Regulation of lettuce hypocotyl elongation by gibberellic acid. Correlation between cell elongation, stress-relaxation properties of the cell wall and wall polysaccharide content

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Abstract
Lettuce hypocotyl elongation caused by gibberellic acid was strongly inhibited by coumarin and dichlobenil, known inhibitors of cellulose biosyndiesis. Stress-relaxation analysis of the cell wall revealed that gibberellic acid induces a decrease in both minimum relaxation time (To) and relaxation rate (b) and an increase in maximum relaxation time (Tm), when gibberellic acid stimulates hypocotyl elongation. Both coumarin and dichlobenil nullified the effect of gibberellic acid on changes in To, Tm and b values. The content of pectic, hemicellulosic and cellulosic substances in the cell wall increased per hypocotyl but decreased per unit hypocotyl length, in response to gibberellic acid treatment. Particularly, gibberellic acid caused a substantial increase in cellulose content per hypocotyl but a decrease per unit length. A good correlation existed between the decrease in To and the decrease in hemicellulose content per unit lengdi of the cell wall. The increase in Tm was correlated with the decrease in cellulose content per unit length of the cell wall. The decrease in b was correlated with the decrease in the content of both cellulose and hemicellulose per unit length. Based on these results, we discuss the role of polysaccharide metabolism of the cell wall in gibberellic acid-induced lettuce hypocotyl elongation and the nature of gibberellic acid-induced biochemical modifications of the cell wall, which are represented by changes in stress-relaxation properties of the cell wall.

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