Studies on the absorption of salts by plant roots

Salt absorption by plants is generally considered to be a two stage process, one phase of which is dependent on the metabolism of the cell. The experimental evidence on the subject is limited and the proposed mechanisms of absorption are based mainly on theoretical considerations. The purpose of this thesis , therefore, is to study the uptake of salts using radioactive tracer techniques and to identify the centre of each phase of the absorption process in plant roots. A technique of autoradiegraphy has been developed for use with water soluble isotopes, giving results at the cellular level with thin sections of root material. Four ions, calcium, rubidium, sulphate and iodide, were used in those experiments. It has been shown by this technique that there are two phases in the process of salt absorption by excised plant roots and the existence of an 'active' phase has been established. However, contrary to an assumption made in the method of isotopic exchange, it has been shown that the material retained after exchange is not all actively absorbed. The passive phase of absorption is associated with the epidermis, the hypodermic and the endodermis. The ions taken up by the epidermis wore found to be readily exchangeable. The nature of the localisation in the hypodermic is assumed to be a binding of the ion which has a low degree of exchangeability. It is proposed that the endodermis acts as a barrier to diffusion in that part of the root beyond the region of calcium accumulation. The active phase of absorption is primarily associated with the parenchyma of the cortex in the apical region of the root, this region being termed the region of calcium accumulation. Cations appeared to be localised in the protoxylem initial cells and in the central cells of the root cap. Cation absorption occurs in the epidermis, hypodermie, endodermis, cortical parenchyma of the apical region of the root and in the protoxylem initial cells. Anion absorption does not appear to occur in the epidermis , protoxylem initials or in the central cello of the root dap. On the autoradlographic evidence, it is proposed that vacuolar accumulation of cations occurs in the cortical parenchyma in the region of calcium accumulation. Anion accumulation does not occur in the vacuoles but in the case of sulphate ions, it is localised in the region of the nucleus of the cell. It is proposed that anions are absorbed into their respective metabolic cycles in the cytoplasm of the coil. The results obtained with sulphate ions suggest that the site of protein metabolism in the cell, is restricted to the region of the nucleus of the cell. The autoradiographs show that there is an intense localisation of non-exchangeable ions in the cortex in a region of the root in which the endodermis does not retain ions. It is possible that these accumulated ions form an osmotic pump which promotes the flow of ions by diffusion from the external solution directly into the stole in this region of the root, The region of maximum salt absorption, therefore, is also the region of maximum translocation