Bioassay for Testing Activity of Gibberellins on Grape Shoots

¹ Department of Viticulture and Enology University of California, Davis

1. The apical 6 inches of Zinfandel and Thompson Seedless shoots were sprayed with GA₃ from 0.01 to 10,000 ppm. A pin was put through the shoot 2 inches from the apex. About 12 days after treatment the length, fresh and dry weights of stems and leaf blades separately, and total weight of shoots were determined for the growth occurring after spraying. The increase in total fresh weight of the shoot was judged to be the best criterion in this bioassay.

2. GA, and GA₃ at 25 ppm sprayed on shoots of Muscat of Alexandria were found to result in about equal shoot response.

3. To study translocation drops containing 0.1 to 1,000 µg were applied to the base of the upper surface of the first mature leaf from the apex. Measurement of subsequent shoot growth showed that fresh weight of new shoot growth was a good criterion for the amount of gibberellin that reached the shoot tip from the leaf. Results were similar when entire leaves were dipped in solutions ranging from 1 to 10,000 ppm.