Water Relations of Plant Cells

ABSTRACrThe volume flux, Jv, and the osmotic driving force, aAi, across excised root systems of Zea mays were measued as a function of AP, the hydrostatic pressure difference applied across the root, using the pressure jump method previously described (Miller DM 1980 Can J Bot 58: 351-360). J, varied from 5.3% to 142% of its value in intact transpiring plants as a result of the application of pressure differences from -2A to 2A bar. The calculated hydraulic conductivity was 5.9 x 10' cubic centi- Several authors (2-5, 10) have devised models ofroot functions in an attempt to predict Jv, the rate of exudation from excised root systems as a function of AP, the hydrostatic pressure difference between the outside surface of the root, Po, and the xylem vessels, Px, (AP = Po -Pr). These models were tested against data obtained from the earlier literature as reveiwed by Fiscus (3) together with more recent observations obtained by him (4). In all this work, the applied pressures were positive, since Px was held at atmospheric pressure, while J, was increased by raising Po.The present work extends these data in two ways. First, not only were measurements of J, made as before by increasing P0, the pressure outside the root, but in addition, by raising the xylem pressure P, above Po, measurements of low values ofJ, at negative AP were obtained. Second, with each measurement of Jv, a corresponding measurement of the osmotic driving force within the root was obtained. These observations were made using an apparatus previously described (6), which allows the recording against time ofeither J, under a fixed pressure gradient AP, or the root pressure, Px, when the flow rate, Jv, is zero. In addition, the apparatus has the ability to determine how great an increase in P, is just necessary to bring about an instantaneous termination of flow. The procedure was to establish a pressure drop across the root system and then to record the volume flow rate, Jv, once it had become steady. This should obey the well known expression: Jv = Lp(AP-fAir) = Lp(P,-Px-ar) (1) where Lp is the hydraulic conductivity of the root system, a is the reflection coefficient of transported ions, and Air is the sum of all osmotic pressure gradients across the root. Next, the pressure at the cut end of the root was increased by a series of pressure jumps until one was found which rendered J, zero (2) and since PO and P. are known, the osmotic driving force aAr can be obtained. Furthermore, it is unlikely that any changes in concentration gradients within the root system will occur during transition between the flowing and nonflowing states, since the application of the pressure jump required a time interval of less than 0.3 s. Thus, the driving force which APX' nullifies, must be the same one which caused the volume flow, Jv, immediately before its application, and it should therefore be valid to rewrite expression 1 as J, = LpAPx0, and to calculate the hydraulic conductivity as L4 = JV/APX0. The value of L4 can be found by this method for each applied pre...

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Studies of Root Function in Zea mays

Miller¹

1985

“…3, 6, 8 (6,14). The importance of the unstirred layer in the interpretation of transport studies has been stressed by a number of investigators (2,12,18). Gutknecht et al (7) have provided a model demonstrating how the apparent facilitation of CO2 diffusion can be explained on the basis of the presence of HCO3 ions and CA in the unstirred layer.…”

Section: Discussionmentioning

confidence: 99%

Nature of the Rate-Limiting Step in the Supply of Inorganic Carbon for Photosynthesis in Isolated Asparagus Mesophyll Cells

Volokita

Reinhold²

1983

Accumulation of acid-stable and acid-labile C has been studied in cells isolated from cladophylis of Asparagm sprengeri regel as a function of the concentrations of the various inorganic C (Ci) species in the external medium.The rate of CO2 fixation was higher when Ci was supplied as CO2 as opposed to HC03-, at the same concentration. Participation of HC03-was indicated when the external CO2 and HC03-concentrations were chosen such that, owing to interconversion between Ci species, the same CO2 concentrations would be reached at a certain point in time, regardiess of which species was suppled hintiafly, at this point, a higher fixation rate was observed in the case of HCO-supply. In the presence of carbonic anhydrase, the apparent affinity Carbonic anhydrase raised the fixation rate over the entire pH range when HC03-was the species supplied. It is concluded that CO2 was the major Ci species permeating from the medium to the carboxylating site. Further, our results bring clear evidence that formation of CO2 from HC03-in the unstirred layer, and the diffusion of both species in this layer, rate limit CO2 fixation by these isolated celis.It is widely accepted that CO2 is the species of C,2 which serves as the substrate of RuBPcase (e.g. 5). However, it is still a controversial question whether HCO3: ions in the medium contribute to the supply of CO2 to this enzyme in higher plant cells. An analysis of the dependence of photosynthesis in leaf slices on pH led to the conclusion that CO2 is the only Ci species which crosses the cell membrane (8,15 First, it might result from the operation of an HCO3 transporting system. Second, there might be a resistance to CO2 diffusion into or within the unstirred layer adjacent to the cells. The CO2 concentration in this layer might thus be considerably lower than that in the bulk medium. Diffusion of HCO3 ions into this layer and its subsequent conversion to CO2 would then have great significance for photosynthesis. Experiments to be reported here were performed to elucidate the nature of the HCO3 contribution. MATERIALS AND METHODSMesophyll cells were mechanically isolated from the cladophylls (1) of Asparagus plants grown in the greenhouse. The cladophylls (1-2 g) were gently ground in 4 ml medium containing: Naascorbate (5 mM), (1% w/v), and 5 mm Hepes-NaOH (pH 7.2). Isolated cells were washed twice by centrifugation (200g) and resuspended in the same medium. The rate of 02 exchange was determined with an 02 electrode (Rank Brothers, Bottisham, Cambridge, U.K.).The photosynthetic accumulation of acid stable 14C as well as the intracellular level of inorganic carbon were estimated as described previously (16). Cells were placed in dim light and aerated with C02-free air to minimize the amount of Ci taken with the cell suspension. Twenty 1d of cell suspension were introduced into microfuge tubes containing (from bottom to top) 20 ,tl NaOH (3 M, killing medium); 60 ,l silicone oil layer composed of F50 and RTV910 (1:2 v/v, respectively, from Silicone Oil Division, Gen...

“…2 Abbreviations: 'P, water potential; A, area of plant element in plane perpendicular to flux; g, growth velocity vector; J, water flux vector; K, hydraulic conductivity tensor; L, relative elemental growth rate (local strain rate); 1, cell length; Q, volumetric flow rate; S, surface area of plant element; t, time; V, volume of plant element; x, distance; r, radial distance from root center line; z, longitudinal distance from root tip. (3) where J2 is the water flux at 2 and g2 is the growth velocity at 2.…”

Section: Derivation Of An Equation Relating Localmentioning

confidence: 99%

“…The element experiences a decrease and then an increase in Ik as it is displaced through the growth zone. These changes are shown for three possible values of K within the suggested range (2). The element has its most negative * at the location of fastest growth rate, 4 mm behind the tip, as shown in Figure 5.…”

mentioning

confidence: 96%

Growth-sustaining Water Potential Distributions in the Primary Corn Root

Silk

Wagner²

1980

( 15) and discuss a cell which is expanding only in the direction of its long axis and which is in contact with water at only one end. For this one-dimensional system (Fig. 1, left) to a first approximation, the rate of volume increase (dV/dt) equals the rate of volumetric water flow (Q) into the cell. Furthermore, Q is proportional to the water flux, and, equivalently, to the growth velocity.d= Q=AJ=Ag (1) where A is the cross-sectional area of the cell face perpendicular to the flow, g is the cell extension rate (growth velocity), and J is the water flux (volumetric flow rate/unit cross-sectional area). Now, diverging from the earlier theory, a cell embedded in a continuum of expanding cells is considered (Fig. 1, right). For this cell, only water which is moving faster than wall 1 will cross the wall and enter the cell. In this case, the amount of water which flows into the cell equals the area of wall 1 times the velocity by which the water flux exceeds the rate of movement of the wall:Flow into the cell = Q, = A(J, -gi) where gi is the growth velocity at 1; similarly Flow out of the cell = Q2 = A(J2 -g2)Much of the large literature on plant water transport concerns flow through nongrowing tissues and deals with the often large +2 gradients in the soil-plant-atmosphere continuum. Recently, Molz and Boyer (13) made a pioneering study of *P distributions which would characterize a growing tissue. They showed that 'I would become more negative along a line extending from a xylem element into the cortex of an elongating soybean hypocotyl. Below, an inhomogeneous distribution of growth rates is considered and a two-dimensional pattern of which can sustain growth of a corn root is found.'This research was supported by National Science Foundation Grant PCM 78-23710.2 Abbreviations: 'P, water potential; A, area of plant element in plane perpendicular to flux; g, growth velocity vector; J, water flux vector; K, hydraulic conductivity tensor; L, relative elemental growth rate (local strain rate); 1, cell length; Q, volumetric flow rate; S, surface area of plant element; t, time; V, volume of plant element; x, distance; r, radial distance from root center line; z, longitudinal distance from root tip.(3) where J2 is the water flux at 2 and g2 is the growth velocity at 2.Since water is highly incompressible, its velocity does not change along the one-dimensional continuum. Therefore J1 = J2. The net flow of water into the expanding cell is equal to the difference between the inflow and the outflow so that the net flow (A Q) into the cell equals the cross-sectional area times the difference in growth rates between the apical and basal end of the cell: AQ = A(g2-gg).(4) As in the simpler example above, the rate of volume change (dV/dt) in the deformable cell is equal to the net volumetric water flow into the cell: where n is the unit normal to the surface. Equation 5a shows that the rate of volume change of the cell which is instantaneously occupying the fixed volume V at a 859 (2) www.plantphysiol.org on May 9, 2018 -Pub...