Senescence is the final stage in organ development, involving a series of physiological and biochemical changes. Generally, it is regarded as a form of cell death, characterised by loss of pigments, lipids, total protein and RNA (Smart, 1994). Programmed senescence, in addition to abscission of existing organs, is used to counter the continuous generation of new organs by plant meristems (Bleecker and Patterson, 1997). It is a highly regulated process in which carbon, nitrogen and other nutrients are mobilised and transported to specific parts of the plant, such as seeds, fruits, roots and young leaves (Weaver et al., 1997). This remobilisation allows recycling of the nutrients to accommodate growth and, ultimately, seed production. Consequently, cells of the vascular system through which nutrients are transported, are the last to senesce (Buchanan-Wollaston, 1997). Senescence of leaves is under strict nuclear control and follows a distinct pattern, in which there is a progressive loss of cellular compartmentalisation.
An early event in cell senescence involves the breakdown of chloroplast membranes, which contain 50% of the protein and 70% of the lipids in leaves. There is also degradation of pigments, particularly chlorophyll, resulting in a yellowing of senescing tissue, together with a progressive loss of other proteins associated with chloroplasts (Bleecker and Patterson, 1997). Such events culminate in a reduction and eventual cessation of photosynthesis. A disproportionate loss of chlorophylls compared to carotenoids accounts for the yellowing of leaves during early senescence (Biswal, 1995). Cytoplasmic volume and the number of cyto-plasmic ribosomes decline, ultimately resulting in a decrease in ribosomal RNA and protein synthesis as the endoplasmic reticulum and polysomes disintegrate. Some organelles, such as mitochondria and nuclei, remain intact until late in senescence (Nooden and Guiamet, 1996).
Since senescence requires energy (Buchanan-Wollaston, 1997), cells must have protective mechanisms to maintain their respiratory and transcriptional machinery during this process. Thus, maintenance of mitochondria allows continued respiration to provide energy (Smart, 1994), while nuclei remain intact in order to permit transcription of genes involved in degradative pathways. Such transcription leads to the recovery of cellular components. Eventually, vacuolar membranes degenerate, releasing proteolytic enzymes into the cytosol. This represents one of the final stages of the degradative processes associated with senescence.
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