A Kinetic and Microautoradiographic Analysis of [14C]Sucrose Import by Developing Wheat Grains

Assimilates enter developing wheat grains via a strand of phloem extending along the crease region of the grain. After phloem unloading, they move several hundred micrometers before being released into the endosperm cavity, from which they are absorbed by the developing endosperm. Extraphloem assimilate pools in the maternal tissue of the crease, therefore, play a central role in post-phloem transport. We investigated the location and turnover of 14C-assimilates in the crease tissues and endosperm cavity sap by pulse labeling the flag leaf with 14CO2. Sucrose accounted for >90% of 14C at all times. Kinetic analysis of the crease sucrose pool and its depletion in excised grains showed that virtually the entire sucrose content of the crease tissues was involved in post-phloem transport and behaved basically as a single well-mixed compartment. Microautoradiographs also showed rapid movement of 14C throughout most of the crease tissues. Quantification of 14C concentration in the tissues showed a relatively shallow gradient of 14C and, presumably, of sucrose through the nucellus and chalaza. The steepest gradient in 14C content occurred in the vascular parenchyma between the chalaza and conducting cells (xylem and phloem).