Electric properties of oat coleoptiles and auxin transport in them

Measurements of electric potential differences between points on the surface of etiolated Avena coleoptiles have been made with a reliable, new technique. These show that an electric wave moves down an intact coleoptile when its apex is illuminated, or down a decapitated coleoptile when indole acetic acid (IAA) is applied to its cut top. It is postulated that auxin-requiring electric oscillators in the coleoptile are the source of the wave and that they are synchron ised by a sudden change in the stream of auxin moving down from the top. It is shown that the electric wave provides a useful technique for studying auxin transport under altered environmental or metabolic conditions. Carbon 14 tracer studies confirm the suggestion, arising from the electric measurements, that IAA, moving from a source on the top of a decapitated coleoptile, possesses a definite front moving at constant speed. This speed depends on the ambient temperature. Graphs have been obtained relating concentration of mobile and immobilised 14C-IAA to both distance and time after application of the IAA at several concentrations to the top of the decapitated coleoptile. At the lowest concentrations of IAA applied the graphs of concentration against distance are linear from near the top of the coleoptile to the front of the IAA stream. Linearity exists in the region of the f'ront even with concentrations above 0.4 mgm/litre applied. Further experiments show that there is much 14c-IAA in the coleoptile that is mobile but is not actually moving with the speed of the front of the stream, so that the very concept of a \stream\" to describe IAA transport must be questioned. Effects of IAA application for a short time are investigated as is the effect of tri-iodobenzoic acid on IAA transport. Design construction and operation of an inexpensive reliable low-background G-M counting system developed specially for the 14c tracer experiments is described. A mathematical treatment of the results shows their inter relationships their dependence on the rate of uptake of IAA from the source into the transport system and on the rate of immobilisation of IAA in the coleoptile. Both the rate of uptake and the rate of immobil isation increase with time. The treatment shows also how results of other workers on the export of IAA from the base of a coleoptile section can be predicted from the present work. Theoretical models of auxin transport are discussed in the light of the results and analysis. It is clear that diffusion cannot play a major part in the normal transport of auxin in the coleoptile and that each cell must pass on to the next nearly all the auxin that it initially receives. A survey of the literature on auxin transport up to 1965 is presented as an appendix."