Translocation from the Flag Leaf of Winter Wheat in the Field

Abstract

Translocation of assimilate from the flag leaf of winter wheat (Triticum aestivum cv. Maris Huntsman) was studied in the field by monitoring the export of photo-assimilated [^l4C]carbon dioxide with a Geiger-Miiller counter placed under the fed area of leaf. The resulting export curve was analysed as a sum of two exponential terms, and interpreted as a two-pool compartmental system. The rate constant for export from the leaf increased slightly from maximum elongation to anthesis, then declined to almost half its peak value just before the leaf lost all visible chlorophyll. The inter-pool transfer rate constants did not change significantly over the same period, but all rate constants varied with time of day. Short-term changes in the environment of the flag leaf had no discernible influence on translocation in the field. The time constants of the two pools of assimilate agreed with those for other species reported in the literature. These results are consistent with the suggestion that sucrose is stored in the vacuole of mesophyll cells. The variations in rate constants with time of day, and deviations of the export data from the two-pool model, suggest that export and inter-pool transport have saturation kinetics. A model with Michaelis-Menten kinetics was formulated, and simulations of this model showed similar deviations from a simple two-pool system to those seen in our data.