Influence of light and sucrose on the Uptake and Transport of Chloride in Vallisneria Leaves

Sucrose and light have a regulating influence on the accumulation of chloride ions in the cells of Vallisneria leaves. The light effect is not only the result of the formation of carbo-hydrates in the photosynthesis, light also being active in the absence of carbon dioxide. Seeing that with light saturation for the chloride uptake sucrose still increases the uptake, the influence of light and sucrose is not due to the formation of the same substance. Light promotes the uptake into the symplasm and as a result increases the transport in the symplasm and indirectly the secretion of chloride ions from the symplasm into the vacuoles. This obtains as much for the vacuoles in the absorbing part as for those in the free part of the leaf. If the uptake takes place in the dark, it has a limiting influence on the accumulation in the exposed free part. This proves that the free part does not take up chloride ions direct from its surroundings. Therefore transport of chlorides through the cell walls to the free part does not take place. Exposure of the free part, the absorbing zone being exposed to light, causes an increase of accumulation in the absorbing zone. This indicates that a substance is formed by the exposed free part and transported to the absorbing part which increases accumulation of chloride ions. Exposure of the free part, the absorbing zone being in the dark with sucrose, may likewise result in an increase of the total uptake, so that the accumulation both in the absorbing and in the free part increases. If, however, the absorbing zone is in the dark without sugar, the effect is variable. The influence of sucrose administered to the absorbing or to the free zone is in the main local, though sometimes an increase in accumulation occurs in the adjoining part. This can be explained by transport of the absorbed sugar. Sucrose administered to the absorbing part in the dark increases the secretion of chloride ions into the vacuoles in the zone of administration. This sometimes takes place at the expense of the accumulation in the free part. This is due to the fact that the uptake of chloride ions in the dark and in the dark with sucrose is limited, so that the absorbing part must compete with the free part to accumulate the ions into the vacuoles. The rate of transport in the symplasm and the presence of carbo-hydrates determine the result. Sucrose in the free zone, if the absorbing zone is in the light, likewise gives a local increase of the secretion into the vacuole; in some cases the secretion into the absorbing part also increases. Addition of sucrose to the free part may have an osmotic effect on the absorbing part, owing to which it accumulates fewer chloride ions in the vacuoles. This influence may be compensated by giving sucrose to the absorbing part. The senior writer is greatly indebted to Mrs. H. Knobbe-Meester and Miss M. J. Schreuder for the careful execution and help with the experiments and the analyses. The experiments marked S have been carried out by Mr. H. H. Sol. The senior author is responsible for the text.