Biomedical subjects

R J Fellows

Publications and source records attributed to R J Fellows.

9 recordsLinked to original sources

PubMed1989

Comparative metabolic behavior and interrelationships of Tc and S in soybean plants.

The comparative behavior of sulfur (S) and technetium (Tc) in soybean seedlings shows gross subcellular distributions to be similar for these oxyanions. More than 75% of the tissue-deposited Tc remains soluble and extractable. Differences in Tc fixation/incorporation were noted for the nuclear and chloroplast fractions of leaf and root cells. Pulse studies showed that soluble protein and nitrate reductase levels rose in response to Tc accumulation by sink leaves but not source leaves. In vitro assay of chloroplast-based S reduction and incorporation systems showed Tc to be reduced and incorporated into amino nitrogen-containing products. A hypothesis related to the metabolic behavior of Tc in plants is presented.

Nitrate Reductases↗

PubMed1987

Nodule activity and allocation of photosynthate of soybean during recovery from water stress.

Nodulated soybean plants (Glycine max [L.] Merr. cv Ransom) in a growth-chamber study were subjected to a leaf water potential (psi w) of -2.0 megapascal during vegetative growth. Changes in nonstructural carbohydrate contents of leaves, stems, roots, and nodules, allocation of dry matter among plant parts, in situ specific nodule activity, and in situ canopy apparent photosynthetic rate were measured in stressed and nonstressed plants during a 7-day period following rewatering. Leaf and nodule psi w also were determined. At the time of maximum stress, concentration of nonstructural carbohydrates had declined in leaves of stressed, relative to nonstressed, plants, and the concentration of nonstructural carbohydrates had increased in stems, roots, and nodules. Sucrose concentrations in roots and nodules of stressed plants were 1.5 and 3 times greater, respectively, than those of nonstressed plants. Within 12 hours after rewatering, leaf and nodule psi w of stressed plants had returned to values of nonstressed plants. Canopy apparent photosynthesis and specific nodule activity of stressed plants recovered to levels for nonstressed plants within 2 days after rewatering. The elevated sucrose concentrations in roots and nodules of stressed plants also declined rapidly upon rehydration. The increase in sucrose concentration in nodules, as well as the increase of carbohydrates in roots and stems, during water stress and the rapid disappearance upon rewatering indicates that inhibition of carbohydrate utilization within the nodule may be associated with loss of nodule activity. Availability of carbohydrates within the nodules and from photosynthetic activity following rehydration of nodules may mediate the rate of recovery of N2-fixation activity.

Acetylene↗

PubMed1983

Comparison of ethylenediaminetetraacetate-enhanced exudation from detached and translocation from attached bean leaves.

A technique for collection of phloem exudate from detached leaves using 20 millimolar EDTA (pH 7.0) has previously been developed (King, Zeevaart 1974 Plant Physiol 53: 96-103). It was the aim of the present study to determine the efficiency of this technique in relation to undisturbed export from attached leaves. Paired primary leaves of bean seedlings (Phaseolus vulgaris L. cv Montcalm) were used to minimize variations in plant material. Attached leaves, exposed to (14)CO(2) for 10 minutes with subsequent excision of one of the leaves and collection of the exudate over a 12-hour period, showed a 25% export of total assimilated (14)C from the attached versus 15% of total assimilated (14)C in the form of exudation from the detached ones. Leaf excision changed the labeling pattern within the leaf, increasing% total leaf (14)C-activity in the ethanolic fraction, while decreasing activity in the starch fraction, as compared to attached leaves. This was presumably caused by a lack of translocation from the detached leaves. Excision did not affect dark respiration. However, measurements of total nonstructural carbohydrates in leaf starch and neutral fractions indicated no significant differences between attached and leaves detached in EDTA. Thus, in terms of actual carbon export, and accompanying distribution of nonexported carbohydrate within the leaf, EDTA-enhanced exudation compares favorably with translocation from attached leaves.

Journal Article↗

PubMed1979

Rapid Changes in Translocation Patterns in Soybeans following Source-Sink Alterations.

The effects of source-sink alterations on the translocation patterns to soybean ("Fiskeby V") pods were studied using a pod leakage technique. The distribution of assimilates from a source leaf using double pulse experiments was followed at the pods at the source node and the node immediately below. Alterations were made by shading, illuminating, or excising two-thirds of the leaf area of the leaf at the node below. In control experiments both pulses exhibited identical time-course patterns at both nodes. Shading the lower leaf during the first half of the experiment and illuminating during the second reduced the distribution of (14)C-assimilate to the lower node's pods from the source leaf by approximately 30 to 50% while having no effect at the source node. Illuminating the lower leaf during the first half of the experiment followed by excision of two-thirds of that leaf's area and shading increased the import from the source leaf by 4- to 33-fold relative to the control while reducing the distribution to the source node by up to 40%. The change in distribution pattern took place in less than 30 minutes with no apparent change in the source leaf net photosynthesis or in the rate of movement to the pods. The results indicate that any alterations in the source-sink balance will quickly produce a change in the distribution patterns to the pods.

Journal Article↗

PubMed1978

A Pod Leakage Technique for Phloem Translocation Studies in Soybean (Glycine max [L.] Merr.).

Radioactive photosynthetic assimilates, translocated to a soybean (Glycine max [L.] Merr. ;Fiskeby V') pod can be measured directly by excising the stylar tip of the pod under 20 mm ethylenediaminetetraacetate solution (pH 7.0) and allowing the material to leak into the solution. Pods at the source node received approximately 50% of the (14)C exported from the source leaf to the pod and leaked approximately 1 to 3% of this into the solution. More than 90% of the (14)C that leaked from the pods was found in the neutral fraction and, of this, about 93% was in sucrose. Fifteen amino acids were identified in the leakage including: alanine, arginine, asparagine, gamma-aminobutyric acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, phenylalanine, serine, threonine, tyrosine, and valine. The majority of the (14)C in the basic fraction was found in serine ( approximately 30%) and asparagine ( approximately 23%). The inorganic ions K, Ca, P, Mg, Zn, and Fe were found in the leakage component. Nitrate was not detectable in the collected leakage solution. The absence of NO(3) (-) and the large proportion of the label in sucrose suggest a possible phloem origin for most of the material. The technique provides an uncomplicated, reproducible means of analyzing the material translocated into and through the soybean pod, as well as following the time course of label arrival at the pod.

Journal Article↗

PubMed1974

Structural and Physiological Changes in Sugar Beet Leaves during Sink to Source Conversion.

The onset of export during leaf development was correlated with changes in metabolism and ultrastructure and with patterns of solute distribution in the developing seventh leaf of sugar beet (Beta vulgaris L.) in order to study the cause of initiation of translocation. Infrared gas analysis of carbon dioxide uptake showed a broad peak for net photosynthesis dm(-2) at 35 to 40% final laminar length. Pulse labeling with (14)CO(2) demonstrated that maximum import of translocate occurred at 25% final laminar length; export was first observed at 35% final laminar length. Between 40 and 50% final laminar length a rapid increase in amount of export occurred, primarily as a result of the increase in the area of leaf which was exporting. Whole leaf autoradiography revealed that onset of phloem loading spread basipetally from the leaf tip; loading was initiated at about 22% final laminar length and was essentially complete by 50% final laminar length. Those areas which clearly exhibited loading no longer imported from other parts of the plant while the area in transition still appeared to import label from source regions.There was little difference between source and sink leaf tissue in the kinetic parameters K(j) and J(max) (30) for uptake of exogenous sucrose supplied via free space. The concentration of solutes in sieve elements and companion cells of the sink leaf was highest in the mature tip area and gradually decreased in the direction of the immature base. There appeared to be no dramatic structural transformation within the phloem of the minor veins that was closely correlated with the time when phloem loading or export began. Rather, there appeared to be a gradual differentiation of phloem which resulted in a sizable proportion of the population of minor vein sieve elements and companion cells attaining maturity in the older sink regions prior to initiation of phloem loading. The area of the leaf undergoing development appeared to exhibit the beginnings of phloem loading 30 to 45 hours prior to onset of export. Import continued into the area in transition until the full level of vein loading was attained. Structural maturation of the phloem and onset of phloem loading are felt to be more preparatory in nature rather than immediately causal events which triggered export.The initiation of export out of a developing leaf, we believe, is the result of the increasing solute content within the sieve element and companion cells of the minor veins, in particular. The higher osmotic pressure in the sieve tubes causes a reversal of the previously inward directed gradient and produces a mass flow, through unobstructed sieve elements, out of the new source region of the leaf.

Journal Article↗

PubMed1974

Evidence for active Phloem loading in the minor veins of sugar beet.

Phloem loading in source leaves of sugar beet (Beta vulgaris, L.) was studied to determine the extent of dependence on energy metabolism and the involvement of a carrier system. Dinitrophenol at a concentration of 4 mm uncoupled respiration, lowered source leaf ATP to approximately 40% of the level in the control leaf and inhibited translocation of exogenously supplied (14)C-sucrose to approximately 20% of the control. Dinitrophenol at a concentration of 8 mm inhibited rather than promoted CO(2) production, indicating a mechanism of inhibition other than uncoupling of respiration. The 8 mm dinitrophenol also reduced ATP to approximately 40% of the level in the control source leaf and reduced translocation of exogenous sucrose to approximately 10% of the control. Application of 4 mm ATP to an untreated source leaf promoted the translocation rate by approximately 80% over the control, while in leaves treated with 4 mm dinitrophenol, 4 mm ATP restored translocation to the control level. No recovery of translocation was observed when ATP was applied to leaves treated with 8 mm dinitrophenol. The results indicate an energy-requiring process for both phloem loading and translocation in the source leaf.Application of (14)C-sucrose solutions in a series of concentrations through the upper surface of a source leaf produced a biphasic isotherm for translocation out of the fed region. A similar dual isotherm was obtained for phloem loading with leaf discs floated on (14)C-sucrose solutions. The first and possibly the second phases were attributed to active, carrier-mediated accumulation in the minor vein phloem. Autoradiography of the tissue confirmed that most of the sucrose was localized in the minor veins. Data from uptake through the abraded surface of intact leaves, the most reliable method, were analyzed by the Hofstee method. Kinetic parameters, analogous to Km and V(max) of enzyme studies, were calculated to be: K(j) = 16 mm and J(max) = 70 mug C/min dm(2) or 490 nmoles sucrose/min.dm(2). Rates for phloem loading and translocation of exogenous sucrose are equal to or greater than those observed for compounds derived from photosynthetically fixed CO(2). The data indicate that a free space sucrose concentration in the region of the minor vein phloem of approximately 20 mm can support translocation at the rates commonly observed for photosynthetically produced sugars.

Journal Article↗

PubMed1974

Role of free space in translocation in sugar beet.

The involvement of the free space in phloem loading of sucrose was studied in sugar beet source leaves (Beta vulgaris, L.). Sucrose, supplied exogenously to the abraded upper surface of leaves at a concentration of 20 mm, was available for translocation at rates similar to those obtained with photosynthesis. The exogenous sucrose substituted as a source of translocate for assimilate derived from photosynthesis when the latter process was disrupted by plasmolysis of the leaf with 0.8 M mannitol. The mesophyll symplast was not completely disrupted by this treatment, however. Data from the sugar uptake experiments indicate that phloem loading can occur from the free space.Isotope trapping of labeled sugars derived from (14)CO(2) was used to intercept and identify sugars passing through the free space prior to phloem loading. Increased translocation rates induced by 4 mm ATP or increased light intensity were accompanied by increased trapping of sucrose but not of glucose. The data support the view that sucrose passes into the free space prior to phloem loading.

Journal Article↗

PubMed1973

Solute distribution in sugar beet leaves in relation to Phloem loading and translocation.

The distribution of solutes in the various cells of sugar beet (Beta vulgaris L.) source leaves, petioles, and sink leaves was studied in tissue prepared by freeze-substitution. The differences in degree of cryoprotection indicated that sieve elements and companion cells of the source leaf, petiole, and sink leaf contain a high concentration of solute. The osmotic pressure of various types of cells was measured by observing incipient plasmolysis in freeze-substituted tissues equilibrated with a series of mannitol solutions prior to rapid freezing. Analysis of source leaf tissue revealed osmotic pressure values of 13 bars for the mesophyll and 30 bars for the sieve elements and companion cells. The osmotic pressure of the mesophyll of sink leaves was somewhat higher.The sharp concentration increase at the membrane of the sieve element-companion cell complex of the source leaf indicates active phloem loading from the free space at this site. Active loading of the phloem is presumably needed to move the sugar from the chloroplasts of the mesophyll to the sieve tubes against the concentration gradient. The osmotic pressure of the mature sieve element-companion cell complex appears to be approximately the same in source leaf, path, and sink leaf tissue. There is a distinct difference in concentration between the mature sieve element-companion cell complex in the sink and the surrounding mesophyll. The solute distribution suggests that sugar is actively accumulated from the free space by the developing sink leaf tissue.The osmotic values observed in the various cells are consistent with the operation of a mass flow mechanism of translocation driven by active phloem loading and by active accumulation of sugar by sink tissues.

Journal Article↗