Sink Strength In Sugar Beet

Parameters determining carbon partitioning and influences of environmental factors are discussed elsewhere (40, and literature cited therein). In general, sink strength may be defined as the competitive ability of an organ to import photoassimilates (40) with sink strength being a product of sink size and sink activity. Sink size could be limited by the genetic determination of cell number (139), whereas sink activity is influenced by important physiological features. These are phloem unloading of photoassimilates and all the possible events following up to their retrieval by sink cells, respiration, build up of cell constituents such as cell walls, storage and others. The emphasis of certain molecular determinants for sink strength may be different according to the type of sink, "utilisation sink", i.e. meristems, growing roots, developing leaves or fruits or "storage sink", i.e. tubers, taproots, petioles, seeds, fruits etc. within the "storage phase". Due to their specific design, characterised by anomalous growth as a result of the formation of supernumerary concentric cambia, beetroots represent a combination of utilisation and storage sinks. The oldest leaves of a Beta plant still carrying all the leaves, are localised at the periphery of the rosette, the youngest ones in the centre. The old source leaves supply predominantly the core region, whereas the youngest source leaves supply first of all the outer regions of the beet root with photoassimilates and nitrogen (140). The phloem parenchyma and meristematic cells of the supernumerary cambia are served first because of their vicinity to the phloem elements, the large "storage parenchyma cells" between the cambia are served relatively late, the core region of a beet root is served latest. However, since the core region is supplied over the longest time period, the sugar content is highest in this region (28, and literature cited therein). Consequently, increase in cell number, cell growth and accumulation of sucrose are the minimum stages characterising the beet taproot as a sink.

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