Composition of Sieve-Tube Sap

Peel, A. J.; Weatherley, P. E.

doi:10.1038/1841955a0

Cited by 67 publications

(20 citation statements)

References 6 publications

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“…However, the amino acid (8) and potassium contents (13,19) of willow exudate can vary considerably. Reported values for K have been as high as 2% (w/v) (14). Hence we pooled some stylet exudates from our plants and measured the molar ratios of sucrose, amino acids, and K. The ratios were 3.5 mol sucrose/1.4 mol ofglutamine equivalents.…”

Section: Methodsmentioning

confidence: 99%

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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Section: Methodsmentioning

confidence: 99%

Direct Measurement of Sieve Tube Turgor Pressure Using Severed Aphid Stylets

Wright

Fisher²

1980

Plant Physiol.

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“…Stylectomy [35] uses the severed stylets (mouthparts) of phloem-feeding insects (e.g. aphids [36,37], planthoppers [38,39] and scale insects [40]). These insects feed on individual sieve elements without disrupting phloem transport, thereby creating a direct tap into functional sieve tubes with their stylets [41].…”

Section: Phloem Sap Sugar Concentration Datamentioning

confidence: 99%

Optimal concentration for sugar transport in plants

Jensen

Savage

Holbrook

2013

J. R. Soc. Interface.

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Vascular plants transport energy in the form of sugars from the leaves where they are produced to sites of active growth. The mass flow of sugars through the phloem vascular system is determined by the sap flow rate and the sugar concentration. If the concentration is low, little energy is transferred from source to sink. If it is too high, sap viscosity impedes flow. An interesting question is therefore at which concentration is the sugar flow optimal. Optimization of sugar flow and transport efficiency predicts optimal concentrations of 23.5 per cent (if the pressure differential driving the flow is independent of concentration) and 34.5 per cent (if the pressure is proportional to concentration). Data from more than 50 experiments (41 species) collected from the literature show an average concentration in the range from 18.2 per cent (all species) to 21.1 per cent (active loaders), suggesting that the phloem vasculature is optimized for efficient transport at constant pressure and that active phloem loading may have developed to increase transport efficiency.

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“…Phloem sap was collected from severed inflorescences of palms and some Agavacae (22) Phloem sap has been collected from the severed stylets of aphids (10) and amino acids were detected in aphid stylets exudate from willows (16,20) and from peas (2). The quantities of sap obtained are small and the difficulty in prepositioning the aphids limit the usefulness of this method in such studies.…”

Section: Introductionmentioning

confidence: 99%

Use of Phloem Exudate Technique in the Study of Amino Acid Transport in Pea Plants

Urquhart¹,

Joy²

1981

Plant Physiol.

119

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The phloem exudation technique using ethylenediaminetetraacetic acid (EDTA) was evaluated In studies of amino acid translocation in Pisum sativnm L. seedlngs. Exudation of phloem sap from cut petioles of fuily expanded leaves was enhanced by EDTA (20 millimolar disodium salt [pH 7.01). Amino acids (mainly asparagine, homoserine, glutamate, and also aspartate and serine) were present in petiole exudates from EDTA-treated leaves at levels which were commonly 5-to 10-fold (or more) higher compared with water-treated controls. Exudation was greater from darkened leaves, and the pattern of amino acids was markedly different from the more uniform mixture leaking from water-treated controls.After feeding 'C-labeled amino acids to the leaf blade, distribution of radioactivity in components of the exudate differed from that of the leaf tissue, suggesting selectivity of amino acid loading. 1'ClAsparagine was converted to 2-hydroxysuccinamic acid and to other amino acids by the leaf, but was recovered in exudate mainly as asparagine (60%) and aspartate (30%). SImilarly, in the exudate, 65 to 70% of the label from 114CI-aspartate was in amino acids, although in the leaf tissue 50% was in the organic acid fraction and only 11% remained as aspartate. Metabolism of asparagine and aspartate was essentially the same in intact leaf blades as in EDTA-treated leaves. Despite the possibility of EDTA damage in the petiole, phloem loading of amino acids appeared to be relatively unimpaired.Although the amount of labeled material appearing in the exudate is less than the amount translocated in the intact plant, the technique is useful in the study of amino acid transport.Mature, transpiring leaves import nitrogen from the roots, with varying proportions of nitrate and organic nitrogen, depending on species and growth conditions (17). In Pisum sativum, the nitrogen level of the growing leaves stabilizes when the leaves are fully expanded (3), thus most of the incoming nitrogen must be reexported. In a continuing study of amino acid metabolism and transport in peas (3), the redistribution of amino acids in phloem has been investigated.Several methods have been used in the past to obtain phloem samples from leaves. Phloem sap was collected from severed inflorescences of palms and some Agavacae (22) Phloem sap has been collected from the severed stylets of aphids (10) and amino acids were detected in aphid stylets exudate from willows (16,20) and from peas (2). The quantities of sap obtained are small and the difficulty in prepositioning the aphids limit the usefulness of this method in such studies.None of these techniques seemed suitable for collection of the phloem sap exported from fully expanded leaves of vegetative plants. Although some leaves do exude a phloem sap through the cut petiole (12), the quantities thus obtained are minute. King and Zeevaart (1 1) described an EDTA-promoted exudation of phloem sap from detached leaves. The method has subsequently been used with a variety of plants by other research groups (6,8,9,...

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Composition of Sieve-Tube Sap

Cited by 67 publications

References 6 publications

Direct Measurement of Sieve Tube Turgor Pressure Using Severed Aphid Stylets

Direct Measurement of Sieve Tube Turgor Pressure Using Severed Aphid Stylets

Optimal concentration for sugar transport in plants

Use of Phloem Exudate Technique in the Study of Amino Acid Transport in Pea Plants

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