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Kenneth A. Shackel - One of the best experts on this subject based on the ideXlab platform.
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seasonal pattern of apoplastic solute accumulation and loss of Cell Turgor during ripening of vitis vinifera fruit under field conditions
Journal of Experimental Botany, 2009Co-Authors: Hiroshi Wada, Mark A Matthews, Kenneth A. ShackelAbstract:Using a novel pressure membrane (PM) apparatus for the extraction of apoplastic fluid from field-grown grape (Vitis vinifera L.) berries, our hypothesis that significant apoplast solutes accumulate at the beginning of the ripening process (i.e. veraison), and that this accumulation might contribute to progressive berry softening due to a progressive loss of mesocarp Cell Turgor pressure (P) was tested. It was necessary to correct the solute potential (Ψs) of fluid collected with the PM for dilution due to the presence of a dead volume in the apparatus, but after correction, the Ψs obtained with the PM agreed with that obtained by low speed centrifugation. A clear decline in fruit apoplastic solute potential (ψSA) began approximately 10 d prior to fruit coloration, and it was found to be coincident with a decline in mesocarp Cell P and fruit elasticity (E). By late in fruit development when berry growth ceased (90 d after anthesis), both apoplast and fruit Ψs reached almost –4 MPa. These results support the hypothesis that a decrease in ψSA is responsible for the observed loss in mesocarp Cell P, and is the mechanistic cause of berry softening.
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direct in situ measurement of Cell Turgor in grape vitis vinifera l berries during development and in response to plant water deficits
Plant Cell and Environment, 2006Co-Authors: Tyler R. Thomas, Mark A Matthews, Kenneth A. ShackelAbstract:Vitis vinifera L. berries are non-climacteric fruits that exhibit a double-sigmoid growth pattern, and at the point known as 'veraison', which is just before the beginning of the second period of rapid fruit growth, these berries undergo several abrupt physiological changes. Cell pressure probe was used to examine the in situ Turgor (P) of Cells in the mesocarp during berry development and in response to plant water deficits. Initial tests comparing attached and detached berries demonstrated that Cell P was stable for up to 48 h after detachment from the vine, provided that water loss from the berry was prevented. Cell P at pre-dawn was on the order of 0.25 MPa pre-veraison (PreV) and was reduced by an order of magnitude to 0.02 MPa post veraison (PostV). Cell P declined slightly but significantly with depth from the berry surface PreV, but not PostV. When water was withheld from potted vines, Cell P declined about 0.2 Mpa, as pre-dawn vine water potential declined about 0.6 MPa over 12 d, whereas Cell P was completely insensitive to a 1.10 MPa decrease in pre-dawn vine water potential after veraison. Rewatering of stressed plants also resulted in a 24 h recovery of Cell P before, but not after veraison. The substantial decline in Cell P around veraison is consistent with the decline in berry firmness that is known to occur at this time, and the PostV insensitivity of P to changes in vine water status is consistent with current hypotheses that the PostV berry is hydraulically isolated from the vine. The fact that a measurable P of about 0.02 MPa and typical Cell hydraulic/osmotic behaviour were exhibited in PostV berries, however, indicates that Cell membranes remain intact after veraison, contrary to many current hypotheses that veraison is associated with a general loss of membrane function and Cellular compartmentation in the grape berry. We hypothesize that Cell P is low in the PostV berry, and possibly other fleshy fruits, because of the presence of regulated quantities of apoplastic solutes.
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seed coat Cell Turgor in chickpea is independent of changes in plant and pod water potential
Journal of Experimental Botany, 2000Co-Authors: Kenneth A. Shackel, Neil C TurnerAbstract:maintain carbon import to the developing embryo. This Turgor–homeostat model (Patrick and Offler, 1995) proTurgor pressure in Cells of the pod wall and the seed poses that above some minimum level of seed coat Cell coat of chickpea (Cicer arietinum L.) were measured Y p , the rate of assimilate unloading (efflux) from the seed directly with a pressure probe on intact plants under coat Cells into the embryonic and seed coat apoplast is initially dry soil conditions, and after the plants were positively correlated with the Y p of the seed coat Cells. irrigated. The Turgor pressure in Cells of the pod wall Hence if the assimilate demand by the embryo increases, was initially 0.25 MPa, and began to increase within a the concentration of solutes in the apoplast will decrease, few minutes of irrigation. By 2–4 h after irrigation, pod leading to an increase in the Y p of the seed coat Cells and wall Cell Turgor had increased to 0.97 MPa. This a corresponding increase in the rate of assimilate efflux increase in Turgor was matched closely by increases from the seed coat Cells. This increased efflux could in the total water potential of both the pod and the regulate both the apoplastic Y s and the seed coat Cell Y p stem, as measured by a pressure chamber. However, back to approximately the initial level (Patrick and Offler, Turgor pressure in Cells of the seed coat was relatively 1995). Consistent with this hypothesis, it has been low (0.10 MPa) and was essentially unchanged up to reported that the Y p of seed coat Cells which were bathed 24 h after irrigation (0.13 MPa). These data demon- in a solution initially responded to a step change in the strate that water exchange is relatively efficient Y s of the bathing solution, but then returned within throughout most of the plant body, but not between about 20 min to the initial level of Y p ( Thorpe et al., the pod and the seed. Since both the pod and the seed 1993; Zhang et al., 1996). In both cases the measurements coat are vascularized tissues of maternal origin, this were made on surgically modified seed coats using the indicates that at least for chickpea, isolation of the empty seed coat ( Thorne and Rainbird, 1983) or a similar water relations of the embryo from the maternal plant technique, and the step changes in Y
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Seed coat Cell Turgor responds rapidly to air humidity in chickpea and faba bean
Journal of Experimental Botany, 1998Co-Authors: Kenneth A. Shackel, Neil C TurnerAbstract:The Turgor pressure in Cells of chickpea (Cicer arietinum L.) and faba bean (Vicia faba L.) seed coats was measured with a pressure probe. Measurements were made under in situ conditions by removing a section of wall from a pod, which remained attached to the plant, and exposing the intact seed. If the pod wall was removed and the Turgor measurements made under ambient laboratory conditions of 50% to 70% relative humidity (RH), Cell Turgor pressure declined over time, typically reaching 0 MPa. If the pod wall was removed and the Turgor measurements made under conditions of 100% RH, however, Cell Turgor pressure was stable over time, relatively uniform within the seed coat tissue, and was found to be 0.1-0.3 MPa for chickpea, and 0.1-0.2 MPa for faba bean. In both species there was a marked decline in Cell Turgor, beginning within about 60 s, when humidification was discontinued. The decline in Cell Turgor occurred regardless of the depth of the Cell within the seed coat tissue, and this decline could be stopped, but not entirely reversed, when humidification was restored. An increase in Cell Turgor could also be caused by wetting of the seed. These responses indicate that a very rapid water exchange can occur within the seed coat tissue in situ, The rapid and, in some cases, relatively permanent loss of seed coat Cell Turgor in the absence of humidification raises serious concerns regarding desiccation artefacts which may be involved in the empty seed coat technique, often used to study seed carbon and water relations in grain legumes.
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Cell Turgor changes associated with ripening in tomato pericarp tissue
Plant Physiology, 1991Co-Authors: Kenneth A. Shackel, Carl Greve, John M Labavitch, Hamid AhmadiAbstract:The pressure microprobe was used to determine whether the Turgor pressure in tomato (Lycopersicon esculentum Mill., variety "Castelmart") pericarp Cells changed during fruit ripening. The Turgor pressure of Cells located 200 to 500 micrometers below the fruit epidermis was uniform within the same tissue (typically +/- 0.02 megapascals), and the highest Turgors observed (<0.2 megapascals) were much less than expected, based on tissue osmotic potential (-0.6 to -0.7 megapascals). These low Turgor values may indicate the presence of apoplastic solutes. In both intact fruit and cultured discs of pericarp tissue, a small increase in Turgor preceded the onset of ripening, and a decrease in Turgor occurred during ripening. Differences in the Turgor of individual intact fruit occurred 2 to 4 days before parallel differences in their ripening behavior were apparent, indicating that changes in Turgor may reflect physiological changes at the Cell level that precede expression of ripening at the tissue level.
K. Köhler - One of the best experts on this subject based on the ideXlab platform.
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Cell elongation Turgor and osmotic pressure in developing sunflower hypocotyls
Journal of Experimental Botany, 1994Co-Authors: Ulrich Kutschera, K. KöhlerAbstract:The relationship between Cell elongation, change in Turgor and Cell osmotic pressure was investigated in the sub-apical region of hypocotyls of developing sunflower seedlings (Helianthus annuus L.) that were grown in continuous white light. Cell Turgor was measured with the pressure probe. The same hypocotyl sections were used for determination of osmotic pressure of the tissue sap. Acceleration of Cell elongation during the early phase of growth was accompanied by a 25% decrease in both Turgor and osmotic pressure. During the linear phase of growth both pressures remained largely constant. The difference between Turgor and osmotic pressure (water potential) was -0.10 to -0.13 MPa
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Turgor pressure and elongation growth in developing sunflower hypocotyls
Journal of Plant Physiology, 1993Co-Authors: Ulrich Kutschera, K. KöhlerAbstract:Summary The average Turgor pressure of hypocotyl Cells of 2-day-old etiolated sunflower seedlings ( Helianthus annuus L.) was 0.59 MPa. Turgor pressure decreased during acceleration of growth in darkness and reached a value of 0.5 MPa at days 4 and 5 after sowing (linear phase of growth). Cessation of hypocotyl elongation at day 6 was accompanied by a 40 % drop in Cell Turgor pressure. We suggest that this loss of Cell Turgor is the cause of the decline in hypocotyl growth in darkness. Irradiation of the etiolated seedlings with white light inhibited hypocotyl growth, but Turgor pressure was unaffected.
U Zimmerniann - One of the best experts on this subject based on the ideXlab platform.
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diurnal changes in xylem pressure and mesophyll Cell Turgor pressure of the lianatetrastigma voinierianum the role of Cell Turgor in long distance water transport
Protoplasma, 1999Co-Authors: Frank Thurmer, Rainer Benkert, P Gesner, Notburga Gierlinger, B Herrmann, Friedrich-wilhelm Bentrup, H. Schneider, U ZimmerniannAbstract:Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the Cell Turgor and the osmotic pressure of leaf Cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 ∶ 1 decrease in the Cell Turgor pressure of the mesophyll Cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the Cell Turgor pressure (ratio of about 1 ∶ 1). The Cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the Turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and Cell Turgor pressure. From the xylem, Cell Turgor, and Cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the Turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero Turgor pressure.
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diurnal changes in xylem pressure and mesophyll Cell Turgor pressure of the liana tetrastigma voinierianum the role of Cell Turgor in long distance water transport
Protoplasma, 1999Co-Authors: Frank Thurmer, Rainer Benkert, P Gesner, Notburga Gierlinger, B Herrmann, Friedrich-wilhelm Bentrup, H. Schneider, J J Zhu, U ZimmerniannAbstract:Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the Cell Turgor and the osmotic pressure of leaf Cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 ∶ 1 decrease in the Cell Turgor pressure of the mesophyll Cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the Cell Turgor pressure (ratio of about 1 ∶ 1). The Cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the Turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and Cell Turgor pressure. From the xylem, Cell Turgor, and Cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the Turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero Turgor pressure.
Ulrich Kutschera - One of the best experts on this subject based on the ideXlab platform.
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Tissue Pressure and Cell Turgor in Axial Plant Organs: Implications for the Organismal Theory of MultiCellularity
Journal of Plant Physiology, 1995Co-Authors: Ulrich KutscheraAbstract:Summary It has been postulated that the rate of growth of axial plant organs (stems, coleoptiles) is controlled by wall-loosening processes that are largely restricted to the expansion-limiting outer epidermal wall (OEW). If this hypothesis is correct it follows that the hydrostatic pressure exerted by the thin-walled, extensible inner tissues against the OEW (tissue pressure) must be similar to the average Turgor of the internal Cells. Experiments with 5 different plant organs (sunflower hypocotyl, Helianthus annuus L.; zucchini hypocotyl, Cucurbita pepo L.; pea epicptyl, Pisum sativum L.; oat coleoptile, Avena sativa L.; maize coleoptile, Zea mays L.) have shown that tissue pressure, estimated with an osmotic equilibrium method, and Cell Turgor, as measured with the pressure probe, are of very similar quantities. The osmotic pressure of the tissue sap was significantly larger than tissue pressure and Cell Turgor. It is concluded that the axial organ consists of a unified, turgid protoplast that is chambered into individual Cells (inner tissues) and a thick, growth-controlling, supraCellular organ wall (OEW) that bears the longitudinal wall stresses of the internal Cells. The results support the organismal concept of multiCellularity in plants.
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Cell elongation Turgor and osmotic pressure in developing sunflower hypocotyls
Journal of Experimental Botany, 1994Co-Authors: Ulrich Kutschera, K. KöhlerAbstract:The relationship between Cell elongation, change in Turgor and Cell osmotic pressure was investigated in the sub-apical region of hypocotyls of developing sunflower seedlings (Helianthus annuus L.) that were grown in continuous white light. Cell Turgor was measured with the pressure probe. The same hypocotyl sections were used for determination of osmotic pressure of the tissue sap. Acceleration of Cell elongation during the early phase of growth was accompanied by a 25% decrease in both Turgor and osmotic pressure. During the linear phase of growth both pressures remained largely constant. The difference between Turgor and osmotic pressure (water potential) was -0.10 to -0.13 MPa
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Turgor pressure and elongation growth in developing sunflower hypocotyls
Journal of Plant Physiology, 1993Co-Authors: Ulrich Kutschera, K. KöhlerAbstract:Summary The average Turgor pressure of hypocotyl Cells of 2-day-old etiolated sunflower seedlings ( Helianthus annuus L.) was 0.59 MPa. Turgor pressure decreased during acceleration of growth in darkness and reached a value of 0.5 MPa at days 4 and 5 after sowing (linear phase of growth). Cessation of hypocotyl elongation at day 6 was accompanied by a 40 % drop in Cell Turgor pressure. We suggest that this loss of Cell Turgor is the cause of the decline in hypocotyl growth in darkness. Irradiation of the etiolated seedlings with white light inhibited hypocotyl growth, but Turgor pressure was unaffected.
H. Schneider - One of the best experts on this subject based on the ideXlab platform.
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diurnal changes in xylem pressure and mesophyll Cell Turgor pressure of the lianatetrastigma voinierianum the role of Cell Turgor in long distance water transport
Protoplasma, 1999Co-Authors: Frank Thurmer, Rainer Benkert, P Gesner, Notburga Gierlinger, B Herrmann, Friedrich-wilhelm Bentrup, H. Schneider, U ZimmerniannAbstract:Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the Cell Turgor and the osmotic pressure of leaf Cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 ∶ 1 decrease in the Cell Turgor pressure of the mesophyll Cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the Cell Turgor pressure (ratio of about 1 ∶ 1). The Cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the Turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and Cell Turgor pressure. From the xylem, Cell Turgor, and Cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the Turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero Turgor pressure.
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diurnal changes in xylem pressure and mesophyll Cell Turgor pressure of the liana tetrastigma voinierianum the role of Cell Turgor in long distance water transport
Protoplasma, 1999Co-Authors: Frank Thurmer, Rainer Benkert, P Gesner, Notburga Gierlinger, B Herrmann, Friedrich-wilhelm Bentrup, H. Schneider, J J Zhu, U ZimmerniannAbstract:Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the Cell Turgor and the osmotic pressure of leaf Cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 ∶ 1 decrease in the Cell Turgor pressure of the mesophyll Cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the Cell Turgor pressure (ratio of about 1 ∶ 1). The Cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the Turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and Cell Turgor pressure. From the xylem, Cell Turgor, and Cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the Turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero Turgor pressure.
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xylem and Cell Turgor pressure probe measurements in intact roots of glycophytes transpiration induces a change in the radial and Cellular reflection coefficients
Plant Cell and Environment, 1997Co-Authors: H. Schneider, U ZimmermannAbstract:Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCI or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σ r ) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σ r assumed values of the order of about 0.2-0.4. The σ r values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m -2 s -1 ; relative humidity 20-40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σ r = 1. Further increase of the light intensity to about 400 μmol m -2 s -1 resulted in a slight but significant decrease of the σ r values to about 0.8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906-912) that, in intact transpiring plants at low light intensities of about 10 μmol m -2 s -l and at relative humidities of 20-40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σ r values 0.3-0.5). In contrast to wheat and barley, very high light intensities (about 700 μmol m -2 s -1 ) were needed to shift the radial reflection coefficients of maize roots to values of about 0.9. Osmotically induced xylem pressure changes were apparently linked to changes in Turgor pressure in the root cortical parenchyma Cells, as shown by simultaneous measurements of xylem and Cell Turgor pressure. In analogy to the σ r values of the respective glycophytes, the σ C values of the root cortical Cells of wheat and barley were close to unity, whereas σ C for maize was significantly smaller (about 0.7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m -2 s -1 , the Cellular reflection coefficient of maize roots increased to about 0.95. In contrast to the σ r values, the σ C values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the Cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly in terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21-31). The farreaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.