An Electron-Microscopical Study of the Plasmodesmata in the Roots of Young Barley Seedlings*

Helder, R. J.; Boerma, J. T.

doi:10.1111/j.1438-8677.1969.tb00575.x

Cited by 35 publications

(10 citation statements)

References 9 publications

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“…The idea that the aging effect on cell PD is associated with plasmodesmata has considerable merit and is supported by the extensive evidence which led to the concept of a symplast (2,13,24). Cutting through the symplast upon excision of a segment would expose plasmodesmata directly to the external solution.…”

Section: Discussionmentioning

confidence: 78%

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Electrical Potential Differences in Cells of Barley Roots and Their Relation to Ion Uptake

et al. 1971

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Single cell electropotentials of barley (Hordeum vulgare L., cv. 'Compana') root cortex were measured at different external concentrations of KCl in the presence of Ca2+. The roots were low in salt from seedlings grown on 0.5 mM aerated CaSO4 solution. Thus, the conditions were equivalent to those used to define the dual nmechanisms found with radioactive tracer-labeled ion uptake. In 0.5 mM CaSO4 alone, there is an increase with time of cell negativity from about -65 millivolts 13 minutes after cutting segments to about -185 millivolts in 6 to 8 hours. Two possible hypotheses, not mutually exclusive, are offered to explain this aging effect: that cutting exposes plasmodesmata which are leaky initially but which seal in time, and that some internal factors, e.g., hormones diffusing from the apex, have a regulatory effect on the cell potential, an influence which becomes dissipated in isolated segments and permits the development of a higher potential difference. In any case changes in selective ion transport must be involved. The cell potentials at KCI concentrations above 2.0 mM are more negative than would be expected for a passive diffuision potential. It is suggested that this discrepancy may be due to an electrogenic pump or to a higher K+ concentration in the cytoplasm than in the remainder of the cell, or perhaps to both. Whether there is a clear relationship between cell potential and mechanisms 1 and 2 of cation transport depends upon whether the cell potentials of freshly cut or of aged tissue represent the values relevant to intact roots. low concentrations (Km = 0.02 mM) and is the major process at concentrations below 0.5 mm. The other component, mechanism 2, is identifiable only at concentrations exceeding 0.5 to 1.0 mM. The two components also differ in selectivity for K+ and Na+: mechanism 1 is highly specific for K+ versus Na+, whereas mechanism 2 shows little discrimination between K+ and Na+. The characteristics of the salt absorption isotherm have been explained by assuming that ion transport takes place via specific sites on a carrier. When the sites are saturated, further increase in concentration, within limits, fails to increase the velocity of uptake unless new sites are brought into action. Consequently, it has been held by some workers that independent ion diffusion does not occur, i.e., ions move only in a combined neutral form (5, 34). This view has not been accepted by others (1,4,21,25). Independent diffusion of some ions may be significant in plants inasmuch as the cell interior is electronegative by a value that can be related to external salt concentration by equations based on passive ion diffusion. In oat coleoptiles, for example, increasing the external KCl or NaCl concentration makes the cell potential less negative by an amount expected if the cell were much more permeable to K+ and Na+ than to 15).With passive transport the potential difference, PD3 (or E, in equations), between the cell sap and the external solution is a diffusion potential which assumes a value s...

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Section: Discussionmentioning

confidence: 78%

“…Recently Helder and Boerma (13) studied the fine structure of plasmodesmata in barley roots. They calculated that about 20,000 to 30,000 plasmodesmata may occur in the tangential wall of an endodermal cell and that such a cell might transport 3.1 ,ueq Fig.…”

Section: Discussionmentioning

confidence: 99%

Electrical Potential Differences in Cells of Barley Roots and Their Relation to Ion Uptake

et al. 1971

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“…5) (20). With an elegant fluorescent staining technique, the Casparian strip in roots of Avena and Ipomoea has been shown to be complete (36 (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Thereafter, the experimental regime was identical in both runs. A root approximately 20 cm in length was excised, rinsed for 1 min with 0.5 mm CaSO., and submerged for 10 min in an aerated solution at 30 C which contained 0.5 mm KCl, the Cl being labeled with TCl, and 0.5 mM CaSO4, for pulse labeling. The root was then rinsed for 1 min with 0.5 mm CaSO,, and a length of polyethylene tubing was attached to it and trimmed to leave a 2-mm sleeve.…”

Section: Methodsmentioning

confidence: 99%

Lateral Transport of Ions into the Xylem of Corn Roots

1971

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When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with 'Cl and then transferred to unlabeled chloride, the activity in the xylem exudate reached a maximum about 4 minutes after pulse labeling was discontinued and then declined sharply. The rate at which labeled chloride was transported across the root into the xylem and basipetally therein was on the order of 75 to 250 centimeters per hour. Consequently, symplasmic movement of chloride in corn roots is fast and may not be rate-limiting in transfer from the root surface to the xylem. Experiments on pulse labeling with 'Na gave similar results. A large fraction of the absorbed 'Na was not translocated into the exudate but was tightly sequestered in a cell compartment, probably the vacuole.Electron probe analysis was used to reveal the pattern of potassium distribution in cross sections taken 10 to 11 millimeters from the tip. The cytoplasm and vacuoles of the xylem parenchyma cells accumulated potassium to a much greater extent than cortical and other stelar cells. Ultrastructural studies showed that the cytoplasm of the xylem parenchyma cells contains numerous membrane systems. It was concluded that the xylem parenchyma cells secrete ions from the symplasm into the conducting vessels, and it was suggested that this secretion is driven across the plasmalemma by a carrier-mediated transport.According to the classical Crafts-Broyer hypothesis on lateral transport of ions in roots (11), the stele does not participate actively in transfer from the symplasm to the xylem vessels. The membranes of the stelar cells are deemed permeable to ions, and hence ions simply leak into the vessels. The Casparian strip of the endodermis functions to prevent ions from diffusing backwards into the outer space of the cortex.Laties (25), Laties and Budd (26), and Luittge and Laties

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“…They observed velocities for symplasmic movement of chloride of 1-4 cmh -1 which is of the same order as the rate of protoplasmic streaming. Helder and Boerma (1969) showed plasmodesmata in the walls of the endodermis, their presence suggests that the symplasm may be continuous from cortex to stele. The mode of transport in the symplasm is not known but it is generally thought to be by cytoplasmic streaming in the cells and by diffusion through the connecting plasmodesmata.…”

Section: Transport Of Nutrients Across the Rootmentioning

confidence: 93%

Uptake and Transport of Nutrients

Bowling

1980

Mineral Nutrition of Fruit Trees

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An Electron-Microscopical Study of the Plasmodesmata in the Roots of Young Barley Seedlings*

Cited by 35 publications

References 9 publications

Electrical Potential Differences in Cells of Barley Roots and Their Relation to Ion Uptake

Electrical Potential Differences in Cells of Barley Roots and Their Relation to Ion Uptake

Lateral Transport of Ions into the Xylem of Corn Roots

Uptake and Transport of Nutrients

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