Abstract. The phloem sap of Ricinus seedlings was analyzed for cytokinins and the concentration was compared with that in cotyledons and xylem sap. The dominant cytokinin in the phloem sap was isopentenyladenine (70 nM) when the endosperm was attached to the cotyledons; zeatin, dihydrozeatin and cytokinin-ribosides were present at relatively low concentrations (1-2 nM). Removal of the endosperm and incubation of the cotyledons in buffer led to a sharp decrease in the level of isopentenyladenine in the phloem sap, down to the value for zeatin, namely 1-2 nM. Similar low cytokinin concentrations were found in the xylem sap, too, whereas in the cotyledons the cytokinin content was at least 10-fold higher. Incubation of the cotyledons with various cytokinins (isopentenyladenine, zeatin and their ribosides) led to an increase of each of the applied cytokinins in the phloem sap, including also the metabolically closely related cytokinins. Zeatin was especially well loaded. It is concluded that the phloem translocates most free bases and ribosides of the various cytokinin species, if they are offered to the phloem. The data also show that the cytokinin levels in the phloem, which may be far higher than in the xylem, are subject to strong fluctuations depending on the physiological situation.

A b s t r a c tChanges in the patterns of gene expression were analyzed in ultraviolet-B (UV-B; 280-320 nm) -treated seedlings of European beech by differential display reverse transcriptase polymerase chain reaction (DDRT-PCR) of mRNA. Utilizing 12 different downstream primers and 27 degenerated 10-mer upstream primers 50 induced and 29 repressed cDNA bands were detected. Nineteen bands were subcloned and sequenced. Two clones revealed significant homologies to known database sequences: putative nascent polypepetide-associated complex alpha and 60S ribosomal protein L37. However, only the latter was confirmed to be UV-B-induced by northern blot analysis.List of abbreviations: CHS = chalcone synthase, DDRT-PCR = differential display reverse transcriptase polymerase chain reaction, EST = expressed sequence tag, PAL = phenylalanine ammonia-lyase, PPFR = photosynthetic photon fluence rate, UV-B = ultraviolet B, 280-320 nm I n t r o d u c t i o n

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