Relationships between Adsorption, Chemical State and Fluxes of Cadmium Applied as Cd(NO3)2in Isolated Xylem Cell Walls of Tomato

Abstract

Wolterbeek, H. Th. 1987. Relationships between adsorption, chemical state and fluxes of cadmium applied as Cd(N03)2 in isolated xylem cell walls of tomato.—J. exp. Bot. 38: 419-432. Isolated xylem cell wall pieces were applied as membranes in ion diffusion experiments. The cell walls were isolated from tomato internodes (Lycopersicon esculentum Mill, cv. Tiny Tim) and sealed in a two-compartment diffusion system. In flux and adsorption calculations, the cell wall was regarded as a leaky membrane with parallel fluxes through Donnan Free Space (DFS) and Water Free Space (WFS). During the experiments absorption into and diffusion across the walls was determined of Cd2+, applied as 115Cd(N03)2. Flux experiments with 82Br indicated that excluded volume effects and path tortuosity resulted in apparent WFS diffusion coefficients in the walls which were 0 012 times as high as in water. The free proton concentration in the DFS was shown to be related to a complex formation between fixed charges and Cd2 + . The cell wall permeability for Cd2+ and NO, varied with applied and absorbed concentrations, and the Cd2+ flux curve showed an inflexion point coinciding with a buffered degree of dissociation of fixed charges in the DFS. The necessary coupling of fluxes of opposite charges resulted in relatively high N03 and small Cd2+ permeability of the DFS for strongly diluted solutions (P = 104 m s' and I CP11 m s-1 for NOf and Cd2+ respectively). The results demonstrate the possible regulatory effects of the cell wall in processes of ion transfer from xylem vessels, or ion uptake in plant tissues. Key words—Cadmium, chemical state, DFS, WFS, ion flux, permeability, xylem cell walls, tomato, bromium, nitrate. Correspondence to: Interuniversitair Reactor Institute, Mekelweg 15,2629 JB Delft, The Netherlands.