Biomedical subjects

A L Christy

Publications and source records attributed to A L Christy.

9 recordsLinked 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.

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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.

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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.

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PubMed1974

A simpler iterative steady state solution of münch pressure-flow systems applied to long and short translocation paths.

A simple steady state iterative solution of Münch pressure-flow in unbranched sieve tubes containing only water and sucrose is derived. The iterative equations can be solved on a programmable desk calculator. Solutions are presented for steady state transport with specific mass transfer rates up to 1.5 x 10(-5) mole second(-1) centimeters(-2) (= 18.5 grams hour(-1) centimeters(-2)) over distances in excess of 50 meters. The calculations clearly indicate that a Münch pressure-flow system can operate over long distances provided (a) the sieve tube is surrounded by a semipermeable membrane; (b) sugars are actively loaded in one region and unloaded at another; (c) the sieve pores are unblocked so that the sieve tube hydraulic conductivity is high (around 4 centimeters(2) second(-1) bar(-1)); (d) the sugar concentration is kept high (around one molar in the source region); and (e) the average sap velocity is kept low (around 20-50 centimeters hour(-1)). The dimensions of sieve cells in several species of plants are reviewed and sieve tube hydraulic conductivities are calculated; the values range from 0.2 to 20 centimeters(2) second(-1) bar(-1). For long distance pressure-flow to occur, the hydraulic conductivity of the sieve cell membranes must be about 5 x 10(-7) centimeters second(-1) bar(-1) or greater.

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PubMed1973

A Mathematical Treatment of Munch's Pressure-Flow Hypothesis of Phloem Translocation.

The steady state solutions of two mathematical models are used to evaluate Münch's pressure-flow hypothesis of phloem translocation. The models assume a continuous active loading and unloading of translocate but differ in the site of loading and unloading and the route of water to the sieve tube. The dimensions of the translocation system taken are the average observed values for sugar beet and are intended to simulate translocation from a mature source leaf to an expanding sink leaf. The volume flow rate of solution along the sieve tube, water flow rate into the sieve tube, hydrostatic pressure, and concentration of sucrose in the sieve tube are obtained from a numerical computer solution of the models. The mass transfer rate, velocity of translocation, and osmotic and hydrostatic pressures are consistent with empirical findings. Owing to the resistance to water flow offered by the lateral membranes, the hydrostatic pressure generated by the osmotic pressure can be considerably less than would be predicted by the solute concentration. These models suggest that translocation at observed rates and velocities can be driven by a water potential difference between the sieve tube and surrounding tissue and are consistent with the pressure-flow hypothesis of translocation.

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PubMed1972

Solution-flow in the Phloem: I. Theoretical considerations.

A mathematical model for the reversible exchange of THO between the sieve tube lumen and its surrounding phloem tissue is used to explain the difference between the apparent velocities of THO and (14)C-sucrose transport observed when both are supplied simultaneously. Theoretically predicted results show a close correlation with those obtained experimentally. This model may be used in evaluating previous work in which THO was used as a tracer. The calculations support the existence of a mass flow of sugars in aqueous solution along the path.

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PubMed1972

Solution-Flow in the Phloem: II. Phloem Transport of THO in Beta vulgaris.

Translocation profiles along the path were studied using a modified flap-feeding technique for the simultaneous application of THO and (14)C-sucrose. A re-evaluation of a mathematical model for phloem transport with reversible lateral exchange of tracer along the path indicates that lower apparent velocities for THO as compared to labeled carbohydrate are primarily due to extensive lateral exchange of THO along the conduction path. Path-chilling experiments support the concept that THO and (14)C-sucrose exhibit different lateral exchange characteristics. The data presented are consistent with a solutionflow mechanism.

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PubMed1972

Carbohydrate translocation in sugar beet petioles in relation to petiolar respiration and adenosine 5'-triphosphate.

Earlier studies have shown that the retarding effect of low petiolar temperatures on sucrose transport through sugar beet (Beta vulgaris L.) petioles is markedly time-dependent. Although the initial effect of chilling the petiole to near 0 C is severely inhibitory, translocation rates soon recover (usually within about 2 hours) to values at or near the control rate. In the present studies, selected metabolic parameters were measured simultaneously with translocation. No stoichiometric relationships among petiolar sucrose transport, petiolar respiration (CO(2) production), and calculated petiolar ATP turnover rates were evident. It appears that the major sources of energy input energizing carbohydrate transport in sieve tubes function mainly at either loading or unloading sites and not at the level of individual sieve-tube elements.

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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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