Experimental work on bidirectional transport was motivated to elucidate the mechanism of assimilate translocation. At a time when “ascending” (water) and “descending” (assimilates) sap streams were still spoken of by most botanists, Curtis (1920a, b) suggested that carbohydrates may be transferred longitudinally through the phloem in either direction. Later Mason et al. (1936) and Phillis and Mason (1936), suggested that carbohydrates and nitrogenous compounds in cotton plants could move through the phloem simultaneously in opposite directions. They advanced the concept of source-to-sink movement of assimilates (Mason and Phillis, 1937), to replace that of a “descending sap stream”. Meanwhile, potassium fluorescein was found to be a valuable tracer for sap movement in sieve tubes (Schumacher, 1933). Palmquist (1938) used it to try to demonstrate simultaneous movement of carbohydrates and fluorescein in opposite directions in the phloem. Experimental times then were still very long (two days in Palmquist’s case), which made interpretation difficult. When radioactive tracers became available as tools for biological research, shorter experiments with two different tracers became possible. Chen (1951) applied 14C and 32P to an upper and lower Pelargonium leaf respectively and found both tracers in bark strips, separated from the xylem, in the stem between the two points of application, and concluded that bidirectional movement had taken place in the phloem. But his experimental times were still 12 to 17 h, ample time for repeated upward movement in xylem and downward movement in phloem.
KeywordsSieve Tube Sieve Element Leaf Trace Single Bundle Secondary Phloem
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