Effects of some Cyclic ‘Crown’ Polyethers on Potassium Uptake, Efflux and Transport in Excised Root Segments and Whole Seedlings

The ionophores benzo-18-crown-6 (18-C-6), t-butylbenzo-18-crown-6 (TBB) and di-t-butyldibenzo-30-crown-10 (30-C-10) were tested for their effects on potassium ion absorption in onion root segments, and in wheat and mung bean seedlings. Potassium uptake, efflux and transport were progressively reduced in onion root segments and seedlings by 18-C-6 over the range 0.1–1.0 mM. The effects of TBB (up to 0.3 mM) were more severe but otherwise generally similar to those of 18-C-6 in seedlings. Both ionophores reduced growth, and at the highest concentrations, resulted in root potassium ion content falling below initial values after 48 h treatment. The effects of 30-C-10 were evident at much lower concentrations, inhibition of net potassium uptake occurring above 10−4 M. Between 10−4 and 10−4 M 30-C-10, however, a modest but significant stimulation of potassium uptake was observed in onion roots and seedlings; growth of seedlings was largely unaffected. The reductions in potassium absorption were attributed to the promotion, by the ionophores, of facilitated diffusion down the electrochemical diffusion gradient, countering the efficiency of the potassium ion influx pump. Stimulation of uptake at certain concentrations of 30-C-10 was considered more likely to be due to an inhibition of passive potassium efflux, rather than a stimulation of active influx. The importance of stability constants, bonding and lipophilicity, in determining the relative effectiveness of the ionophores, is discussed.