Osmotic responses of maize roots

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Abstract
Water and solute relations of excised seminal roots of young maize (Zea mays L) plants, have been measured using the root pressure probe. Upon addition of osmotic solutes to the root medium, biphasic root pressure relaxations were obtained as theoretically expected. The relaxations yielded the hydraulic conductivity Lpr) the permeability coefficient (Psr), and the reflection coefficient (σsr) of the root. Values of Lpr in these experiments were by nearly an order of magnitude smaller than Lpr values obtained from experiments where hydrostatic pressure gradients were used to induce water flows. The value of Psr was determined for nine different osmotica (electrolytes and nonelectrolytes) which resulted in rather variable values (0.1·10-8–1.7·10-8m·s-1). The reflection coefficient σsr of the same solutes ranged between 0.3 and 0.6, i.e. σsr was low even for solutes for which cell membranes exhibit a σs≈1. Deviations from the theoretically expected biphasic responses occured which may have reflected changes of either Psr or of active pumping induced by the osmotic change. The absolute values of Lpr, Psr, and σsr have been critically examined for an underestimation by unstirred layer effecs. The data indicate a considerable apoplasmic component for radial movement of water in the presence of hydrostatic gradients and also some solute flow byppassing root protoplasts. In the presence of osmotic gradients, however, there was a substantial cell-to-cell transport of water. Cutting experiments demonstrated that the hydraulic resistance for the longitudinal movement of water was much smaller than for radial transport except for the apical ends of the segments (length=5 to 20 mm). The differences in Lpr as well as the low σsr values suggest that the simple osmometer model of the root with a single osmotic barrier exhibiting nearly semipermeable properties should be extended for a composite membrane model with hydraulic and osmotic barriers arranged in series and in parallel.