Abstract
The selectivity of cation uptake by radish (Raphanus sativus L.) growing in the field near Chernobyl varies during the growth season. It is hypothesised that this is a consequence of variation in Cs-137 (Cs-ss) and potassium (K-ss) concentrations in the soil solution or the amount of dissolved potassium available to the plants. In the experiments reported here, it was observed that (1) Cs-ss and K-ss were positively correlated, (2) the selectivity for uptake of Cs-137 versus potassium (r) increased exponentially with decreasing Cs-ss and K-ss, and (3) the Cs-137 concentration, but not the potassium concentration, in plant material, increased abruptly upon the simultaneous reduction of K-ss and Cs-ss below about 10 mu g ml(-1) and 6.7 Bq l(-1) respectively. It is thought that potassium enters root cells from the soil solution through constitutively expressed, inward rectifying K+ channels (KIRC) and K+/H+-symporters, whose abundance increases when plants become potassium-deficient. Cesium is thought to enter root cells through non-specific cation channels (NSCC) and, in plants lacking sufficient potassium, through K+/H+-symporters. It is argued that the increase in r, together with the abrupt increase Cs-137 concentration in plant tissues, when K-ss and Cs-ss decrease simultaneously cannot be attributed to competition between Cs+ and K+ for transport though KIRC, NSCC or K+/H+-symporters and that the most plausible explanation of these phenomena is an increase in the abundance of K+/H+-symporters in plants exhibiting incipient potassium deficiency. (C) 2015 Elsevier Ltd. All rights reserved.