Autumn senescence is a visually spectacular phenomenon in which trees prepare for the oncoming winter. The mechanism for regulation of autumn senescence in trees has been very hard to pinpoint. In this thesis the main focus is to investigate how autumn senescence is regulated in aspens (Populus tremula). Previous work has established that autumn senescence in aspens is under daylight control, in this thesis the metabolic status and the effect on autumn senescence was investigated. The metabolic status was altered by girdling which leads to accumulation of photosynthates in the canopy. This resulted in an earlier onset of senescence but also the speed of senescence was changed. At the onset of senescence the girdled trees also accumulated or retained anthocyanins. The nitrogen status of aspens during autumn senescence was also investigated, we found that high doses of fertilization could significantly delay the onset of senescence. The effects of various nitrogen forms was investigated by delivering organic and inorganic nitrogen through a precision fertilization delivery system that could inject solutes directly into the xylem of the mature aspens. The study showed that addition of nitrate delayed senescence, addition of arginine did not have any effect on the autumn senescence in aspens, and furthermore the nitrate altered the trees leaf metabolism that was more profound in high dosages of supplied nitrate.  Cytokinins are plant hormones believed to delay or block senescence, studies have suggested that the decrease of cytokinins and/or cytokinin signalling may precede senescence in some plants. To investigate how cytokinin regulates autumn senescence in aspens we profiled 34 cytokinin types in a free growing mature aspen. The study begun before autumn senescence was initiated and ended with the shedding of the leaves, and spanned three consecutive years. The study showed that the individual cytokinin profiles varied significantly between the years, this despite that senescence was initiated at the same time each year. Senescence was furthermore not connected to the depletion of either active or total cytokinins levels. The gene pattern of genes known to be associated with cytokinin was also studied, but no gene expression pattern that the profile generated could explain the onset of senescence. These results suggest that the depletion of cytokinins is unlikely to explain the tightly regulated onset of autumn leaf senescence in aspen.

<p>During the development and lifecycle of multicellular organisms, cells have to die, and this occurs by a process called programmed cell death or PCD, which can be separated from necrosis or accidental cell death (Pennell and Lamb, 1997). Senescence is the terminal phase in the development of an organism, organ, tissue or cell, where nutrients are remobilized from the senescing parts of the plant into other parts, and the cells of the senescing organ or tissue undergo PCD if the process is not reversed in time. Leaf senescence involves cessation of photosynthesis, loss of pigments and proteins, nutrient remobilization, and degradation of the plant cells (Smart, 1994). Initiation of leaf senescence is triggered by a wide range of endogenous and environmental factors, that through unknown pathways controls the process, and regulates the expression of senescence-associated genes (SAGs) (Buchanan-Wollaston, 1997). Autumn leaf senescence in deciduous trees is regulated by photoperiod and temperature, and is an attractive experimental system for studies on senescence in perennial plants.</p><p>We have studied the process of autumn senescence in a free-growing aspen (Populus tremula) by following changes in pigment, metabolite and nutrient content, photosynthesis, and cell and organelle integrity. All data were combined in a cellular timetable of autumn senescence in aspen. The senescence process started on September 11 with degradation of pigments and other leaf constituents, and once initiated, progressed steadily without being affected by the environment. Chloroplasts were rapidly degraded, and mitochondria took over energy production after chlorophyll levels had dropped by 50%. At the end of remobilization, around 29th of September, some cells were still metabolically active and had chlorophyll-containing plastids. Over 80% of nitrogen and phosphorus was remobilized, and a sudden change in the 15N of the cellular content on September 29, indicated that volatile compounds may have been released.</p><p>We have also studied gene expression in autumn leaves by analysing EST sequences from two different cDNA libraries, one from autumn leaves of a field-grown aspen and the other from young, but fully expanded leaves of a green-house grown aspen. In the autumn leaf library, ESTs encoding metallothioneins, proteases, stress-related proteins and proteins involved in respiration and breakdown of macromolecules were abundant, while genes coding for photosynthetic proteins were massively downregulated. We have also identified homologues to many known senescence-associated genes in annual plants.</p><p>By using Populus cDNA microarrays, we could follow changes in gene expression during the autumn over four years in the same free-growing aspen tree. We also followed changes in chlorophyll content to monitor the progression of leaf senescence. We observed a major shift in gene expression, occuring at different times the four years, that reflected a metabolic shift from photosynthetic competence to energy generation by mitochondrial respiration. Even though autumn senescence was initiated almost at the same date each year, the transcriptional timetables were different from year to year, especially for 2004, which indicates that there is no strict correlation between the transcriptional and the cellular timetables of leaf senescence.</p>

During the development and lifecycle of multicellular organisms, cells have to die, and this occurs by a process called programmed cell death or PCD, which can be separated from necrosis or accidental cell death (Pennell and Lamb, 1997). Senescence is the terminal phase in the development of an organism, organ, tissue or cell, where nutrients are remobilized from the senescing parts of the plant into other parts, and the cells of the senescing organ or tissue undergo PCD if the process is not reversed in time. Leaf senescence involves cessation of photosynthesis, loss of pigments and proteins, nutrient remobilization, and degradation of the plant cells (Smart, 1994). Initiation of leaf senescence is triggered by a wide range of endogenous and environmental factors, that through unknown pathways controls the process, and regulates the expression of senescence-associated genes (SAGs) (Buchanan-Wollaston, 1997). Autumn leaf senescence in deciduous trees is regulated by photoperiod and temperature, and is an attractive experimental system for studies on senescence in perennial plants. We have studied the process of autumn senescence in a free-growing aspen (Populus tremula) by following changes in pigment, metabolite and nutrient content, photosynthesis, and cell and organelle integrity. All data were combined in a cellular timetable of autumn senescence in aspen. The senescence process started on September 11 with degradation of pigments and other leaf constituents, and once initiated, progressed steadily without being affected by the environment. Chloroplasts were rapidly degraded, and mitochondria took over energy production after chlorophyll levels had dropped by 50%. At the end of remobilization, around 29th of September, some cells were still metabolically active and had chlorophyll-containing plastids. Over 80% of nitrogen and phosphorus was remobilized, and a sudden change in the 15N of the cellular content on September 29, indicated that volatile compounds may have been released. We have also studied gene expression in autumn leaves by analysing EST sequences from two different cDNA libraries, one from autumn leaves of a field-grown aspen and the other from young, but fully expanded leaves of a green-house grown aspen. In the autumn leaf library, ESTs encoding metallothioneins, proteases, stress-related proteins and proteins involved in respiration and breakdown of macromolecules were abundant, while genes coding for photosynthetic proteins were massively downregulated. We have also identified homologues to many known senescence-associated genes in annual plants. By using Populus cDNA microarrays, we could follow changes in gene expression during the autumn over four years in the same free-growing aspen tree. We also followed changes in chlorophyll content to monitor the progression of leaf senescence. We observed a major shift in gene expression, occuring at different times the four years, that reflected a metabolic shift from photosynthetic competence to energy generation by mitochondrial respiration. Even though autumn senescence was initiated almost at the same date each year, the transcriptional timetables were different from year to year, especially for 2004, which indicates that there is no strict correlation between the transcriptional and the cellular timetables of leaf senescence.