Osmotic responses of maize roots : Water and solute relations.

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 Lp r) the permeability coefficient (P sr), and the reflection coefficient (σ sr) of the root. Values of Lp r in these experiments were by nearly an order of magnitude smaller than Lp r values obtained from experiments where hydrostatic pressure gradients were used to induce water flows. The value of P sr was determined for nine different osmotica (electrolytes and nonelectrolytes) which resulted in rather variable values (0.1·10(-8)-1.7·10(-8)m·s(-1)). The reflection coefficient σ sr of the same solutes ranged between 0.3 and 0.6, i.e. σ sr was low even for solutes for which cell membranes exhibit a σ s≈1. Deviations from the theoretically expected biphasic responses occured which may have reflected changes of either P sr or of active pumping induced by the osmotic change. The absolute values of Lp r, P sr, and σ sr 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 Lp r as well as the low σ sr 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.