A general method for determining translocation velocity in Laminariales

The nondestructive pulse method for studying 14C-labelled photoassimilate translocation in Alaria esculenta has been adapted for species of Laminariales with flat, thick blades, in this case, Laminaria digitata. A 1-h pulse of [14C]bicarbonate(25 μCi) was applied to one side of the blade surface via a 12 mm diameter by 22 mm high, closed cylindrical incubation chamber. The upper 20-mm closed portion of the chamber was removed after the incubation period, the lower 2-mm open portion remained glued to the blade surface during the subsequent translocation period. A Geiger-Müller (G-M) detector probe with a 50 mm diameter end-window was used to measure the disappearance of radioactive organic matter from the pulsed region over the next 11–12 days. Accurate monitoring of the movement of 14C-labelled solutes through the cortex and into the medulla was confounded by changing absorption of radioactivity by the cortex. Uniform absorption was achieved once the translocatable radioactivity reached the medullary conducting cells (sieve filaments). Thus arrival and accumulation of 14C-labelled assimilate in the blade sink (meristem) was reliably measured with the G-M probe. For Laminaria digitata, the translocation velocity of the moving solute front was 1.7 cm∙h−1. The specific mass transfer of carbon was estimated at 0.4 mg C∙week−1∙mm−2 cross-sectioned medulla. Fifty-seven percent of assimilated carbon was exported in 10 days; 97% of the 14C remaining in the source was in insoluble matter and 3% was in soluble matter. Cutting sieve filaments on the sink side of the incubation chamber did not stop short distance transport through the cortex, but significant 14C-labelled photoassimilate was apparently unable to be re-routed around the cut as no radioactivity was detected in the sink area, the meristematic region at the base of the blade.