Longitudinal Gradients of Indole-3-Acetic Acid and Abscisic Acid in the Hypocotyl of Etiolated Bean Seedlings

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Abstract
The longitudinal distribution of abscisic acid (ABA) and indole-3-acetic acid (IAA) along the hypocotyl of 5-d-old etiolated Phaseolus vulgaris L. cv. Limburg seedlings was measured. IAA was analysed by the L-methyl-indole-α-pryone assay (2-MIP) and ABA by electron capture gas chromatography (ECD-GC). Length and width of the inner parenchyma cells, growth rate and protein content were also measured. Cell expansion occurred predominantly in a region 20 mm below the centre of the hook where elongation rate was maximal and where protein concentration decreased rapidly with distance from the hook. The ratio between ABA and IAA was constant along the length of the hypocotyl. On a fresh weight basis the concentration of both growth substances was maximal in the upper (youngest) part, decreased in slightly older sections where cell expansion was proceeding and was smallest in the basal regions where cell expansion was complete. However, when expressed on a protein basis the concentration gradient of the hormones was the reverse of that described on a fresh weight basis.