The Short-term Growth Stimulation Induced by External Supply of IAA in Internodes of Intact Pea Seedlings

Abstract

Indole-3-acetic acid (IAA) mixed with lanolin was applied to a defined region of the top elongating internode (third internode, initially 21–24 mm long) of intact pea (Pisum sativum L. cv. Alaska) seedlings, and the effects of applied IAA on growth of internode zones were investigated. Experiments were conducted with plants grown in red light (R), with supplementary use of plants grown in white light (W). Growth of the internode was first characterised without IAA application. Internode zones elongated at an identical and constant rate per unit length until each zone sharply ceased elongation at a certain distance from the apex. The elongation rate was similar even between R-grown and W-grown plants, the major difference being an earlier cessation of zone elongation in W-grown plants. Following IAA application, the growth rate of zones increased (maximally about two times) but, after a few hours, it declined to the pre-stimulation rate. This short-term growth stimulation (SGS) was observed when IAA was applied directly to the zone monitored for growth and when IAA was applied to an apically separated region. It was demonstrated that SGS was caused not because radial growth was enhanced in place of elongation, nor because energy became limiting. Based on the kinetic differences observed by applying IAA to different regions of the internode and also on the data obtained by applying IAA again at the post-stimulation phase, two independent regulatory elements could be resolved for SGS: (1) reduction of the tissue responsiveness for IAA, and (2) reduction of IAA supply to the target tissue. These regulations are considered to operate in response to increases in the tissue content of IAA.