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The Osmometer Model of the Root: Water and Solute Relations of Roots of Phaseolus coccineus
Author(s) -
Steudle E.,
Brinckmann E.
Publication year - 1989
Publication title -
botanica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.871
H-Index - 87
eISSN - 1438-8677
pISSN - 0932-8629
DOI - 10.1111/j.1438-8677.1989.tb00071.x
Subject(s) - hydraulic conductivity , mannitol , chemistry , phaseolus , water transport , urea , analytical chemistry (journal) , botany , chromatography , water flow , soil water , biochemistry , soil science , environmental science , biology
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 ( Lp r ), the permeability coefficient ( P sr ), and the reflection coefficient (σ sr ) of the roots could be evaluated. Lp r was 1.8 to 8.4 . 10 −8 m . s −1 . MPa −1 and P sr (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; NaNO 3 , 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 Lp r ( 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; NaNO 3 : 0.54. The transport coefficients ( Lp r , P sr , σ sr ) have been critically examined for influences of unstirred layers and active transport. The low σ sr suggests that the common treatment of the root as a rather perfect osmometer (σ sr = 1) analogous to plant cells should be treated cautiously. The reasons for the low σ sr and the possible implications of the absolute values of the transport parameters for the absorption of water and nutrients are discussed.