Gibberellic Acid Stimulation of Cucumber Hypocotyl Elongation

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Abstract
Recently developed techniques have been used to reinvestigate the mechanism by which gibberellic acid (GA3) stimulates elongation of light-grown cucumber (Cucumis sativus L.) seedlings. Osmotic pressure and turgor pressure were slightly reduced in GA3-treated seedlings, which elongated 3.5 times faster than control seedlings. This indicated that GA3 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 GA3-treated seedlings increased by up to 35% above the control values. Stress-relaxation measurements on frozen-thawed tissue showed that T0 the minimum relaxation time, was reduced following application of GA3. 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 GA3 affected both the mechanical (viscoelastic) and biochemical (chemorheological) properties of the cell walls of light-grown cucumber. The previous hypothesis, that GA3 stimulates cucumber hypocotyl growth by increasing osmotic pressure and cell turgor, is contradicted by our results.