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Biomedical subjects

D R Geiger

Publications and source records attributed to D R Geiger.

At least 37 records · Page 2Linked to original sources
PubMed1984

Effects of decreased net carbon exchange on carbohydrate metabolism in sugar beet source leaves.

The relationship between CO(2) concentration and starch synthesis and degradation was studied by measuring leaf starch content and disappearance of (14)C-starch. At a concentration of 340 microliters CO(2) per liter, starch accumulated without degradation of previously synthesized starch. Degradation of starch began when CO(2) concentration was lowered, but its synthesis continued. At 120 microliters CO(2) per liter rates of synthesis and degradation were equal. Even at the CO(2) compensation point, synthesis of starch continued. Concomitant starch synthesis and mobilization supported export from the leaf. Changes in starch metabolism that occur when photosynthesis is CO(2)-limited provide a means to study regulation of starch metabolism and carbon allocation in translocating leaves.

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PubMed1983

Relation of increased potassium nutrition to photosynthesis and translocation of carbon.

Effects of supplying K(+) at 2 or 10 millimolarity concentration on net carbon exchange and translocation of products of photosynthesis were studied in plants of Beta vulgaris L. (var. Klein E). Transport of K(+) into and out of leaves was studied with (42)K over a 3-day period. Increasing the K(+) supplied to the roots from 2 millimolarity, a level just sufficient to overcome obvious deficiency symptoms, to 10 millimolarity resulted in a gradual accumulation of K(+) per unit area and an increased export of K(+) to sink regions. No significant increase in net carbon exchange was observed in leaves that had accumulated a high level of K(+) per unit area. Initiation rate, total area, and total fresh weight of leaves of plants with K(+) supplied at 10 millimolarity was similar to that for leaves from plants at a 2 millimolarity level. Shoot/root ratio and dry weight accumulation, which are indicative of translocation and partitioning over the long term, were independent of K(+) supply in the 2 to 10 millimolarity range. Accumulation of K(+) by exporting leaves and its subsequent recirculation to sinks increased when K(+) supply was increased in this range but did not appear to affect carbon nutrition even after a long period.

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PubMed1983

Sources of sucrose translocated from illuminated sugar beet source leaves.

A search for source leaf sucrose pools that differed in their relation to export was carried out in photosynthesizing leaves of Beta vulgaris L. The time course of depletion of [(14)C]sucrose in a leaf in unlabeled CO(2) following steady state labeling provided evidence for two distinct sucrose pools. After the start of the light period, leaf blade sucrose remained constant although it exchanged between the two pools. Newly synthesized sucrose destined for export passed through one pool more rapidly than through the other. All of the leaf blade sucrose appeared to exchange with export sucrose. Modeling and regression analysis of [(14)C]sucrose data provided a means for estimating the size of the two pools. From 20 to 40% of the sucrose was calculated to be present in the pool that provided the less direct path to export; this was likely vacuolar sucrose. The remainder of the sucrose in the blade was probably in the cytoplasm and veins. Added amounts of leaf blade sucrose, produced in response to elevated CO(2), appeared to be stored mainly in the vacuolar compartment.

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PubMed1982

Potassium nutrition and translocation in sugar beet.

The effect of increased net foliar K(+) accumulation on translocation of carbon was studied in sugar beet (Beta vulgaris, L. var. Klein E and US H20) plants. Net accumulation of recently absorbed K(+) was studied by observing arrival of (42)K(+) per unit area of leaf. Labeled K(+) was added to give an initial concentration at 2 or 10 millimolar K(+) in mineral nutrient solution. Because the newly arrived K(+) constitutes a small part of the total leaf K(+) in plants raised in 10 millimolar K(+), export of (42)K(+) by phloem was negligible over the 2- to 3-day period; consequently, accumulation is a measure of arrival in the xylem. In leaves from plants in 2 millimolar K(+), export by the phloem was estimated to be of the same order as import by the xylem; K(+) per area was observed to remain at a steady-state level. Increasing the supply of K(+) to 10 millimolar caused arrival in the xylem to increase 2- to 3-fold; K(+) per area increased gradually in the mature leaves. Neither net carbon exchange nor translocation of sugar increased in response to a faster rate of arrival of K(+) over a 6- to 8-hour period. In the absence of short-term effects, it is suggested that K(+)-promoted increase in synthetic metabolism may be the basis of the increased carbon assimilation and translocation in plants supplied with an above-minimal level of K(+).

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PubMed1982

Diurnal Pattern of Translocation and Carbohydrate Metabolism in Source Leaves of Beta vulgaris L.

Transitions in carbohydrate metabolism and translocation rate were studied for evidence of control of export by the sugar beet (Beta vulgaris L. Klein E.) source leaf. Steady-state labeling was carried out for two consecutive 14-hour light periods and various quantities related to translocation were measured throughout two 24-hour periods. Starch accumulation following illumination was delayed. Near the end of the light period, starch stopped accumulating, whereas photosynthesis rate and sucrose level remained unchanged. At the beginning of the dark period there was a 75-minute delay before starch was mobilized. The rate of import to the developing sink leaves at night was similar to that during the day, whereas export decreased considerably at night.Starch accumulation and degradation seemed to be initiated in response to the level of illumination. Cessation of starch accumulation before the end of the light period was initiated endogenously. Exogenous control appeared to be mediated by the level of sucrose in the source leaf while endogenous control seemed to be keyed to photoperiod or photosynthetic duration.

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PubMed1981

Uptake and distribution of N-phosphonomethylglycine in sugar beet plants.

Glyphosate (N-phosphonomethylglycine) was readily transported in sugar beet plants (Beta vulgaris L., Klein E type, monogerm). Concentrations in sink leaves reached 2.5 to 13.7 micromolar in 10 hours from a 15 millimolar solution supplied to one mature leaf. Distribution of glyphosate followed that of [(3)H]sucrose used as a marker for materials transported by phloem, indicating that this is the primary means for distribution of glyphosate. Possible mechanisms of entry into the sieve tubes were evaluated using isolated leaf discs. Concentration dependence of uptake and kinetics of exodiffusion from tissue indicate a passive, nonfacilitated mechanism. Uptake was not affected by pH, eliminating the passive, weak acid mechanism. Permeability of the plasmalemma to glyphosate was calculated as 1.7 x 10(-10) meters per second. This characteristic would allow slow entry and exit from the phloem, and together with other physiological parameters of the plant, is postulated to allow accumulation and transport in the phloem.

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PubMed1980

Effect of Rapid Changes in Sink-Source Ratio on Export and Distribution of Products of Photosynthesis in Leaves of Beta vulgaris L. and Phaseolus vulgaris L.

Effects of increasing sink-source ratio on rate of translocation and net carbon exchange were studied by darkening all but one source leaf of Beta vulgaris L. or one primary leaf of Phaseolus vulgaris L. Rates of export of labeled material and patterns of its distribution among sinks were studied by means of GM detectors. Changes in export and import rates were compared with adjustments in starch, sucrose, and glucose levels in sugar beet source leaves before and during treatment.Sugar beet source leaf treatments which increased sink-source ratio had no sustained effect on rates of net carbon exchange, export, accumulation of starch, or the levels of sucrose and glucose on the day of treatment. However, the rate of import into a developing sugar beet leaf increased. Similar treatment of primary leaves of bean either had no effect on the rate of export of labeled material or caused up to a 20% increase. The increase in export was sufficient to account for the increase in import of labeled material into the far-sink leaflet. Nevertheless, distribution of labeled material exported to the sink regions changed; import of labeled material into the near-sink leaflet remained nearly unchanged and that into the roots decreased.The data indicate that rapid changes in sink-source ratio cause a redistribution of the products of photosynthesis but need not involve a change in the rate of export.

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PubMed1979

A method for continuous measurement of export from a leaf.

Export of labeled material derived by continuous photosynthesis in (14)CO(2) was monitored with a Geiger-Müller detector positioned next to an exporting leaf blade. Rate of export of labeled material was calculated from the difference between rates of retention and net photosynthesis of labeled carbon for the observed leaf. Given certain conditions, including nearly constant distribution of labeled material among minor veins and various types of cells, count rate data for the source leaf can be converted to rate of export of carbon. Changes in counting efficiency resulting from changes in leaf water status can be corrected for with data from a transducer which measures leaf thickness.Export data agreed with data obtained by monitoring the arrival of (14)C in the sink region; isolated leaves gave values near zero for export. The method allows continuous nondestructive measurement of export of labeled carbon from a given leaf on an intact plant. The technique detects changes in export with a resolution of 10 to 20 minutes.

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PubMed1979

Effect of Exogenously Supplied Foliar Potassium on Phloem Loading in Beta vulgaris L.

The effect of foliar application of K(+) on processes associated with phloem loading was investigated in source leaves of sugar beet (Beta vulgaris L.). KCI was supplied exogenously at concentrations of up to 100 millimolar in the solution bathing the abraded upper epidermis of source leaves. K(+) added at concentrations below 30 millimolar generally promoted the rate of export of material derived from (14)CO(2) but not from exogenously applied [(14)C]sucrose. Paralleling promotion of export, the level of material derived from photosynthesis, which was released into the bathing solution, also increased in response to addition of K(+) to the free space. Net photosynthetic rate was not affected. K(+) at 5 and 15 millimolar concentrations did not stimulate uptake of [(14)C]sucrose into source leaf discs.The data suggest that the promotion of export rate by K(+) results primarily from an effect on the site regulating efflux of sucrose into the apoplast prior to loading into the minor veins, rather than on the loading site itself. A change in the level of sucrose in the free space appears to precede a change in export rate.

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PubMed1977

Sugar Selectivity and Other Characteristics of Phloem Loading in Beta vulgaris L.

The rate of phloem loading, its selectivity, and the disposition of labeled carbon were studied following application of (14)C-labeled sugars to the free space of source leaves of sugar beet (Beta vulgaris L.). Buffered 10 mm solutions of (14)C-labeled sucrose, fructose, stachyose, mannitol, 3-0-methyl glucose or l-glucose were applied to the abraded epidermis of source leaves held in the dark. Distribution of the labeled carbon from sugar taken up from the free space was studied by micro-densitometry of autoradiographs. Uptake of labeled sugar from the free space, partition between mesophyll and minor veins, metabolic conversions, export and respiration were followed during the 3-hr time course studies. Rates of sugar uptake into the minor veins, flux rates through the sieve element-companion cell complex membrane and concentration ratios between free space and the interior of the minor vein phloem cells were compared for the six sugars studied for evidence of active uptake. The composition of the free space solution in leaves photosynthesizing in (14)CO(2) was studied by vacuum infiltration of the source leaf air spaces and removal of the solution by centrifugation. Labeled compounds in this solution were compared to those in an aqueous ethanol extract of the same leaf pieces.The results in sugar beet source leaves support the concept of direct, active uptake of sucrose from free space into minor veins. This is not the case for fructose, 3-0-methyl glucose, mannitol, or stachyose. The latter two sugars, which are translocated in some plants, are not loaded into the minor veins at a rate sufficient to make them a significant component of the material translocated. The rate of phloem loading is controlled in part by mesophyll metabolism, especially as it affects the availability of sucrose to the free space. Both the rate and selectivity of export are controlled by uptake from the free space into the sieve element-companion cell complex of the minor veins.

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PubMed1974

Effects of light intensity and oxygen on photosynthesis and translocation in sugar beet.

The mass transfer rate of (14)C-sucrose translocation from sugar beet (Beta vulgaris, L.) leaves was measured over a range of net photosynthesis rates from 0 to 60 milligrams of CO(2) decimeters(-2) hour(-1) under varying conditions of light intensity, CO(2) concentration, and O(2) concentration. The resulting rate of translocation of labeled photosynthate into total sink tissue was a linear function (slope = 0.18) of the net photosynthesis rate of the source leaf regardless of light intensity (2000, 3700, or 7200 foot-candles), O(2) concentration (21% or 1% O(2)), or CO(2) concentration (900 microliters/liter of CO(2) to compensation concentration). These data support the theory that the mass transfer rate of translocation under conditions of sufficient sink demand is limited by the net photosynthesis rate or more specifically by sucrose synthesis and this limitation is independent of light intensity per se. The rate of translocation was not saturated even at net photosynthesis rates four times greater than the rate occurring at 300 microliters/liter of CO(2), 21% O(2), and saturating light intensity.

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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.

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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.

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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.

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PubMed1973

Mechanism of inhibition of translocation by localized chilling.

Arrhenius plots of translocation velocity as a function of petiole temperature show a marked increase in temperature dependence below 10 C in bean (a chilling-sensitive species) but not in sugar beet (chilling-resistant). The increased temperature dependence below 10 C was not observed for cytoplasmic streaming or oxygen uptake in bean. Bean petioles were served to release pressure in order to determine whether sieve tubes are obstructed in cold-treated petioles. The resulting pressure release caused serious displacement of the crystalline protein bodies in the sieve tubes of petioles at 25 C, but in those locally cooled to 0 C for 30 minutes little displacement occurred, indicating obstruction in the latter. An ultrastructural study of sieve tubes in tissue frozen rapidly in situ and dehydrated by freeze substitution revealed that treatment at 0 C for 30 minutes caused structural alteration and displacement of the cytoplasmic material lining the sieve tube wall resulting in occlusion of sieve plates. The sieve plates of the control petioles at 25 C were generally clear of obstructions. The results indicate that inhibition of translocation by chilling in chilling-sensitive plants results from physical blockage of sieve plates rather than from direct inhibition of a metabolic process which drives translocation.

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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.

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PubMed1971

Effect of sink region anoxia on translocation rate.

Translocation rate, ATP level, and CO(2) production of a developing leaf (sink leaf) were studied in sugar beet (Beta vulgaris) plants prior to and during anaerobic treatment of the sink leaf. Within 3 to 5 minutes after onset of treatment with a N(2) atmosphere, translocation into the sink leaf decreased to near zero and then recovered to a level of about 50% of the control over the next 2 hours. A decline in CO(2) output and ATP levels coincided with the attainment of the new translocation rate. All three quantities returned to near control levels within 60 to 120 minutes after the sink leaf was returned to air. Swelling and ultrastructural changes in mitochondria coincided with the observed ATP level changes during inhibition and recovery periods. The first phase of marked inhibition of translocation did not coincide with low ATP level and appeared to be caused by decreased membrane permeability during the transition to anaerobic metabolism, possibly as a result of a temporary build up of toxic products. The correlation between ATP level and translocation rate suggests that ATP-dependent active transport in the sink leaf augments the driving force for translocation.

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