Long Distance Transport in Macrocystis integrifolia: III. MOVEMENT OF THO

Movement of THO and tritium-labeled photoassimilate was studied in intact fronds and frond cuttings of Macrocystis integrifolia following labeling of a mature blade by tritiated water. Both THO and tritium-labeled assimilate moved from the source blade to sink areas at velocities comparable to those recorded earlier for ¹⁴C- and ³²P-labeled compounds. In intact fronds and frond cuttings, THO and tritium-labeled assimilate showed a declining gradient with increasing distance from the source. In the exudate collected from the basal cut end of the frond, there was a marked increase in radioactivity with time in the photoassimilate, but no such gradient was evident for THO. These results support the idea that, although both tritium-labeled assimilate and THO move in the sieve elements, THO is rapidly exchanged with water in the tissues surrounding the sieve elements. Finally, it is shown that THO is transported to the sink and there “unloaded”; indeed, it can move out of the plant itself. The data on velocity and directionality of transport as well as unloading of THO at the sink are discussed, along with computations on specific mass transfer, and favor the idea that Münch's pressure-flow hypothesis is applicable in Macrocystis for long distance translocation of photoassimilates.