V. I. Korobkov

Label Distribution in Tissues of Wheat Seedlings Cultivated with Tritium-Labeled Leonardite Humic Acid.

Humic substances (HS) play important roles in the biotic-abiotic interactions of the root plant and soil contributing to plant adaptation to external environments. However, their mode of action on plants remains largely unknown. In this study the HS distribution in tissues of wheat seedlings was examined using tritium-labeled humic acid (HA) derived from leonardite (a variety of lignites) and microautoradiography (MAR). Preferential accumulation of labeled products from tritiated HA was found in the roots as compared to the shoots, and endodermis was shown to be the major control point for radial transport of label into vascular system of plant. Tritium was also found in the stele and xylem tissues indicating that labeled products from tritiated HA could be transported to shoot tissues via the transpiration stream. Treatment with HA lead to an increase in the content of polar lipids of photosynthetic membranes. The observed accumulation of labeled HA products in root endodermis and positive impact on lipid synthesis are consistent with prior reported observations on physiological effects of HS on plants such as enhanced growth and development of lateral roots and improvement/repairs of the photosynthetic status of plants under stress conditions.

Tritium labeling: A unique tool for studying the behavior of humic substances in living systems

A range of labeled humic substances was obtained by the tritium thermal activation method. The high specific radioactivity and radiochemical purity allowed direct determination of the hydrophobicity and surface activity of humic substances, and investigation of the behavior of humic substances in bacteria and plants.

Polymer films as indicator of hydrogen spillover through the gas phase

Films of poly-ε-caproamide (PA), polyethylene (PE), and polyethylene terephthalate (PET) were used for detection of hydrogen spillover through the gas phase. The hydrogen used in the experiments contained tritium activated by two procedures (W wire, 2000 K; 5% Pd/C, 335 K). The radioactivity of the films was recorded by classical and digital autoradiography and by liquid scintillation counting. Under the action of “hot” atoms generated on a W wire, the maximal specific radioactivity of the films, equal to 420, 415, and 330 mCi cm–2 for PA, PE, and PET, respectively, was reached in 100 s. Preliminary thermalization of the atoms to a temperature of 77–335 K influenced the decrease in the film radioactivity differently. The effective activation energy of the reaction in the range 298–318 K was 21, 30, and 12.5 kJ mol–1 for PA. PE, and PET, respectively. Under the conditions of heating 5% Pd/C to 335 K for 25 min, the radioactivity of PA, PE, and PET was 1.6, 0.05, and 0.15 μCi cm–2, respectively. The revealed difference in the radioactivity of the films suggests different mechanisms of the interaction of tritium with organic molecules at different activation methods.

Interaction of tritium atoms with solid composite targets: Adsorption layers of nonionic surfactants on hydrophobic supports

The capability of tritium atoms to penetrate deep into solids was used for revealing nonuniformity of coating of polymeric [polyethylene, poly(ethylene terephthalate)] supports by adsorption layers of a nonionic surfactant Brij-35, formed by adsorption from aqueous solutions. The permeability to atomic tritium of Brij-35 adsorption layers on polyethylene appeared to be close to that obtained for hydrocarbon fragments of cationic surfactants in adsorption layers on the water/air boundary. A decrease in the coefficients of attenuation of the tritium atom flow at formation of adsorption polylayers was revealed. This decrease suggests less ordered structure of the polylayers relative to the monolayer. The developed approach involving preparation of adsorption coatings of tritium-labeled surfactants, their treatment with tritium atoms, and subsequent removal of the surfactant from the support, with the autoradiographic detection of tritium distribution on the plate in all the steps of the work, can be used for studying the structure of surfactant adsorption layers on flat solid surfaces.

The Endodermis Is the Major Control Point for Radial Transport of Humic Substances into the Vascular System of Plants

In order to understand humic substance (HS) uptake by plants and its entrance into the plant vascular system, a microautoradiography approach using tritium-labeled HS was applied. Wheat seedlings were used as test plants. The data obtained clearly demonstrated preferential radial transport of HS by the apoplastic pathway. Epidermal and cortex cells did not remove significant proportions of humic substances, and the endodermis was shown to be the major control point for their radial transport.