Translocation from the Flag Leaf of Winter Wheat in the Field

Translocation of assimilate from the flag leaf of winter wheat(Triticum aestivum cv. Maris Huntsman) was studied in the fieldby monitoring the export of photo-assimilated [^l4C]carbon dioxidewith a Geiger-Miiller counter placed under the fed area of leaf.The resulting export curve was analysed as a sum of two exponentialterms, and interpreted as a two-pool compartmental system. Therate constant for export from the leaf increased slightly frommaximum elongation to anthesis, then declined to almost halfits peak value just before the leaf lost all visible chlorophyll.The inter-pool transfer rate constants did not change significantlyover the same period, but all rate constants varied with timeof day. Short-term changes in the environment of the flag leaf had nodiscernible influence on translocation in the field. The timeconstants of the two pools of assimilate agreed with those forother species reported in the literature. These results areconsistent with the suggestion that sucrose is stored in thevacuole of mesophyll cells. The variations in rate constants with time of day, and deviationsof the export data from the two-pool model, suggest that exportand inter-pool transport have saturation kinetics. A model withMichaelis-Menten kinetics was formulated, and simulations ofthis model showed similar deviations from a simple two-poolsystem to those seen in our data.