Some evidence for the existence of turgor pressure gradients in the sieve tubes of willow

Rogers, Sharon A.; Peel, A. J.

doi:10.1007/bf00388967

Cited by 13 publications

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“…A number of attempts have been made to determine whether the pressure gradients found in the sieve elements are of sufficient magnitude to drive mass flow (5,7,8,10,(15)(16)(17)19). In these studies, two methods have been utilized to evaluate sieve-tube turgor and its gradient.…”

Section: Introductionmentioning

confidence: 99%

“…In these studies, two methods have been utilized to evaluate sieve-tube turgor and its gradient. (a) Turgor pressure can be calculated from other parameters of the system, specifically, water potential of the phloem and osmotic potential of the sieve-tube sap (5,8,10,15). (b) Turgor pressure can be measured directly (1, 2, 7,16,17,19).…”

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confidence: 99%

“…Since an 4, measured in this manner is an average for the entire phloem, it is likely that turgor pressures calculated from these values reflect an average phloem turgor, rather than turgor in the sieve elements alone. Rogers and Peel (15) repeated these experiments in 1975, using small willow trees and cuttings. While 4 of the phloem was measured in the same way as in the previous study, 4,, was determined from sieve-tube sap, collected either from severed aphid stylets or from incisions made in the phloem.…”

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confidence: 99%

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Direct and Indirect Measurements of Phloem Turgor Pressure in White Ash

Sovonick-Dunford

Lee²,

Zimmermann³

1981

Plant Physiol.

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Direct determinations and indirect calculations of phloem turgor pressure were compared in white ash (Fraxinus americana L.). Direct measurements of trunk phloem turgor were made using a modified Hammel-type phloem needle connected to a pressure transducer. Turgor at the site of the direct measurements was calculated from the osmotic potential of the phloem sap and from the water potential of the xylem. It was assumed that the water potentials of the phloem and xylem were close to equilibrium at any one trunk location, at least under certain conditions. The water potential of the xylem was determined from the osmotic potential of xylem sap and from the xylem tension of previously bagged leaves, measured with a pressure chamber. The xylem tension of bagged leaves on a branch adjacent to the site of the direct measurements was considered equivalent to the xylem tension of the trunk at that point. While both the direct and indirect measurements of phloem turgor showed clear diurnal changes, the directly measured pressures were consistently lower than the calculated values. It is not clear at present whether the discrepancy between the two values lies primarily in the calculated or in the measured pressures, and thus, the results from both methods as described here must be regarded as estimates of true phloem turgor.A knowledge of the turgor pressure gradients which exist in the sieve elements of higher plants is essential to any evaluation of the pressure flow mechanism of phloem translocation. This mechanism, first proposed by Munch in 1930 (14), states that phloem transport consists of a mass flow of solutes and water, driven by an osmotically generated pressure gradient in the sieve elements.A number of attempts have been made to determine whether the pressure gradients found in the sieve elements are of sufficient magnitude to drive mass flow (5,7,8,10,(15)(16)(17)19). In these studies, two methods have been utilized to evaluate sieve-tube turgor and its gradient. (a) Turgor pressure can be calculated from other parameters of the system, specifically, water potential of the phloem and osmotic potential of the sieve-tube sap (5,8,10,15). (b) Turgor pressure can be measured directly (1, 2,7,16,17,19). Kaufmann and Kramer in 1967 (10) samples, 4, of the same discs was determined. Since an 4, measured in this manner is an average for the entire phloem, it is likely that turgor pressures calculated from these values reflect an average phloem turgor, rather than turgor in the sieve elements alone. Rogers and Peel (15) repeated these experiments in 1975, using small willow trees and cuttings. While 4 of the phloem was measured in the same way as in the previous study, 4,, was determined from sieve-tube sap, collected either from severed aphid stylets or from incisions made in the phloem. Turgor calculated from these measurements is more likely to estimate sieve-tube turgor specifically. Using this refined technique, Rogers and Peel demonstrated that calculated sieve-tube turgor is higher at the apex than at ...

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Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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Direct and Indirect Measurements of Phloem Turgor Pressure in White Ash

Sovonick-Dunford

Lee²,

Zimmermann³

1981

Plant Physiol.

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“…Such calculations have yielded values ranging from 1 to 15 bars (e.g. 3,7,16). A somewhat more direct approach has been to calculate the turgor from the exudation rate from severed aphid stylets, using Poiseuille's equation.…”

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confidence: 99%

“…A somewhat more direct approach has been to calculate the turgor from the exudation rate from severed aphid stylets, using Poiseuille's equation. This method has yielded values ranging from 8 to 40 bars (1,11,16,19), but is fairly inaccurate, since the channel is tapered (11) proportional to the fourth power of its radius. In 1968, Hammel (4) introduced a "phloem needle" technique for the direct measurement of sieve tube turgor and found pressures ranging from 7 to 24 bars at various heights in a red oak tree.…”

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Direct Measurement of Sieve Tube Turgor Pressure Using Severed Aphid Stylets

Wright

Fisher²

1980

Plant Physiol.

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Turgor pressure in individual sieve tubes was measured directly by gluing capillary micromanometers over exuding aphid stylets with cyanoacrylate adhesive. Pressures of up to 10 bars were measured in sieve tubes of Salix babylonica, with an estimated accuracy of ± 03 bars or better.For comparison with the direct measurements of sieve tube turgor, calculated values of turgor pressure were also obtained from the difference between leaf water potential and phloem exudate solute potential, estimated from its refractive index and sucrose content. In most cases the measured turgor pressure was greater than the calculated value. The discrepancy between the two values was most likely due to the presence of appreciable concentrations of potassium and amino acids in the phloem exudate.Sieve tube turgor plays a central role in the physiology of phloem transport. Movement of solutes along the sieve tubes is almost certainly driven by osmotically generated pressure flow (3,6,10). Measurements of the turgor pressure are needed to estimate this driving force and its variation under different conditions. Also, although the evidence is fragmentary, several observations support the suggestion by Peel and Hoad (13) proportional to the fourth power of its radius. In 1968, Hammel (4) introduced a "phloem needle" technique for the direct measurement of sieve tube turgor and found pressures ranging from 7 to 24 bars at various heights in a red oak tree. Sheikholeslam and Currier (17) used Hammel's technique to measure sieve tube turgors of 0.3-10 bars in the squirting cucumber, Ecballium.The above techniques suffer various shortcomings. The first two are indirect and may be fairly inaccurate, while the phloem needle technique destroys the sieve tubes, allowing only a single measurement. The method presented in this report allows the direct measurement of sieve tube turgor over extended periods of time in intact, functioning sieve tubes. MATERIALS AND METHODSColonies of the willow aphid (Tuberolachnus salignus Gmelin) were maintained on weeping willow saplings (Salix babylonica L.) in a growth chamber on a 17-h photoperiod with 340 ,tE m-2 s-1 illumination (400-700 nm). Day temperature was 25 C and night temperature was 20 C.Plants used for turgor measurements were well established saplings, 2-3 m in height, rooted in large pots of soil, and grown in a greenhouse under a natural photoperiod. Turgor measurements were made in the laboratory, during which time illumination was provided by normal fluorescent room lights (about 40 ,uE m-2 s-'). A number of aphids were transfered to the cutting and, after they had begun to produce honeydew several hours later, they were anesthetized with CO2 and their stylets were cut with a razor blade fragment. A water droplet was placed around the stylet to prevent the exudate from becoming too viscous.The micromanometers for pressure measurements were square glass capillaries (Vitro Dynamics, Inc.)

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Sieve Elements: The Favourite Habitat of Phytoplasmas

Bel

2018

Phytoplasmas

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Some evidence for the existence of turgor pressure gradients in the sieve tubes of willow

Cited by 13 publications

References 12 publications

Direct and Indirect Measurements of Phloem Turgor Pressure in White Ash

Direct and Indirect Measurements of Phloem Turgor Pressure in White Ash

Direct Measurement of Sieve Tube Turgor Pressure Using Severed Aphid Stylets

Sieve Elements: The Favourite Habitat of Phytoplasmas

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