The initiation of leaf senescence is regulated by various internal and environmental factors. Such environmental cues include day length, extremes of temperature, drought, water logging, nutrient deficiency and infection by pathogens (Smart, 1994). Leaf senescence allows reallocation of resources to reproductive organs to ensure that plants complete their life cycles, even under conditions of stress. Internal factors can also induce senescence, including leaf age, reproductive development and the concentrations of phytohormones. Consequently, senescence occurs even in the absence of environmental stress (Gan and Amasino, 1997). A range of phytohormones have been suggested to have a possible role in the initiation of leaf senescence, including auxins, gibberellins, ethylene, abscisic acid and cytokinins (Smart, 1994). Indeed, cytokinins have been implicated for some time in leaf senescence in many species (Nooden and Leopold, 1978; Nooden, 1980), with depletion of cytokinins in these organs being thought to trigger the cascade of events constituting this process.
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