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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

A Cellular Timetable of Autumn Senescence¹

Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, S–901 87 Umea, Sweden

We have studied autumn leaf senescence in a free-growing aspen (Populus tremula) by following changes in pigment, metabolite and nutrient content, photosynthesis, and cell and organelle integrity. The senescence process started on September 11, 2003, apparently initiated solely by the photoperiod, and progressed steadily without any obvious influence of other environmental signals. For example, after this date, senescing leaves accumulated anthocyanins in response to conditions inducing photooxidative stress, but at the beginning of September the leaves did not. Degradation of leaf constituents took place over an 18-d period, and, although the cells in each leaf did not all senesce in parallel, senescence in the tree as a whole was synchronous. Lutein and -carotene were degraded in parallel with chlorophyll, whereas neoxanthin and the xanthophyll cycle pigments were retained longer. Chloroplasts in each cell were rapidly converted to gerontoplasts and many, although not all, cells died. From September 19, when chlorophyll levels had dropped by 50%, mitochondrial respiration provided the energy for nutrient remobilization. Remobilization seemed to stop on September 29, probably due to the cessation of phloem transport, but, up to abscission of the last leaves (over 1 week later), some cells were metabolically active and had chlorophyll-containing gerontoplasts. About 80% of the nitrogen and phosphorus was remobilized, and on September 29 a sudden change occurred in the ¹⁵N of the cellular content, indicating that volatile compounds may have been released.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Johanna Keskitalo (johanna.keskitalo{at}plantphys.umu.se).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.066845.

^* Corresponding author; e-mail johanna.keskitalo{at}plantphys.umu.se; fax 46–786–66–76.

Received June 10, 2005; returned for revision August 26, 2005; accepted September 13, 2005.

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