Uptake and Distribution of Sodium and Potassium by Corn Seedlings : II. Ion Transport within the Mesocotyl.

In this paper, uptake and distribution of sodium and potassium within the mesocotyl are considered in excised, 8-day-old corn (Zea mays L.) seedlings supplied with label via the transpiration stream. The stele and cortex were dissected following uptake and analyzed separately. At equal concentrations, sodium uptake by the stele was much more rapid than potassium uptake, and sodium was preferentially retained within the stele. Transport of sodium to the cortex halted when the supply of ions in the transpiration stream was interrupted. Potassium would not substitute for sodium in restoring this transport but neither did it compete with sodium for transport to the cortex. In the presence of continued sodium supply, transport was temperature sensitive.By labeling first with (22)Na for 2 hours and subsequently with (24)Na for up to 21 hours, three sodium pools were identified within the stele. The first was rapidly transportable to the cortex. The second equilibrated rapidly with the first but was not itself directly available for transport. We postulate that these represent the stelar symplasm and apoplasm, respectively. A third pool was not transported and probably represents sequestration within the vacuoles of some cell type. Transport of label acquired during the initial 2 hours proceeded with a half-time of approximately 10 hours with 10 millimolar sodium present during the redistribution period, and with a half-time of approximately 30 hours at 1 millimolar sodium.A working model is presented which explains these characteristics and supplies approachable questions for subsequent study.