Test for Activity of Plant Growth-Regulators on Grapes

Methods for testing the activity of herbicidal, plant growth-regulating substances-2,4-dichloro-phenoxyacetic acid (2,4-D), 4-(2-methyl-4-chlorophenoxy) butyric acid (4-MCBP), and 3,4-dichloro-phenoxyacetic acid (3,4-D)-on grapes were studied. When drops containing 0.1-5 μg. of different growth-regulators were placed on different apexes of young Tokay shoots, subsequent growth was highly variable within each treatment. 2. The apical 6 inches of Black Monukka shoots were sprayed with 2,4-D in the range of 1-750 p.p.m. A ring of India ink was applied on the shoots 2 inches below the apexes as a marker. After 8 days 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 degree of suppression of the total fresh weight of shoots was judged to be the best criterion for use in this bio-assay. Similar results were obtained with Thompson Seedless. 3. The bio-assay was utilized to determine the relative toxicity of various regulators and to test differences in varietal sensitivity. There was an indication that 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) at 25 p.p.m. was less toxic than 2,4-D or 4-chlorophenoxyacetic acid (4-CPA) when tested on Zinfandel. Thompson Seedless proved to be more sensitive than Delaware to 2,4-D at 25 or 50 p.p.m.