Gibberellic Acid Stimulation of Cucumber Hypocotyl Elongation

Recently developed techniques have been used to reinvestigate the mechanism by which gibberellic acid (GA(3)) stimulates elongation of light-grown cucumber (Cucumis sativus L.) seedlings. Osmotic pressure and turgor pressure were slightly reduced in GA(3)-treated seedlings, which elongated 3.5 times faster than control seedlings. This indicated that GA(3) enhancement of growth was not controlled by changes in the osmotic properties of the tissues. Stress/strain (Instron) analysis revealed that plastic extension of the cell walls of GA(3)-treated seedlings increased by up to 35% above the control values. Stress-relaxation measurements on frozen-thawed tissue showed that T(0) the minimum relaxation time, was reduced following application of GA(3). In vivo wall relaxation (measured by the pressure block technique) showed that the wall yield coefficient was increased, and the yield threshold was slightly reduced. Thus GA(3) affected both the mechanical (viscoelastic) and biochemical (chemorheological) properties of the cell walls of light-grown cucumber. The previous hypothesis, that GA(3) stimulates cucumber hypocotyl growth by increasing osmotic pressure and cell turgor, is contradicted by our results.