Uptake of Potassium and Sodium by Seedlings of Sinapis Alba

Abstract

Seedlings of Sinapis alba (mustard) have a lower potassium selectivity than those of barley, as shown by the lower ratio of potassium to sodium in the shoots of plants grown on the same solution. The ratio in the shoots is usually lower than in the roots, whereas in barley it is higher. In spite of this difference in selectivity, the uptake of potassium and sodium by mustard has much in common with that by barley. The total potassium and sodium uptake to the shoot is independent of the ratio in the solution; the ratio of potassium to sodium in the shoot is proportional to that in the roots, but not to that in solution; and potassium selectivity can be reduced by transpiration. Thus it appears that the same general model for ion uptake can be used for both plants, although they have different levels of selectivity. In this model it is suggested that total potassium and sodium uptake is controlled by an active anion transport coupled with plant growth by means of metabolism, but selectivity is determined during movement of ions into the stele through cytoplasm and cell walls.