Transfer of Solutes Across the Young Root

In considering the results obtained in the investigations concerning the intake of solutes by the roots of the normal higher plant, two principles come to the fore, which, in the first instance, might seem to be contradictory. The process that nowadays attracts the main attention of research workers in this field is that of accumulation, which can only be brought about by the active participation of aerobic metabolism. Besides this fact we cannot neglect the statements in which a passive method of intake is suggested. We have to accept the fact that the plasm of cells has proved to be permeable and besides this, there have been several results published which are explained on the basis of permeability of the root tissue. As yet, exact measurements of the permeability of the whole root tissue did not exist. It has been the main purpose of this investigation to examine the importance of passive transfer in the symplasm of the root. Our first object has been to devise a method in which we could use the total living tissues between the xylem-vessels and the medium as a membrane between two solutions, which could be varied at will. This was attained by building an apparatus in which we could place pieces of root of about 6% cm length, cut from the young secondary roots of Vida Faba. The xylem-tracts having been opened at both ends, the living tissues formed a membrane enclosing the pathways through which we could pass a solution by means of a suction-tension. In this way we could examine lateral transport of solutes by dissolving them in the outer surroundings and analyzing for them in the water we had caused to flow through the wood-vessels. Also the set up could be reversed by flowing through a solution containing the solute to be examined and then following up its outward transport across the stele cells, endodermis and cortex by analyzing the outer medium. Presence of the Casparian strips in the endodermis cells or a suberin layer in the cell wall necessitates that all transport passes through the protoplasm, in this place at least. By means of this new technique we have been able to ascertain that a number of ions can pass through the root symplasm. This is valid for Cl, NG3 and H2P04, although we are not absolutely certain that no active processes have partaken in the transfer. For K and Ca we have been able to demonstrate that these substances can penetrate the root tissue from the xylem to the surroundings. By applying anaerobic conditions and also respiration-poisons we were able to establish the passive nature of this transport. The root tissue showed even a far higher degree of permeability for Ca in its outward transfer than for K. This can be accounted for by the low-salt condition of the roots, which would contain Ca to a fair extent, as they were grown in tap-water. Also three organic substances were investigated. As could be expected, urea proved to permeate well. The other substances examined were glucose and sucrose. The latter especially was again extensively analyzed in its transfer from the vessels to the outer medium. Conforming with our expectations, although it may be somewhat surprising, sucrose was able to penetrate through the tissue easily. Finally, we have come to regard the root tissue as easily permeable to a number of solutes and we suggest that this conclusion will be valid to a much wider extent. This conclusion may seem to be somewhat doubtful in view of the many statements on the impermeability of the protoplasm, obtained by plasmolysis experiments. But following the example of Arisz, we wish to make a clear distinction between permeability sensu stricto, where only plasmalemma and the cytoplasm are penetrated, and transmeability, where also the tonoplast has to be passed. Our conclusions only refer to permeability of the protoplasm. As the pieces of root were cut at different distances from the root apex, we could confirm the opinion that the whole zone of the root, where there are no suberized cell walls, is capable of uptake. But we were also able to demonstrate that a crosstransfer and a secretion into the xylem can occur throughout a zone corresponding to the length of the surface, where root haircells are able te develop. Concerning the function of the process of intake and transfer we arrive at the following point of view. Transportation across the root tissue may be of two different types. Passive transport is made possible by permeability of the tissue, and here the direction can be controlled by changing the driving forces, i.e. concentration- or activation-gradients. This passive transport is not subject to polarity. Far more important for the normal supply of the shoot with nutrients is the active transport of solutes. This can only be brought about by energy delivered in aerobic respiration and is of a strictly polar character. This accumulation process only operates an inward bound transfer from surface to vacuole or xylem-vessel. In the last chapter some theoretical considerations are discussed. We propose to consider the amount of salts absorbed by a cell or tissue the result of a dynamic equilibrium. The “accumulation- level” a cell attains is the result both of leakage of solutes, as a consequence of the possibility for permeation, and the intensity of active accumulation. It seems to us that in agreement with the hypothesis of Crafts and Broyer we might arrive at a good description of absorption by accepting a different “accumulationlevel” between cortex and stele. As the latter cells have the lower capacity for retaining salts, they ooze away into the xylem, where they can give rise to the phenomena of exudation or are carried off by the transpiration stream. This scheme has its limitations, however, and so we propound the following hypothesis. The whole root tissue has the capability for an active inwardly directed transfer of solutes — probably in bound state — and secretes them into the xylem. Loss of particles can occur at any place, also by interchange, and the free particles may leak away to the medium again, as the tissue is permeable.