NMR Studies of Radial Exchange and Distribution of Water in Maize Roots: The Relevance of Modelling of Exchange Kinetics

Bacic, G. and Ratkovic, S. 1987. NMR studies of radial exchange and distribution of water in maize roots: the relevance of modelling of exchange kinetics.—J. exp. Bot. 38: 1284-1297. Radial transport of water in primary maize roots has been studied by following the kinetics of the H20-D20 exchange using proton magnetic resonance. The method is based on the use of an impermeant relaxation agent, dextran-magnetite, which made possible separation of the surrounding proton signal from the root water proton signal. Two pulsed NMR techniques for continuously following the exchange are described. The exchange curve was fitted to several theoretical models in order to evaluate the dominant radial water pathway in roots. It was found that the choice of the appropriate model could not be made solely on the basis of goodness of fit and thus the previously calculated value of membrane water permeability, which is not dependent on the water transport pathways, was used as an additional testing criterion. It is concluded that the major radial water flux in root occurs through the cells, and not through the intercellular space, as previously assumed. The relatively low calculated apparent diffusion coefficient (D' = 3 x 10"6 cm2 s~l) of water is mostly determined by water permeation through a series of membranes. A minor fraction of water in roots (10-15%), exchanged at a slower rate than the rate at which bulk diffusion would allow and was assumed to correspond to the water having restricted mobility due to the influence of macro molecules. The NMR measurements of relaxation of water in roots also suggest different water fractions and it is concluded, from both experimental approaches, that most of the cytoplasmic water has significantly different mobility from the bulk water.