Radial hydraulic conductivity along developing onion roots

Although most studies have shown that water uptake varies along the length of a developing root, there is no consistent correlation of this pattern wi th root anatomy. In the present study, water movement into three zones of o nion roots was measured by a series of mini-potometers. Uptake was least in the youngest zone (mean hydraulic conductivity, Lp(r) = 1.5 x 10(-7) +/- 0 .34 x 10(-7) m MPa-1 s (-1); +/- SE, n = 10 roots) in which the endodermis had developed only Casparian bands and the exodermis was immature. Uptake w as significantly greater in the middle zone (Lp(r) = 2.4 x 10(-7) +/- 0.43 x 10(-7) m MPa-1 s(-1); +/- SE, n = 10 roots) which had a mature exodermis with both Casparian bands and suberin lamellae, and continued at this level in the oldest zone in which the endodermis had also developed suberin lame llae (Lp(r) = 2.8 x 10(-7) +/- 0.30 x 10(-7) m MPa-1 s(-1); +/- SE, n = 10 roots). Measurements of the hydraulic conductivities of individual cells (L p) in the outer cortex using a cell pressure probe indicated that this para meter was uniform in all three zones tested (Lp = 1.3 x 10(-6) +/- 0.01 x 1 0(-6) m MPa-1 s(-1); +/- SE, n = 60 cells). Lp of the youngest zone was low ered by mercuric chloride treatment, indicating the involvement of mercury- sensitive water channels (aquaporins), Water flow in the older two root zon es measured by mini-potometers was also inhibited by mercuric chloride, des pite the demonstrated impermeability of their exodermal layers to this subs tance. Thus, water channels in the epidermis and/or exodermis of the older regions were especially significant for water flow. The results of this and previous studies are discussed in terms of two models. The first, which de scribes maize root with an immature exodermis, is the 'uniform resistance m odel' where hydraulic resistances are evenly distributed across the root cy linder. The second, which describes the onion root with a mature exodermis, is the 'non-uniform resistance model' where resistances can be variable an d are concentrated in a certain layer(s) on the radial path.