Despite the central role of sinks in determining photosynthetic assimilate partitioning in plant growth, very little is known about the control of assimilate import into a sink tissue. The overal goal of this research is to understand how import by a sink is controlled. Because the developing wheat grain offers significant experimental advantages, Dr. Fisher's work has focused on that system. However, findings from this research have important implications for all sinks in which unloading of the sieve element/companion cell complex follows a symplastic pathway. All of the transport steps, from within the sieve tubes to the endosperm cavity, are passive, reversible and relatively nonspecific. Assimilate movement out of the grain phloem into surrounding parenchyma cells is accompanied by the largest turgor and concentration differences over any step of the source-to-sink pathway. Because solute movement out of the sieve tubes occurs via pressure-driven flow, while grain growth rate is constant, flow rate must be inversely related to the pressure and concentration gradients across this part of the pathway. This, and the high resistance involved, implicate this step as an important control point for assimilate import into the grain. This research will focus more closely on this step of assimilate import. Important questions remain to be addressed in developing wheat grains, and they will continue to be the main object of the work. However, to test whether import might be controlled at this step in other sinks, some work with growing root tips will also be initiated. The main questions to be addressed in growing roots are whether sieve tube unloadiong is reversible, the pressure and concentraiton gradients involved, and their relationship to root growth rate.
