Diurnal Changes in Volume and Solute Transport Coefficients of Phaseolus Roots

Abstract

Volume (J,) and solute (J,) fluxes through Phaseolus root systems were observed over a 24-hour period. The volume flux was varied in a pressure chamber by altering the hydrostatic pressure in 10 steps, from 0 to OA meppascals. All root systems showed strong diurnal peaks in volume flux. The five transport coefficients (,, w, J,*, L., and x*) were estimated from a nonlinear least squares algorithm. Analysis of the data revealed that all the coefficients exhibited a diurnal rhythm. When the total differential of the volume flux was considered it was possible to show that the diurnal changes in volume flux were due to a complex interaction between the diurnally shifting coefficients with the role of each highly dependent on the level of volume flux. At low volume fluxes, w, J,*, and T* accounted for nearly all the diurnal change in volume flux.At high volume fluxes, however, the major influence shifted to L. and i*, while w and J,* became relatively unimportant. Thus, w* was the only coefficient of interest across the entire range of J, and appeared to be the single most important one in determining the diurnal rhythm of J, under conditions of a constant applied pressure.Since the first report of Hofmeister in 1862 (see Vaadia [27]) of the diurnal fluctuations of both root pressure and exudation rates, the phenomenon has been repeatedly demonstrated (more recently [11-13, 18, 21, 24, 27 (0600-2000) at the top of the plants. The plants, selected for size at the time of the experiments, averaged about 2230 cm2 projected leaf area as measured with a LI-CORI model 3100 leaf area meter, and about 3150 cm2 root surface area. Each root system was decapitated and sealed into a pressure chamber with the cut stump protruding through the lid and the roots surrounded by aerated nutrient solution (70 = 35 kPa)2 as previously described (5). The plants were placed in the chamber at about 1600 h, brought to the specified pressure and temperature (25 ± 0.25C), and allowed to equilibrate. Beginning at midnight, measurements of volume flow and ion flow from the cut stump were started and continued at 10-min intervals for the next 24 h. The volume flow was measured by weighing the exudate on an electronic balance. Concentration of the exudate was estimated from the electrical conductivity and expressed as KCI equivalents. The ion flow was then calculated as the product of the volume flow and the concentration. Using Newman's technique (19) on each root size class, surface areas were measured as described earlier (7)

Some Compartmental Models of the Root: Steady-state Behavior

“…Application of ABA somehow renders these solutes mobile, and their consequent release to the xylem abolishes the internal concentration gradient. It can be shown from eqns (4) and (5) in Fiscus (1986) that consistent with observation, V(R)"(1! ) M( M where is the re#ection coe$cient of the composite root membrane.…”

“…This would lead to a decrease in P and a &&burst'' of solute release into the xylem associated with increased &&solvent drag''. Fiscus (1986Fiscus ( , 1988 proposed a di!erent explanation: that the o!set in the P}Q T curve ( P ' M) seen in the absence of ABA results from an asymmetric distribution of nonmobile solutes within an intermediate compartment (by &&nonmobile'' Fiscus apparently meant solutes not normally released to the xylem). Application of ABA somehow renders these solutes mobile, and their consequent release to the xylem abolishes the internal concentration gradient.…”

“…It is shown that a two-compartment model with solute build-up can account for the fact that P ' M. But in doing so, it also predicts that the xylem-sap osmotic pressure approaches M as Q T PR, which is inconsistent with observation. This leads to a consideration of three-compartment models (Ginsburg, 1971;Newman, 1976;Fiscus 1981Fiscus , 1986Fiscus , 1988Miller, 1985), and, "nally, the introduction of a new four-compartment model. The essentially new feature of this model is the inclusion of solute unloading by the phloem.…”

Endogenous NO3− in the Root as a Source of Substrate for Reduction in the Light

“…3), both lend support to a general change occurring in translocation efficiency in response to increased transpiration. In addition, there is evidence indicating that properties of the root directly governing water and nutrient fluxes also are altered in the dark (1,4,21,22). Thus, a complex system of effects is implied, possibly being triggered by changes in turgor which would accompany adjustments in transpiration.…”

A Model Based on Facilitated Passive Diffusion is Needed to Describe Root Water Entry through Aquaporins

“…[1][2][3][4][5][6][7][8] Much of this evidence has documented diurnal rhythms either in water flux or in measures of conductivity. 3,4,[9][10][11] Emery and Salon (2002) 11 reported the existence of such a rhythm in Lp of Pisum sativum. We recently followed this up by measuring changes in PsPIP2-1 gene expression and root hydraulic conductivity (Lp r ) in response to time of day as well as treatment of the roots with a compound that reduced Lp r (i.e., mercuric chloride, HgCl 2 ).…”