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Biomedical subjects

D B Fisher

Publications and source records attributed to D B Fisher.

At least 37 records · Page 2Linked to original sources
PubMed1978

Kinetics of C-photosynthate translocation in morning glory vines.

The movement of (14)C-photosynthate in morning glory (Ipomea nil Roth, cu. Scarlet O'Hara) vines 2 to 5 meters long was followed by labeling a lone mature leaf with (14)CO(2) and monitoring the arrival rate of tracer at expanding sink leaves on branches along the stem. To a first approximation, the kinetic behavior of the translocation profiles resembled that which would be expected from movement at a single velocity ("plug flow") without tracer loss from the translocation stream. There was no consistent indication of a velocity gradient along the vine length. The profile moved along the vine as a distinct asymmetrical peak which changes shape only slowly. The spatial distribution of tracer along the vine reasonably matched that predicted on the basis of the arrival kinetics at a sink, assuming plug flow with no tracer loss. These observations are in marked contrast to the kinetic behavior of any mechanism describable by diffusion equations.However, a progressive change in profile shape (a symmetrical widening) was observed, indicating a range of translocation velocities. A minimum of at least two factors must have contributed to the observed velocity gradient: the exchange of (14)C between sieve elements and companion cells (demonstrated by microautoradiography) and the range of velocities in the several hundred sieve tubes which carried the translocation stream. Possible effects of these two factors on profile spreading were investigated by means of numerical models. The models are necessarily incomplete, due principally to uncertainties about the exchange rate between sieve elements and companion cells and the degree of functional connectivity between sieve tubes of different conductivities. However, most of the observed profile spreading may be reasonably attributed to the combined effects of those two factors.The mass average velocity of translocation (calculated from the mean times of (14)C arrival at successive sink leaves) was about 75% of the maximum velocity (calculated from the times of initial detection at the same sink leaves), which was usually between 0.6 and 1 cm min(-1). Owing to tracer exchange between sieve elements and companion cells, the mass average velocity of tracer in the sieve tubes was probably closer to 86% of the maximum velocity, a figure which agreed with a predicted velocity distribution based on calculated sieve tube conductivities and the size distribution of functional sieve tubes.

Journal Article↗
PubMed1978

Source pool kinetics for C-photosynthate translocation in morning glory and soybean.

The kinetic behavior of translocation profiles indicates that their shape is determined largely by the rate at which tracer enters the sieve tubes in the source leaf. Confirmation of this relationship was sought by investigating the kinetics of (14)C in the immediate source pool for translocated sucrose in soybean (Glycine max L., cv. Bragg) and morning glory (Ipomea nil Roth, cv. Scarlet O'Hara) leaves. Quantitative microautoradiography was used to follow the water-soluble (14)C contents of the companion cells in minor veins after pulse-labeling with (14)CO(2). In both morning glory and soybean, the observed kinetics in the companion cells matched reasonably well those expected from the shape of the translocation profiles.Marked compartmentation of sucrose was evident in soybean leaves in that the specific radioactivity of total leaf sucrose was greatest immediately after labeling and quickly declined, whereas labeling in the companion cells was low at first and did not reach a maximum for about 35 minutes. In morning glory leaves, the kinetics of sucrose specific radioactivity and of companion cell-labeling more closely paralleled one another.

Journal Article↗
PubMed1977

Estimation of Osmotic Gradients in Soybean Sieve Tubes by Quantitative Autoradiography: Qualified Support for the MUnch Hypothesis.

An attempt was made to evaluate Münch's hypothesis of osmotically generated pressure flow in soybean (Glycine max L.) sieve tubes from velocity measurements and calculations of pressure potentials and sieve tube resistances. Pressure potential was estimated from values for water potentials and osmotic potential. Leaf water potential measurements were made by isopiestic thermocouple psychrometry, while the water potential of the nutrient solution was made with a vapor pressure osmometer. Osmotic potential was measured by first bringing the sucrose pools in the entire plant to the same specific radioactivity by steady-state-labeling of the shoot with constant specific radioactivity (14)CO(2) for 5 to 8 hours. Sucrose concentrations in sieve tubes were calculated from the disintegration rate per unit volume in sieve elements as measured by absolute quantitative microautoradiography of freeze-substituted, Eponembedded source (leaf) and sink (root) tissues.Conductivity of the sieve tubes was calculated from measurements of their dimensions in the petiole, stem, and root. The total pressure drop required for pressure flow at the observed velocities was calculated from the conductivity, velocity, and path length.In all experiments, the calculated sucrose concentration in source sieve tubes was greater than that in sink sieve tubes, with an average ratio (source to sink) of 1.79:1. However, the absolute sucrose concentrations (average values of 46.4 mg cm(-3) in the source and 23.9 mg cm(-3) in the sink) would have been insufficient to maintain positive turgor in the sieve elements, and the expected pressure differences would not have accounted for movement at the observed velocities. However, the low values for sucrose concentrations almost certainly were due to loss of sucrose during tissue preparation but, for technical reasons, such loss could not be accurately quantified.Assuming a sucrose concentration sufficient to maintain zero turgor in the root sieve tubes, a xylem water potential gradient (psi(w) [sink] - psi(w) [source]) of 2 bars between source and sink, and the measured ratio of sucrose concentrations in source and sink (1.79:1), the average turgor gradient between source and sink (psi(p) [sink] - psi(p) [source]) would have been about -1.6 to -3.5 bars, which compares favorably with the -1.07 to -2.41 bars average gradient that would have been required to drive translocation at the observed velocities.

Journal Article↗
PubMed1975

Compartmentation in Vicia faba Leaves: I. Kinetics of C in the Tissues following Pulse Labeling.

Leaflets of Vicia faba were pulse-labeled with (14)CO(2) to follow the subsequent movement of photosynthate between leaf tissues. Samples were taken during a (12)CO(2) chase, quick frozen, freeze-substituted, and embedded in methacrylate. Paradermal sections provided tissue samples consisting only of upper epidermis, palisade parenchyma, spongy parenchyma and veins, spongy parenchyma, or lower epidermis. Most CO(2) fixation occurred in the palisade parenchyma, but its (14)C content declined rapidly after labeling. Concomitant with the decline of activity in the palisade parenchyma, there was an increase in activity in the spongy parenchyma and upper epidermis and a slow increase in the lower epidermis. Activity in the palisade parenchyma and spongy parenchyma eventually reached similar levels and remained constant. Tissue samples containing veins were consistently the most radioactive, and activity in those samples showed a decline. Very little change occurred in the insoluble fraction from any tissue. The results support previous assumptions regarding the pathway of assimilate transport to the veins, and demonstrate the rapidity of such transport. Sucrose is apparently the principal mobile compound.

Journal Article↗
PubMed1975

Compartmentation in Vicia faba Leaves: II. Kinetics of C-Sucrose Redistribution among Individual Tissues following Pulse Labeling.

Leaflets of Vicia faba L. were pulse labeled with (14)CO(2) and the kinetics of (14)C-sucrose redistribution among individual tissues was followed. Sucrose specific activity in the whole leaf peaked about 15 minutes after labeling and declined with a half-time of about 80 minutes. In one experiment, leaflet discs taken at various times during the (12)CO(2) chase were quick frozen, freeze-substituted, and embedded in plastic. The tissue was sectioned paradermally and sections of palisade parenchyma, of spongy parenchyma, and of spongy parenchyma that contained veins were collected. Water extracts from these sections were assayed for sucrose specific activity. Sucrose specific activity in the palisade parenchyma was higher than that of the spongy parenchyma and reached a maximum in both tissues 9 to 15 minutes after labeling. Sucrose specific activity initially declined rapidly in the palisade parenchyma followed by a period during which little or no loss occurred. Sucrose specific activity in sections containing veins peaked at 15 minutes with a maximum value substantially higher than either mesophyll tissue, indicating that recently synthesized sucrose was preferentially exported from the mesophyll. Decline of activity in these sections containing veins continued for the remainder of the experiment. Sucrose specific activity in lower epidermal peels peaked several minutes after that of the whole leaflet and remained lower. Sucrose specific activity in upper epidermal peels was variable (probably due to contamination), but the limited data suggest that the sucrose specific activity there reached somewhat higher values than those of the lower epidermis. The experiments indicate that each leaf tissue contains a kinetically identifiable sucrose pool (which we refer to as "histological compartmentation"), and that further compartmentation may occur at the intracellular level. A simulation of leaf sucrose compartmentation is presented.

Journal Article↗
PubMed1975

Structure of functional soybean sieve elements.

Soybean (Glycine max cv. Bragg) petiolar tissue containing translocated (14)C-sucrose was quick frozen, freeze-substituted in acetone or propylene oxide and embedded in Epon. This procedure allowed cytological observations on sieve elements whose functional condition could be verified by microautoradiography. Sieve elements and companion cells were essentially free of ice damage. Aside from a P-protein crystal, the central portion of the sieve tube lumen was devoid of stainable content except in the vicinity of sieve plates. Various sized clumps of stacked endoplasmic reticulum (ER) lined the wall. Superficially, the ER "membranes" seemed to consist of parallel arrays of 100 A protein fibrils. Although that possibility could not be excluded, it seemed more likely that the fibrils were actually between ER cisternae and that the lipoprotein ER membrane could not be detected readily due to the loss of lipids during tissue preparation. The amount and distribution of proteinaceous material in the vicinity of sieve plates was variable but, when present, still consisted almost entirely of 100 A fibrils organized into membrane-like arrays. Stacks of ER in various degrees of disorganization and a few 100 A fibrils were found near sieve plates, with some fibrils extending through the pores. However, most (70%) of the sieve plate pores were essentially free from obstruction. The observations favor an osmotically generated pressure flow mechanism of translocation in soybean.

Journal Article↗
PubMed1975

The efficiency of 14C detection in autoradiographs of semithin plastic sections.

Sections of uniformly labeled 14C-methylmethacrylate were used to determine the efficiency of 14C detection by Ilford L4 emulsion layers, 0.22 mu in thickness, in light microscope autoradiographs. When efficiency was expressed as a function of radiation dose (i.e., the number of decays/mu2) there was a strong dependency on radiation dose up to about 6 decays/mu2, after which detection efficiency was fairly constant. For most emulsion batches, the detection efficiency was about 5%, but it ranged from 8 to 3%.

Autoradiography↗
PubMed1973

Detection of hepatic phenylalanine 4-hydroxylase in classical phenylketonuria.

An extract from a liver biopsy of a patient with classical phenylketonuria contains phenylalanine 4-hydroxylase (EC 1.14.3.1) with a specific activity 0.27% of the normal activity of the hydroxylase. This enzyme from a patient with phenylketonuria is a structurally altered form of phenylalanine 4-hydroxylase that probably results from a mutation in the gene coding for the hydroxylase.

Carbon Isotopes↗
PubMed1972

Artifacts in the Embedment of Water-soluble Compounds for Light Microscopy.

The relative effectiveness of freeze-drying and freeze-substitution in accurately preserving the distribution of water-soluble compounds in plants tissues was evaluated. Specimens with a known distribution of water-soluble materials (sucrose-infiltrated pith blocks and red beet tissue) were freeze-dried or freeze-substituted and embedded in paraffin, Epon, or methacrylate for microscopic examination. Dried water-soluble compounds were extremely sensitive to only traces of water, which caused severe shrinkage of the vacuolar contents. In spite of care taken to maintain dry conditions, freeze-drying was always accompanied by marked shrinkage of the water-soluble compounds away from the cell walls. Freeze-substitution was successful only if the solvents and embedding media were thoroughly dried and if infiltration was done in a dry box. Other artifacts were also noted, although they were less severe. Artifacts comparable to those seen in pith blocks and in red beet tissue were observed in the contents of pumpkin sieve tubes.

Journal Article↗
PubMed1972

The Retention of Water-soluble Compounds during Freeze-Substitution and Microautoradiography.

Freeze-substitution and Epon embedment were quantitatively evaluated for their effectiveness in retaining water-soluble metabolites in plant tissues. Roughly 99% of the 80% (v/v) ethanol-extractable radioactivity in photosynthetically labeled soybean leaf discs and in petiole fragments containing translocated (14)C was retained during freeze-substitution in acetone or propylene oxide and embedment in Epon. Substantially more activity was lost from (14)C-sucrose-infiltrated pith blocks, but most or all of this loss came from the block surface. The procedure was effective for a sucrose concentration as low as 0.004%. Sections floated on water retained most of their (14)C-sucrose, and high resolution autoradiographs could easily be prepared without resorting to dry procedures. Embedded (14)C-sucrose was apparently chemically unreactive, since there was no loss of radioactivity when sections were stained with the periodic acid-Schiff reagent, nor did the embedded sucrose show staining.

Journal Article↗