Translocation of 14C Sucrose in Sugar Beet during Darkness

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Abstract
The time-course of arrival of 14C translocate in a sink leaf was studied in sugar beet (Beta vulgaris L. cultivar "Klein Wanzleben") for up to 480 min of darkness. Following darkening of the source leaf, translocation rapidly declined, reaching a rate approximately 25 % of the light period rate by 150 min. Comparison of data from plants that were girdled 1 cm below the crown with data from ungirdled plants indicates that after about 150 min darkness the beet root becomes a source of translocate to the sink leaf. After about 90 min darkness, starch-like reserve polysaccharide from the source leaf begins to contribute 14C to ethanol soluble pools in that leaf. Because of a 15 % isotope mass effect, sucrose, at isotopic saturation, reaches a specific activity which is about 85 % of the level of the supplied CO2. The source leaf sucrose specific activity remains at the isotopic saturation level for about 150 min of darkness, after which time input from polysaccharide reserves causes the specific activity to drop to about 55 % of that of the supplied CO2. Sucrose specific activity determinations, polysaccharide dissolution measurements, and pulse labeling experiments indicate that following partial depletion of the sucrose pool, source leaf polysaccharide contributes to dark translocation. Respired CO2 from the source leaf appears to be derived from a pool which, unlike sucrose, remains at a uniform specific activity.