Effects of Osmotic Pressure on Exudation from Corn Roots

“…There are two major problems involved. First, there is no way at present to account for either standing gradient effects (1) or for reabsorption of solutes from the xylem (17). Both Figure 9 shows the relative contributions of each of the five parameters over the pressure range.…”

Section: Resultsmentioning

confidence: 99%

Diurnal Changes in Volume and Solute Transport Coefficients of Phaseolus Roots

Fiscus¹

1986

Plant Physiol.

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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)

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“…The often quoted suggestion (Klepper, 1967;Kramer and Boyer, 1995) that there is a gradient of osmotic concentration (high at the tip and diluted toward the base) along the xylem lumens in the roots of a guttating plant so that the xylem sap is pressurized osmotically at the distal ends of the roots is not applicable to our observations, nor are those of McCully et al (1998), where refilling, also near the base of the roots, was always observed to be through the lateral walls of the vessels and not basipetally from the root tips (Fig. 1, C-F).…”

Section: Is the Refilling Osmotic?mentioning

confidence: 99%

Root Xylem Embolisms and Refilling. Relation to Water Potentials of Soil, Roots, and Leaves, and Osmotic Potentials of Root Xylem Sap1

McCully

1999

Plant Physiology

124

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Embolism and refilling of vessels was monitored directly by cryomicroscopy of field-grown corn (Zea mays L.) roots. To test the reliability of an earlier study showing embolism refilling in roots at negative leaf water potentials, embolisms were counted, and root water potentials (⌿ root ) and osmotic potentials of exuded xylem sap from the same roots were measured by isopiestic psychrometry. All vessels were full at dawn (⌿ root ؊0.1 MPa). Embolisms were first seen in late metaxylem vessels at 8 AM. Embolized late metaxylem vessels peaked at 50% at 10 AM (⌿ root ؊0.1 MPa), fell to 44% by 12 PM (⌿ root ؊0.23 MPa), then dropped steadily to zero by early evening (⌿ root ؊0.28 MPa). Transpiration was highest (8.5 g cm ؊2 s ؊1 ) between 12 and 2 PM when the percentage of vessels embolized was falling. Embolized vessels were refilled by liquid moving through their lateral walls. Xylem sap was very low in solutes. The mechanism of vessel refilling, when ⌿ root is negative, requires further investigation. Daily embolism and refilling in roots of wellwatered plants is a normal occurrence and may be a component of an important hydraulic signaling mechanism between roots and shoots.Xylem vessels in the roots of corn (Zea mays) embolize and refill daily . At dawn, in plants growing in moist soil in the field, all vessels were sap filled, but by 1 to 2 h after sunrise embolisms began to form in the large, LMX vessels. The proportion of embolized, LMX vessels peaked in the middle of the day at values between 70% and 80%. The percentage declined beginning from mid-to late-afternoon, and reached 0% in most roots by sunset. At all times while embolisms were present some of the empty vessels appeared to be refilling, with liquid entering them from the side. The number of refilling vessels increased as the afternoon progressed. ⌿ leaf s (as determined by pressure-chamber measurement) were less negative (Ϫ0.3 to Ϫ0.4 MPa) when root embolism began in the morning, and more negative (Ϫ0.5 to Ϫ1.2 MPa) when embolism was decreasing during the afternoon. Thus, vessels in the roots were refilling when the water potential in the leaves was Ϫ0.5 to Ϫ1.2 MPa.The customary interpretation is that values for water potentials of plant tissues (determined either by pressure chamber or psychrometer) are also values for the tensions (negative pressures) in the sap-filled tracheary elements.Some drop in tension of xylem sap columns would be expected between the corn leaves and the roots. Nevertheless, in the corn plants measured by McCully et al. (1998), embolized vessels (at atmospheric pressure) in the roots were refilling, whereas intact sap columns in nearby vessels were apparently under considerable tension, as the plants were transpiring.One possible explanation of these discordant data might be that the samples of roots, the embolisms of which were counted, were not representative of the whole population of roots on a plant. They might, for example, have different values of water potential. A second explanation might be that the ⌿ leaf were ...

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Effects of Osmotic Pressure on Exudation from Corn Roots

Cited by 38 publications

References 15 publications

Determination of Hydraulic and Osmotic Properties of Soybean Root Systems

Determination of Hydraulic and Osmotic Properties of Soybean Root Systems

Diurnal Changes in Volume and Solute Transport Coefficients of Phaseolus Roots

Root Xylem Embolisms and Refilling. Relation to Water Potentials of Soil, Roots, and Leaves, and Osmotic Potentials of Root Xylem Sap1

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