Translocation of C14-Labeled Assimilates in Squash Petioles

A qualitative analysis of sieve tube sap was undertaken to help determine the method of assimilate movement in phloem. Hydroponically-grown squash plants (Cucurbita melopepo torticollis Bailey) were trimmed to a mature, fully-expanded leaf (the source) and one or more immature, unexpanded leaves (the sinks). The blade of the source leaf was exposed to C14O2 in air for 10 minutes and allowed to translocate for 20 or 50 minutes in ambient air. Petioles of treated leaves were cut into sections and ground in cold buffer with a chilled mortar and pestle to extract materials from the translocation stream. The brei was subjected to differential centrifugations at 121, 3020, 6780, 12100, and 27000 times gravity (g) for 10 minutes. Duplicate 2-ml samples were taken from the supernatant after each centrifugation. The amount of C14 present in each was determined by liquid scintillation counting. Background radiation was determined by analyzing 2-ml samples of buffer. Sugars were extracted from leaf blade and petiole tissue with hot 80% ethanol in a micro-Soxhlet apparatus. Individual sugars were separated by descending paper chromatography. Radioactivity in the several sugars was determined by cutting the chromatograms into 1-cm sections from the origin to the solvent front and eluting each section for 3 hours with distilled water into scintillation vials. C14 was measured as before. Background radiation was determined by elution of chromatographed and unchromatographed paper with distilled water. Because of short translocation times, all C14 detected was assumed to be in the translocation stream. In every case, the amount of C14 was similar in all samples taken from differential centrifugations. These results indicate the presence of the translocate in the slouble portion of the sieve tube sap as opposed to an association with the cell particulate or insoluble portion. C14 in individual sugars was located mainly in the sucrose and stachyose fractions which are known to be translocated in squash. It is concluded that the sugars in squash phloem are found dissolved in the sap of the sieve tubes and not associated with any kind of particulate matter therein, unless the particles were much smaller than mitochondria. These findings would tend to support a mass flow system of assimilate movement rather than one governed by protoplasmic or transcellular streaming