Calcium in Xylem Sap and the Regulation of its Delivery to the Shoot

Amounts of total and free calcium in root and shoot xylem sap were quantified for a number of species grown in comparable environments and in a rooting medium not deficient in calcium. The potential for the shoot to sequester calcium was also examined, along with the ability for ABA to regulate calcium flux to the leaf. Xylem sap calcium showed considerable interspecific and diurnal variation, even though the plants were grown with similar rhizospheric calcium concentrations. The potential for the shoot to sequester xylem sap calcium was also highly variable between species and implied an ability, at least in some species, to regulate the calcium reaching the shoot in the transpiration stream. Long distance transport of calcium in the xylem was not primarily by mass flow, because neither calcium uptake nor distribution were closely related to water uptake or transpiration. The diurnal changes in xylem sap total ion concentration appeared to be negatively correlated with transpiration while, in contrast, the calcium ion concentration showed two peaks, one occurring in the dark and the other in the light period. The application of ABA to roots caused an increase in the rate of exudation from the xylem of detopped well-watered plants. These experiments suggest that changes in root water relations driven by ionic fluxes were the likely cause for enhanced sap exudation from ABA-treated roots. The steady-state concentration of calcium in the xylem sap was unaffected by ABA when exudation rate increased and, consequently, the flux of calcium must also have increased.