Ask Mr. Smarty Plants

For choice of injection site, the main idea is to get the glucose into the vascular system of the plant. Glucose is produced mainly in the leaves where the majority of photosynthesis takes place but is then moved via the plant's vascular system to other parts of the plant where it is needed. The xylem of the roots transports water and minerals up to the rest of the plant, while the phloem moves sugars and minerals throughout the plant. Since the stems are involved in carrying the water and minerals up from the roots and also in moving the products of photosynthesis from their sources (usually the leaves) to the remainder of the plant, they would seem to be the logical site for injection. Additionally, injecting into the roots would be more disruptive to the plant since the roots would have to be uncovered and disturbed each time the plant is injected. You might consider using various positions on the stem as a variable in the experiment, also. For instance, inject the base of the stem where it emerges from the soil on one set of plants and a measured number of inches above the base for another set.

To get an idea about the variations in glucose concentrations according to plant and various parts of plants, you might like to read the following articles:

Rosa, Eduardo; M. David; and M. H. Gomes. "Glucose, fructose and sucrose content in broccoli, white cabbage and Portuguese cabbage grown in early and late seasons" Journal of Science of Food and Agriculture, vol. 81, no. 12, pp.1145-1149. July 18, 2001.

Mahn, K; C. Hoffmann; and B. Maerlaender. Distribution of quality components in different morphological sections of sugar beet (Beta vulgaris L.). European Journal of Agronomy vol. 17, no. 1, pp. 29-39. July, 2002.

You may, or may not, be able to read these online. If not, you can probably get a copy of them via Interlibrary Loan through your local library.