The Osmometer Model of the Root: Water and Solute Relations of Roots of Phaseolus coccineus

Abstract

Water and solute relations of young roots of Phaseolus coccineus have been measured using the root pressure probe. Biphasic root pressure relaxations were obtained when roots were treated with solutions containing different osmotic test solutes. From the relaxations, the hydraulic conductivity (Lpr), the permeability coefficient (Psr), and the reflection coefficient (σsr) of the roots could be evaluated. Lpr was 1.8 to 8.4 . 10−8 m . s−1 . MPa−1 and Psr (in 10−10 m . s−1): methanol, 27–62; ethanol, 44–73; urea, 5–11; mannitol, 1.5; KCl, 7.1–9.2; NaCl, 2.1; NaNO3, 3.7. The hydraulic conductivity was similar when using osmotic and hydrostatic pressure gradients as driving forces. The hydraulic conductivity of individual root cortex cells (Lp) was by two orders of magnitude larger than Lpr (Lp = 0.3 to 4.7 . 10−6 m . s−1 . MPa−1) which indicated a predominant cell‐to‐cell rather than an apoplasmic transport of water in the Phaseolus root. Except for distances shorter than 20 mm from the root apex, the hydraulic resistance of the roots was limited by the radial movement of water across the root cylinder and not by the hydraulic resistance within the xylem. Reflection coefficients were low: methanol: 0.16–0.34; ethanol: 0.15–0.47; urea: 0.41–0.51; mannitol: 0.68; KCl: 0.43–0.54; NaCl: 0.59; NaNO3: 0.54. The transport coefficients (Lpr, Psr, σsr) have been critically examined for influences of unstirred layers and active transport. The low σsr suggests that the common treatment of the root as a rather perfect osmometer (σsr = 1) analogous to plant cells should be treated cautiously. The reasons for the low σsr and the possible implications of the absolute values of the transport parameters for the absorption of water and nutrients are discussed.

Effects of Salinity on Water Transport of Excised Maize (Zea mays L.) Roots

“…Their results showed that, in hydrostatic experiments, Lpr was constant along the root segment except for an apical zone of about 20 mm in length which was hydraulically isolated because of a high axial resistance. In barley and bean roots, hydrostatic and osmotic LPr values were similar (24,26 (4-6, 8, 30). Shortterm experiments performed by adding supplemental calcium to the salinized growth media of maize roots had ameliorative effects on the stressful conditions.…”

Effects of Mercuric Chloride on the Hydraulic Conductivity of Tomato Root Systems (Evidence for a Channel-Mediated Water Pathway)

“…The authors suggested that 80 to 90% of radial flux could occur via the cell-to-cell pathway, an estimation in substantial agreement with the results shown here for roots treated with HgC1,. Utilizing the pressure probe technique, Steudle and Jeschke (1983) and Steudle and Brinckmann (1989) each concluded that the cell-to-cell pathway rather than the apoplastic route is predominant in young roots of Hordeum distichon and Phaseolus coccineus, respectively.…”

Effects of NaCl and CaCl2 on Water Transport across Root Cells of Maize (Zea mays L.) Seedlings

“…They concluded that there was a considerable apoplasmic bypass during water flow similar to the situation found in maize roots (see above, 26). However, in other systems, such as in bean and barley roots, both the hydrostatic and osmotic flows were from cell to cell, as indicated by a large cell Lp compared with root Lpr (17,19). Thus, depending on the species and on the nature of the driving force (hydrostatic or osmotic), there can be large differences in the pattern of radial water transport across roots.…”