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The leaf is the main photosynthetic organ of plants and its development a complex process governed by a combination of environmental factors and intrinsic and genetically regulated signals (Van Lijsebettens & Clarke, 1998). Usually, leaf ontogeny includes an early phase of increasing photosynthetic rates while the leaf is actively expanding, a mature phase where such rates peak and a senescence phase where they decline (Gepstein, 1988; Miller et al., 2000). During early development, the leaf is a sink receiving nutrients from the rest of the plant; however, as soon as it reaches full photosynthetic capacity, it becomes the main source organ of the plant. After this productive period, the leaf enters the senescence phase, during which most compounds present in it are removed and reused (Hortensteiner & Feller, 2002; Buchanan-Wollaston et al., 2003a). Leaf senescence, which is last stage in leaf development, is a highly regulated and programmed degeneration process governed by a variety of developmental and environmental signals (Lim et al., 2003). This important phase in the leaf lifespan period may last as long as leaf maturation and involves a shift from nutrient assimilation to nutrient remobilization and recycling (Guiboileau et al., 2010). In senescent leaf metabolism, carbon and nitrogen assimilation are replaced by catabolism of chlorophyll and macromolecules such as proteins, RNA and membrane lipids, the degradation of which marks the senescence phase. Unsurprisingly, senescence alters the expression of many genes. These senescence-associated genes include regulatory genes encoding transcription factors; genes involved in degradative processes that code for hydrolytic enzymes such as proteases, lipases and ribonucleases; and genes with secondary functions in senescence that code for proteins involved in nutrient remobilization (e.g. glutamine synthetase, which catalyses the conversion of ammonium into glutamine to enable nitrogen recycling in senescing cells) (Taiz & Zeiger, 2010). Environmental cues such as day length and temperature, and various biotic and abiotic sources of stress, can also affect the initiation and progress of such a high complex as leaf senescence.

Metabolic Regulation of Leaf Senescence in Sunflower (Helianthus annuus L.) Plants