The Uptake of Growth Substances

In a previous paper it was established that during the course of uptake of radioactive 2,3,6-trichlorobenzoic acid (2,3,6-TCBA) by mesocotyl segments of Avena, the rate, initially positive, became negative within six hours. This phase of net loss is prevented by streptomycin and by synthalin, while an enhancement of accumulation is brought about by cetyl trimethylammonium bromide (CTAB). It was postulated that the initial accumulation is governed by an unstable accumulatory process (Type 1) which involves adsorption by some cell membrane system through an interaction between the carboxyl anion of the growth-regulator molecule and the quaternary ammonium group of the choline moiety of α-lecithin. Hydrolysis of lecithin by phospholipase-D destroys this Type 1 binding, while cationic nitrogen compounds maintain positive uptake by competing with the choline quaternary ammonium group of α-lecithin for the anionic site of phospholipase-D. The effects of pretreatment at a low temperature on the subsequent uptake of 2,3,6-TCBA and the influence of pH on the course of uptake, as well as studies of the egress of choline, provide some support for the role of phospholipase-D in determining the instability of the accumulatory system. Synthalin and CTAB inhibit the activity of phospholipase-D in vitro. However, other investigations with this enzyme also emphasize that such inhibition can only partially account for the great enhancement of the uptake of TCBA produced by CTAB. Related experiments on the uptake of alkyl pyridinium compounds by Avena segments and on their adsorption to lecithin in vitro favour a suggestion that quaternary ammonium compounds, such as CTAB, act largely by providing aftificial Type 1 sites. The mechanism and significance of Type 1 accumulation are discussed and compared with similar postulates for the binding of auxins and salts.