Cytokinins are plant hormones that typically block or delay leaf senescence. We profiled 34 different cytokinins/cytokinin metabolites (including precursors, conjugates and degradation products) in leaves of a free-growing mature aspen (Populus tremula) before and after the initiation of autumnal senescence over three consecutive years. The levels and profiles of individual cytokinin species, or classes/groups, varied greatly between years, despite the fact that the onset of autumn senescence was at the same time each year, and senescence was not associated with depletion of either active or total cytokinin levels. Levels of aromatic cytokinins (topolins) were low and changed little over the autumn period. Diurnal variations and weather-dependent variations in cytokinin content were relatively limited. We also followed the expression patterns of all aspen genes implicated as having roles in cytokinin metabolism or signalling, but neither the pattern of regulation of any group of genes nor the expression of any particular gene supported the notion that decreased cytokinin signalling could explain the onset of senescence. Based on the results from this tree, we therefore suggest that cytokinin depletion is unlikely to explain the onset of autumn leaf senescence in aspen.

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany CAS and Faculty of Science Palacký University Šlechtitelů 27 Olomouc CZ 78371 Czech Republic

Umeå Plant Science Centre Department of Forest Genetics and Plant Physiology Swedish University of Agricultural Sciences Umeå 901 83 Sweden

Umeå Plant Science Centre Department of Plant Physiology Umeå University Umeå 901 87 Sweden

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