The Absorption of Labelled Chloride and Bromide Ions by Young Intact Barley Plants

The techniques for cultivating the young intact barley seedlings used in these absorption experiments are described in some detail. Sets of twelve selected seedlings were used and all data obtained refer to these units. In the experiments, proper absorption took place from dilute solutions of either potassium chloride alone or a mixture of potassium bromide and chloride, the total concentration of which was 0.001 M. Calcium sulphate was also present in all experimental solutions. Chloride is absorbed from the medium at an almost constant speed for a number of hours. This absorption is not influenced by subsequent distribution. At first, the root tissue tends to retain a large proportion of the chloride absorbed at the expense of the transfer of ions to the shoots. Gradually the transfer increases until a constant value is reached within a few hours. Identical results were obtained for total halides if absorption took place from a mixture of chloride and bromide. In addition it was found that chloride and bromide were absorbed at a ratio, which was constant and about twice as high as the ratio or the ions present in the medium. This ratio remained constant even in the dark ■vhen total absorption tended to decline slightly. In the transport processes following primary absorption, further discrimination oetween chloride and bromide ions takes place. The additional screening effect of the root tissue is very small. Nevertheless, important conclusions could be drawn from the behaviour of the Cl/Br ratios of the halides found in roots and shoots. For root tissue this ratio, which must equal the ratio for total absorption at the beginning of the experiment, decreases slightly until a somewhat lower but constant level is reached after a few hours. Although there is no doubt about this decrease for the root tissue, it proved to be so small that the simultaneous increase of the ratio for the shoot tissue was hardly discernable under normal conditions. It became so, if the transfer of ions to the shoot was reduced by a dark treatment. These results could be explained by assuming that the root tissue consists of at least two compartments, differing with respect to the Cl/Br ratio. Ions are brought from the medium into the first compartment by the primary absorption process and then transferred to either the second compartment or via the xylem vessels to the shoots. It is tempting to identify the first compartment with the symplasm of the root tissue, the second one with the vacuole system. The process that takes ions out of the symplasm into the vacuoles prefers bromide ions to chloride ions. Little preference, if any, for either chloride or bromide ions was found for the secretion of ions from the symplasm into the xylem vessels. This is consistent with the idea that the secretion is a simple leakage of a solution out of the tissue. Under conditions of suppressed transfer to the shoots, the accumulation of ions from the symplasm into the vacuoles becomes relatively more important. This causes greater shifts in the Cl/Br ratios found in the different compartments.