The Uptake of Growth Substances

Abstract

The absorption by Lemna minor of 2 : 3 : 5-triiodobenzoic acid (TIBA), labelled in the 2 position with iodine-131, has been investigated. Under constant conditions of light (300 foot candles) and temperature (25°C.) the course of uptake by the whole plant is dependent on the concentration of TIBA in the culture solution (pH 5.I). Up to circa 1 mg./l. a high rate of uptake in the first 30 minutes is followed by a slower but steady accumulation during the next 4.5 hours. When the concentration is increased to 5 mg./1. uptake, at first rapid, falls off progressively so that after 1-2 hours the content of the tissues starts to decrease because TIBA moves out in to the external solution faster than it enters. The magnitude of this reversal from a positive to a negative rate of uptake,.partitularly for the roots, is even more pronounced at 10 mg/l. When plants, first placed in labelled solutions for 2 to 2.5 hours are transferred to culture solution the loss of the labelled compound to the external solution, especially from the roots, is very rapid : the decrease in root content may reach 90 per cent. in the first 80 minutes. The initial rate of egress is little affected either by the hydrogen ion concentration or the presence of unlabelled TIBA in the external solution. Between 5, 15, and 25° C. the temperature quotients range from 1.29 to 1.70 for the root and 1.66-1.37 for the frond. Comparable experiments on the initial phase of uptake during the first 2o to 30 minutes demonstrate that (a) the rates are greatly dependent on the external pH i.e. uptake is largely in the molecular form and (b) between 5 and 15°C. the rates are not temperature dependent but between 15 and 25°C. they are: the quotients for roots and frond are 2.6 and 2.0. Comparisons of the course of uptake of normal plants and plants previously frozen in liquid air showed that at both I° and 25°C. 'pre-frozen' fronds continue to accumulate TIBA and may contain at the end of 5 hours 8 times as much as control fronds. In contrast, the uptake by the pre-frozen roots is depressed. The addition of high (30-50 mg./l.) but not low (10 mg./1.) concentrations of indole-3-acetic acid (IAA) to the external solution also causes a striking accumulation of TIBA in the frond but uptake by the roots is not changed. Lastly, the presence of TIBA in the external solution while not affecting the uptake of cerium-144. in the first 2 hours subsequently depresses uptake by the root but not by the frond. These findings are compared with the similar pattern of uptake for z,4-dichlorophenoxyacetic acid described in the previous paper. The nature of the processes controlling the inward and outward flow of TIBA, their location within the cell and their bearing on the physiological actions of auxins are discussed.