Effects of gibberellins on abscission in cotton seedling explants.

Gibberellic acid (GA3) accelerated abscission when applied, in a wide concentration range, to excised abscission zones of cotton. Abscission was promoted equally by distal or proximal applications of from 10(-3) to 100 μg. A slight, but inconsistent, abscission retardation was obtained with distal applications of 10(-6) and 10(-7) μg.Seven different gibberellins accelerated abscission equally when applied distally at amounts of 5×10(-4) to 5×10(-1) μg per abscission zone. At 5×10(-5) μg there were great differences in effectiveness; their activities can be ranked: A3>A5≥A4>A7=A8>A1=A9.The ready translocatability of GA3 was suggested when 1.0 or 0.01 μg was applied to one petiole, and the opposite untreated petiole abscised at the same time as the treated one. However, 0.001 μg was not effective in moving across the stem and inducing abscission of the untreated petiole.The rate of abscission of petioles treated with 1.0 μg GA3 was not affected by increasing the length of the petiole from 3 to 9 mm. However, abscission of petioles treated with smaller amounts is inversely proportional to petiole length.The rate of abscission of petioles treated with GA3 decreased with increasing seedling age; there was a simultaneous increase in abscission rate of the controls.