Long Distance Transport in Macrocystis integrifolia: II. Tracer Experiments with C and P

Discs from mature regions of Macrocystis blades picked up significantly more [(32)P]phosphate from the ambient medium than similar discs from young meristematic regions, and this uptake was higher in light than in darkness. Double-labeling experiments with NaH(14)CO(3) and [(32)P]phosphate, using intact fronds as well as cut frond segments, indicated that (32)P was translocated from mature blades to sink regions at velocities of 25 to 45 centimeters per hour, velocities comparable to (14)C translocation velocity in the same material. There was a slight delay in transport of (32)P which may be due to a delay in loading or to a high metabolism of (32)P in the transporting channels. Histoautoradiography of stipe segments in the translocation pathway indicated that transport of label occurred in the peripheral parts of medulla. An analysis of (32)P-labeled compounds in the fed blade and in the sieve tube sap, collected from basal cut ends of stipes, indicated major differences in labeling patterns. In the blade, a high proportion of (32)P was recovered as inorganic phosphate and relatively small amounts were found in hexose mono- and diphosphates, UDPG and ATP. In the sieve tube sap, however, only a small amount of (32)P was present as inorganic phosphate, a large proportion was found in hexose mono- and diphosphates, and appreciable amounts were present in ATP and UDPG.