoot to the roots. Apparently increased cytokinin contents in the root can transiently increase nitrate uptake, but this upregulation may be overridden by transcriptional and posttranscriptional down-regulation mediated by amino compounds.