A root pressure probe was employed to measure hydraulic properties of primary roots of maize (Zea mays L.). The hydraulic conductivity (Lp,) of intact root segments was determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. In hydrostatic experiments, Lp, was constant along the segment except for an apical zone of approximately 20 millimeters in length which was hydraulically isolated due to a high axial resistance. In osmotic experiments, Lp, decreased toward the base of the roots. Lp, (osmotic) was significantly smaller than Lp, (hydrostatic). At various distances from the root tip, the axial hydraulic resistance per unit root length (Rx) was measured either by perfusing excised root segments or was estimated according to Poiseuille's law from cross-sections. The calculated RX was smaller than the measured RX by a factor of 2 to 5. Axial resistance varied with the distance from the apex due to the differentiation of early metaxylem vessels. Except for the apical 20 millimeters, radial water movement was limiting water uptake into the root. This is important for the evaluation of Lp, of roots from root pressure relaxations. Stationary water uptake into the roots was modeled using measured values of axial and radial hydraulic resistances in order to work out profiles of axial water flow and xylem water potentials.
Transient responses of cell turgor ( P ) and root elongation to changes in water potential were measured in maize (Zea mays 1.) to evaluate mechanisms of adaptation to water stress. Changes of water potential were induced by exposing roots to solutions of KCI and mannitol (osmotic pressure about 0.3 MPa). Prior to a treatment, root elongation was about 1.2 mm h-' and P was about 0. 67 MPa across the cortex of the expansion zone (3-10 mm behind the root tip). Upon addition of an osmoticum, P decreased rapidly and growth stopped completely at pressure below approximately 0.6 MPa, which indicated that the yield threshold (YtranS,') was just below the initial turgor. Turgor recovered partly within the next 30 min and reached a new steady value at about 0.53 MPa. lhe root continued to elongate as soon as P cose above a new threshold (Ytran,,J of about 0.45 MPa. The time between Yt,ans,l and Vtrans,, was about 10 min. During this transition turgor gradients of as much as 0.15 MPa were measured across the cortex. They resulted from a faster rate of turgor recovery of cells deeper inside the tissue compared with cells near the root periphery. Presumably, the phloem was the source of the compounds for the osmotic adjustment. Turgor recovery was restricted to the expansion zone, as was confirmed by measurements of pressure kinetics in mature root tissue. Withdrawal of the osmoticum caused an enormous transient increase of elongation, which was related to only a small initial increase of P. Throughout the experiment, the relationship between root elongation rate and turgor was nonlinear. Consequently, when Y were calculated from steady-state conditions of P and root elongation before and after the osmotic treatment, Y, was only 0.21 MPa and significantly smaller compared with the values obtained from direct measurements (0.42-0.64 MPa). Thus, we strongly emphasize the need for measurements of short-term responses of elongation and turgor to determine cell wall mechanics appropriately. Our results indicate that the rate of solute flow into the growth zone could become rate-limiting for cell expansion under conditions of mild water stress.In Lockhart's model of plant expansive growth (Lockhart, 1965;Boyer, 1985), P in excess of a Y is the driving force for irreversible cell expansion; the rate of how fast a cell expands per unit of pressure above Y is determined by the m ("cell wall extensibility"). Commonly, the effect of water stress (reduction in q ) on Y and m is evaluated from the analysis of steady turgor and growth rates before and after a perturbation of the environment. This approach has been chal- Sharp et al., 1988; Spollen and Sharp, 1991) and leaves (Saab et al., 1992) illustrate the complexity of modeling total plant growth based on single-cell models such as that of Lockhart.Under steady-state conditions of growth, P remains constant in growing roots of wheat (Pritchard et al., 1989(Pritchard et al., , 1991 Tomos et al., 1989) and maize (Pritchard et al., 1990a; Spollen and Sharp, 1991). The adaptati...
The present state of modelling of water transport across plant tissue is reviewed. A mathematical model is presented which incorporates the cell-to-cell (protoplastic) and the parallel apoplastic path. It is shown that hydraulic and osmotic properties of the apoplast may contribute substantially to the overall hydraulic conductivity of tissues (Lpr) and reflection coefficients (Crsr). The model shows how water and solutes interact with each other during their passage across tissues which are considered as a network of hydraulic resistors and capacitances ('composite transport model'). Emphasis is on the fact that hydraulic properties of tissues depend on the nature of the driving force. Osmotic gradients cause a much smaller tissue Lpr than hydrostatic. Depending on the conditions, this results in variable hydraulic resistances of tissues and plant organs. For the root, the model readily explains the well-known phenomenon of variable hydraulic resistance for the uptake of water and non-linear force/flow relations. Along the cell-to-cell (protoplastic) path, water flow may be regulated by the opening and closing of selective water channels (aquaporins) which have been shown to be affected by different environmental factors.
Responses of cortical cell turgor ( P ) following rapid changes in osmotic pressure (m,,,) were measured throughout the elongation zone of maize (Zea mays 1.) roots using a cell pressure probe and compared with simultaneously measured root elongation to evaluate: yield threshold ( Y ) (minimum P for growth), wall extensibility, growth-zone radial hydraulic conductivity (K), and turgor recovery rate. Small increases in rr,,, (0.1 MPa) temporarily decreased P and growth,;which recovered fully in 5 to 10 min. Under stronger rr,,, Roots adjust their growth rate when subjected to water stress. The phenomenon is commonly studied using Lockhart's concept of growth, which relates growth to cell wall mechanics and hydraulic and osmotic properties of the growing tissue (Lockhart, 1965;Hsiao et al., 1976;Boyer, 1985;Cosgrove, 1986). Of the parameters in the Lockhart equations, the yield threshold Y and wall extensibility m are often evaluated from plots (usually linear) of growth rates versus P, with Y representing the intercept on the P axis and m representing the slope for roots (Pritchard et al., 1990b) as well as leaves (Bunce, 1977;Hsiao and Jing, 1987 , 1990a, 1991, 1993Spollen and Sharp, 1991). Indeed, early work on algae (Green et al., 1971) and more recently on roots (Hsiao and Jing, 1987;Frensch and Hsiao, 1994) showed rapid changes of Y with water stress. Measurements of P and root growth during osmotic transitions revealed values of Y much closer to P (Frensch and Hsiao, 1994). This indicates that m is large and factors different from changes in m could be equally or more important in determining rates of elongation. The rate of cell expansion along the elongation zone is not uniform. For maize (Zea mays L.) roots, local relative Abbreviations: A, surface area (m'); AT,,,, change of osmotic pressure in the medium (MPa); E, volumetric elastic coefficient (MPa); ER, elongation rate (mm h-'); k, rate constant of water exchange (s-'); K, tissue hydraulic conductivity coefficient (m2 s-' MPa-I); LRER, local relative elongation rate (h-'); LVDT, linear variable differential transducer; m, cell wall extensibility (MPa-' h-'); P, cell turgor (MPa); Pmin (P,,,), minimum (maximum) turgor measured during the response of P to a decrease (increase) in V of the medium; ( P -Y), growth-effective turgor; T, osmotic pressure (MPa); V, water potential (MPa); r, radius (mm); t,,2, half-time of water exchange (s); V, volume (m3); Y, yield threshold (MPa); Ytrans,', Y evaluated from the turgor decline phase during the biphasic pressure-response curve; Ytrans,2, similar to Ytrans,' but evaluated from the turgor recovery instead of the decline phase; z, length of the root elongation zone; subscript "c" associates a parameter with a cell; subscript "m" associates a parameter with the medium; subscript "r" associates a parameter with the root; subscript "t" indicates the dependency of a parameter on time; subscript "z" associates a parameter with respect to its position along the elongation zone.www.plantphysiol.org on May 12, 2018 -P...
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