Translocation of sugar in stressed corn

​Movement of materials within a plant is called translocation.  Minerals are translocated from the roots to leaves through the xylem portion of the plant vascular system. Soluble sugars produced in chloroplasts in leaf cells moves through cytoplasmic connections into the living phloem cells of vascular tissue.  The flow direction is guided by hormones such as cytokinins.  After pollination, much of the translocation of sugar is directed to the kernels. Tracing the direction and timing of flow is often done by using radioactivity such as radioactive N in nitrate uptake in roots (Plant physiology 100(3):1251-8 · December 1992).
 
A study published nearly 40 years ago (Plant Physiol. (1978) 52, 436-439) compared the movement of sugar movement from leaves to developing kernels when the plants were under drought stress. The section of leaf tissue was fed radioactive 1CO2 in plants either stressed by root pruning or restriction of water to the plant.  Radioactive carbon was incorporated in sugar by photosynthesis at about the same rate in stressed and non-stressed plants. This indicated that rate of photosynthesis was not being affected by the drought stress. 
 
Sugars were translocated to the kernels, stalk near the ear node and the ear shank and husk.  Two hours after treatment with the radioactive carbon dioxide, 7.6% of the radioactive carbon remained in the treated leaf area of the non- stressed plants but 20.5% remained in the water stressed leaf tissue.  18.6% of the carbon was moved to the kernels in the non-stressed plants but only 11.0% in the stressed plants.  Similar differences were found for the stalk and ear shank distribution.

This study indicated that the effect of drought stress after pollination was greater on the movement of the sugars from leaf tissue than reduction in photosynthesis. Maintenance of living tissue in stalks by translocation to that tissue during drought stress is probably significant as well.