Even Distribution of Endogenous Indole-3-acetic Acid in Phototropism of Pea Epicotyls

Abstract

Summary The curvatures and the differential flank growth rates of etiolated and de-etiolated pea epicotyls were determined in response to various phototropic stimulations. The phototropic curvatures of pea epicotyls were caused only by growth inhibition at the lighted side, except in the first positive curvature of etiolated epicotyls, in which a slight growth promotion was observed at the shaded side next to the growth inhibition at the lighted side. De-etiolation magnified the response of the epicotyls to phototropic stimulation both at the first and second positive curvatures. The distribution of endogenous indole-3-acetic acid (IAA) was determined in relation to phototropism using an HPLC with fluorescence detector. Endogenous IAA was equally distributed over the lighted and shaded sides of peripheral cell layers of the epicotyl, both at the first and second positive curvatures of etiolated and de-etiolated pea seedlings. De-etiolation did not change the even distribution of IAA but induced a decrease in the amount of IAA in the peripheral cell layers. Apparently, as in hypocotyls and coleoptiles, the Cholodny-Went theory, which describes phototropic curvature in terms of a lateral gradient of auxin, does not apply to the phototropism of the pea epicotyl.