Propagation of Turgor and Other Properties Through Cell Aggregations.

Experiments show that the rate of water uptake by living tissues external to mature xylem of cotton stems (Gossypiunt hirsutum L. Auburn 7-683) is very similar to the corresponding curves for leaf tissue. In both cases one obtains a twophase curve with phase I corresponding to passive rehydration and phase II pertaining to active growth.A theory of water movement in plant tissue first proposed by Philip allows one to make a more rigorous distinction than made previously between phase I and phase II. This theory is applied explicitly to water uptake by leaf disks and results in a simple expression for the time required for phase I completion. Because the time required varies as the square of the disk radius, it is essential to use a standad disk size in water uptake studies of a particular tissue.Additional analysis indicates that clear temporal distinction cannot be made between phase I and phase II. Different portions of the leaf disk rehydrate at significantly different rates, resulting in a grey zone with phase I and phase II occurring simultaneously in different parts of the disk.It has been reported that when a nonturgid leaf disk is floated on water, the water uptake curve (plot of water uptake versus time) is composed of two phases (1, 2, 5). According to the experiments and analysis of Barrs and Weatherley (1), phase I is in response to the initial water deficit, and phase IL results from the continued water uptake due to growth effects. It is not clear when rehydration ends and growth effects begin, and the present writers contend that such a separation cannot be made. For practical purposes, such as the determination of relative turgidity of leaves, it has been suggested that during the first 4 hr of the uptake process, rehydration is completed with little growth contribution (1). However, this must be recognized as a more or less arbitrary criterion.The first objective of the present paper is to describe experiments which show that a similar uptake phenomenon occurs in the rehydration of water-stressed cotton stems. A second objective is to discuss a theory of moisture movement in plant tissue which allows for a more rigorous distinction than that 'This study was supported in part by the Engineering Experi- made previously between phase I and phase II of the water uptake process. MATERIALS AND METHODSThe basic experimental procedure was to sever the stems of water-stressed cotton plants (Gossypium hirsutum L. Auburn 7-683) beneath degassed aqueous solutions, thereby releasing the tension in the xylem. Water would then begin to flow radially into the phloem and associated living tissues, causing them to swell (6,8). Except for the solutes present in the xylem sap and the geometrical-mechanical differences, this tension release process is analogous to that occurring when a nonturgid leaf disk is placed on a free water surface.The volumetric water uptake of the phloem and associated tissues was determined as a function of time by monitoring the stem diameter approximately 12 cm below the cut with...

Section: Discussionmentioning

confidence: 99%

Rehydration versus Growth-induced Water Uptake in Plant Tissues

Molz¹,

Klepper²,

Peterson³

1973

“…GROWTH RATE TO LOCAL ,I One can start with the approach of Philip ( 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.…”

Section: Derivation Of An Equation Relating Localmentioning

confidence: 99%

“…Local equilibrium between walls and cytoplasm is possible for many values of permeability, and Molz (12) has also shown that the diffusivity coefficient in the more complete and more complex model is numerically not too different from the diffusivity which would have been computed on Philip's (15) assumption of a noncompartmented cylinder. Thus, the treatment of the corn root as a noncompartmented system reported here can be expected to lead to reasonable predictions for growth-sustaining i' distributions.…”

mentioning

confidence: 99%

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...

“…Shoots were then covered with plastic bags to decrease transpiration, and the covering around the shoot was manipulated to control transpiration so that there was no change in stem diameter for 2 to 3 hr. This was sufficient time for water potential in the bark tissues to equilibrate with the low water potential in the xylem (13 (15). Data were analyzed to determine diffusivity using a semi-log plot of fractional volume increase versus time, described in detail in a previous paper (13).…”

Section: Methodsmentioning

confidence: 99%

Temperature Effects on Radial Propagation of Water Potential in Cotton Stem Bark

Klepper

Molz²,

Peterson³

1973

Low temperature affects the lateral movement of water across the xylem-phloem boundary in intact cotton stems. There is a reduction in the effective diffusion coefficient relating free energy flux to water potential gradients with an associated increase in resistance to water flow. Detached phloem and excised leaves do not show this effect of low temperature. Experiments on stem section halves indicate that the effect is probably associated with the cambial region.Recent experiments on water relations of cotton stems have shown that diurnal changes in stem diameter result almost entirely from lateral movement of water out of and into the phloem and associated living tissues external to mature xylem (12). This water movement can be considered to occur in response to water potential gradients in these tissues (11), and the pattern of stem diameter change brought about by a change in xylem water potential depends in part on the diffusivity for free energy transfer in these living tissues. The tissues of concern are largely phloem and cambium but may include greater or lesser amounts of young xylem elements which still have living protoplasts. For the sake of brevity, the term "bark" will be used to describe the tissues being measured. The "bark" unless otherwise stated includes the vascular cambium, primary and secondary phloem, cortex, and periderm. A distinction has been made (13) between D, the diffusion coefficient relating free energy flux to the gradient in water potential, and D,,, a similar coefficient relating water flux to water potential gradient. According to the theory of Philip (15), these two coefficients are proportional to each other and measurements of the increase in bark volume during hydration can be used to estimate D, the apparent diffusivity of the bark. During rehydration of a droughted stem, the curve relating bark volume to time is two-phased, similar to curves obtained for leaf discs, with a passive rehydration phase preceding an active growth phase (14). The temperature of 31 C selected for our previously reported experiments on cotton stem rehydration was chosen because it approximates midsummer field conditions. The experiments reported here were done at several tempera-'This study was tures in order to determine the effect of temperature on rehydration characteristics of droughted cotton stem tissues. MATERIALS AND METHODSMethods for Experiments on Intact Plants. Soil-grown cotton plants (Gossypium hirsutum L. Auburn 7-683) were raised in a growth chamber at 31 C/21 C on a 12-hr photoperiod for approximately 3 months. The experiments were done in a series of growth chambers at different temperatures. For about 24 hr before an experiment, plants were equilibrated at the experimental temperature, with the exception that plants were equilibrated at 8 C only a few hours to preclude tissue damage by such a low temperature. Several hours before each experiment, a plastic reservoir was attached around the stem so that the main stem could be severed under water held in the reservoi...