Plant senescence

The senescence (from the Latin senescere = " getting old, aging ") is a genetically controlled and energy-dependent aging process in plants. To what extent this definition applies also for aging in humans and animals, is a subject of research in biogerontology.

Senescence affects the whole plant, individual organs, such as flowers, fruits and leaves or individual cells, such as trichomes, tracheids and vessel elements. At least in leaf senescence is a reversible process. Thus, the removal of the shoot leads to Wiederergrünung the lower leaves. For the beans has been shown that the truncation of flowers and pods for delaying leaf senescence leads. The Blütenseneszenz is delayed by removal of young flower buds. The nutrients from senescent tissues are transported in non- senescent tissues of shoot axis or reproductive organs ( flowers, seeds and fruits).

Proteins in the course of senescence to maintain the reduced synthesized photosynthesis, whereas other proteins are strongly expressed. These include

• Hydrolytic enzymes: Degradation of proteins, nucleic acids and lipids
• Glutamine synthetase Synthesis of Glutamine
• The asparagine Synthesis of asparagine
• Enzymes for the synthesis of ethene: acceleration of fruit ripening and senescence; Abscission ( separation) of leaves, flowers and fruits

While lipids are converted via the glyoxylate cycle and gluconeogenesis in sucrose, leads to the degradation of nucleic acids and proteins in the synthesis of glutamine and asparagine. These amino acids are transported as sucrose from senescent tissues and can be reused by other parts of plants.

The senescence and abscission of leaves are regulated by plant hormones: ethylene accelerated senescence, cytokinins inhibit it ( Antiseneszenz ). Trigger of senescence could be reduced photosynthetic activity. The process begins with the reversible conversion of chloroplasts to Gerontoplasten, which are characterized inter alia by reduced chlorophyll content and lack of mitotic activity, while the nuclei are destroyed at the end of senescence. The final products of chlorophyll breakdown are stored in the vacuole, the cell sap with proteolytic enzymes to release later in the cytoplasm.

After the breakdown of chlorophyll, the carotenoids give further preserved autumn leaves a yellow to orange color that was previously masked by the green of chlorophyll. Some deciduous tree species synthesize especially on cold sunny autumn days anthocyanins, which cause a red leaf coloration. Anthocyanins provide this protection for the cells against too high light intensities and thus against oxidative stress. This prevents the recovery of nutrients from senescent leaves ends prematurely. As part of the leaf abscission at the end of the senescence phase, the reduction of auxin synthesis in the leaf blade causes an increased ethylene production. Ethene induced subsequently in the small thin-walled parenchyma cells of the petiole Abszissionszone the production and release of enzymes that destroy the cell walls and thus bring about the separation of the leaves.