Transpiration stream & ascension of calcium

Abstract

The degree of dependence between water movement and Ca movement in the bean plant was determined by the simultaneous use of THO and Ca . The root water reached only 77% of equilibration with nutrient water in 24 hours while equilibration in the xylem cylinder of the stem was complete to the 3d trifoliate leaf in 3 hours. The bark and leaves reached 87% and 65%, respectively, of equilibration in 3 hours, and neither exceeded this value in 72 hours. The time course curve for Ca45 entry into specific stem sections showed 2 phases a reversible exchange phase and an irreversible accumulation phase. The exchange phase was complete in 3 hours, and the exchangeable fraction constituted less than 10% of the total Ca. The Ca45 on exchange sites could be removed by a transpiration stream carrying unlabeled Ca, the loss falling through 3 half values in 6 hours. The non-exchangeable Ca45 was mostly in the calcium oxalate crystal system, the low solubility constituting the driving force for accumulation. The data showed the xylem cylinder of the bean stem fundamentally to operate on an exchange column for Ca. En masse flow in the vessels was inadequate for explaining the rapid, deep-seated exchanges observed for this tracer.