Studies on the Absorption, Distribution and Release of Labelled Rubidium Ions in young intact Barley Plants

Young intact barley plants proved to absorb labelled rubidium at an almost constant speed during the first six hours of an experiment. Afterwards severe depressions in the absorption ability occurred followed by a resumption of the uptake at about its initial speed. Only a very small fraction, if any, of the rubidium absorbed could have been exchangeable, as no significant loss of labelled rubidium to unlabelled solutions was detected. However, on rubidium sulphate, potassium sulphate and potassium nitrate solutions marked amounts of labelled rubidium, previously absorbed, were given off. A loss to water could also be observed, provided previous rubidium absorption had taken place in the absence of calcium sulphate. The ionic composition of the medium also had an influence on the transfer of rubidium ions to the shoots. It was assumed that at any moment part of the rubidium ions in the symplasm of the roots are in a mobile state, i.e. they can be either more definitely fixed within the root tissue, or transferred to the shoots or exceptionally given back to the medium. The external solution was thought to have an influence on the total amount of mobile rubidium ions, on the structural features of the symplasm (permeability) and on its functional features (active absorption into the symplasm, fixation in the symplasm, secretion to the vacuoles, and transfer to the shoots). Stress was laid on the importance of the ionic composition of the medium and the plant system used for studies on free space and exchange phenomena. It was found necessary to postulate a complex physiological system relating the absorption and distribution of rubidium in intact plants, in order to be able to account for the divergent effects observed.