Biomedical subjects

A H Markhart

Publications and source records attributed to A H Markhart.

9 recordsLinked to original sources

PubMed1990

Temperature Effects on Mitochondrial Respiration in Phaseolus acutifolius A. Gray and Phaseolus vulgaris L.

Electron transport, using succinate as a substrate, was measured polarographically in mitochondria isolated from Phaseolus vulgaris and P. acutifolius plants at 25 degrees C and 32 degrees C. Mitochondria isolated from P. vulgaris plants grown at 32 degrees C had reduced electron transport and were substantially uncoupled. Growth at 32 degrees C had no effect on electron transport or oxidative phosphorylation in P. acutifolius compared to 25 degrees C grown plants. Mitochondria isolated from 25 degrees C grown P. vulgaris plants measured at 42 degrees C were completely uncoupled. Similarly treated P. acutifolius mitochondria remained coupled. The uncoupling of P. vulgaris was due to increased proton permeability of inner mitochondrial membrane. The alternative pathway was more sensitive to heat than the regular cytochrome pathway. At 42 degrees C, no alternative pathway activity was detected. The substantially greater heat tolerance of P. acutifollus compared to P. vulgaris mitochondrial electron transport suggests that mitochondrial sensitivity to elevated temperatures is a major limitation to growth of P. vulgaris at high temperatures and is an important characteristic conveying tolerance in P. acutifolius.

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PubMed1987

Stomatal aperture oscillations of Abutilon theophrasti Medic. and Hordeum vulgare L. examined by three techniques.

Temporal fluctuations of stomatal aperture are important to water use efficiency. Techniques that use point measurements widely spaced in time such as diffusive resistance porometry (DRP) or pot weighing (PW) may not provide a complete description of changes in stomatal aperture. Three techniques, DRP, PW, and continuous flow porometry (CFP), were used to examine stomatal aperture changes in Hordeum vulgare L. and Abutilon theophrasti Medic. Plants maintained in controlled environment chambers at 23 degrees C under LD 12:12 were monitored during 2 days of continuous illumination (LL) and a subsequent LD cycle. Oscillations with periods of about 24 hr were observed in both leaf surfaces in the two species. In addition, during LL, high-frequency fluctuations were sometimes superimposed on the 24-hr oscillation. There was better agreement between the results of CFP and DRP in Abutilon than in Hordeum. The reason for this difference is unclear. CFP revealed rapid fluctuations in stomatal aperture that were not observed with the other techniques.

Activity Cycles↗

PubMed1985

Comparative Water Relations of Phaseolus vulgaris L. and Phaseolus acutifolius Gray.

Leaf area expansion, dry weight, and water relations of Phaseolus vulgaris L. and P. acutifolius Gray were compared during a drying cycle in the greenhouse to understand the characteristics which contribute to the superior drought tolerance of P. acutifolius. Stomates of P. acutifolius closed at a much higher water potential than those of P. vulgaris, delaying dehydration of leaf tissue. P. acutifolius had a more deeply penetrating root system, which also contributes to its drought tolerance. Root-shoot ratios did not differ between the two species either under well watered or water stressed conditions. Leaf osmotic potential was also similar in the two species, with no apparent osmotic adjustment during water stress. These results indicate that P. acutifolius postpones dehydration and suggest that sensitive stomates and a deeply penetrating root system are characteristics which, if incorporated into cultivated beans, might increase their drought tolerance.

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PubMed1985

Quantifying Apoplastic Flux through Red Pine Root Systems Using Trisodium, 3-hydroxy-5,8,10-pyrenetrisulfonate.

The fluorescent compound trisodium, 3-hydroxy-5,8,10-pyrenetrisulfonate (PTS) was used to quantify the apoplastic flux through red pine (Pinus resinosa Ait.) root systems-that portion of the total water flux reaching the xylem without ever crossing a semipermeable membrane. Flow was induced by pressure through detopped root systems, and by transpiration through intact seedlings. Apoplastic flux was determined by multiplying total flux by the ratio of PTS concentration in the xylem exudate to PTS concentration in the bathing medium.Under aeration, apoplastic flux was less than 1% of total flux. Under anaerobic conditions, up to 50% of total flux was apoplastic suggesting that anaerobic conditions change the pathway of water flow into root xylem. The change under anaerobic conditions was reversible. Detopped root systems under pressure and intact seedlings under transpiration gave similar results. In detopped root systems, the magnitude of the pressure gradient may alter the apoplastic contribution to total flux.

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PubMed1984

Amelioration of chilling-induced water stress by abscisic Acid-induced changes in root hydraulic conductance.

Pretreatment of soybean (Glycine max L. var Ransom) root systems with abscisic acid (ABA) ameliorates the deleterious effect of low temperatures on root hydraulic conductance. ABA treatment of root systems subsequently chilled to 10 degrees C with shoots at 25 degrees C resulted in higher leaf water potentials and lower stomatal resistances. If the root systems are left at 25 degrees C, ABA causes stomatal closure. Membrane alterations are suggested as a mechanism for the ABA action in plant response to chilling stress.

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PubMed1982

Penetration of soybean root systems by abscisic Acid isomers.

The penetration of soybean (Glycine max L. cv. Ransom) root systems by exogenously applied isomers of abscisic acid was monitored by measuring the concentration of the chemical in the xylem exudate of root systems exposed to a three bar hydrostatic pressure difference. The cis-trans isomer penetrated more readily than the trans-trans isomer; however, up to 6 hours was needed to reach steady-state values. Exogenous abscisic acid also decreased volume flux through the root system and increased total carbon dioxide efflux from the vessel containing the root system.

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PubMed1980

Effect of valinomycin and gramicidin d on the reflection coefficient of soybean root systems.

Valinomycin and gramicidin D were used to test the hypothesis that a lipoprotein membrane is the osmotic barrier in mature root systems. Hydroponically grown soybean (var. Ranson) root systems were pressurized in steps between 0.2 and 5.0 bars at 25 C. Steady-state flow rates and exudate osmotic potentials were measured at each pressure. Valinomycin or gramicidin D to a final concentration of 2.6 or 5.0 micromolar, respectively, was injected into the nutrient solution, and steady-state values were again measured between 0.2 and 5.0 bars. Both ionophores reduced the x intercept of the straight line portion of the flux rate versus pressure curve, and increased the exudate osmotic potential at infinite flow rate. Valinomycin lowered the reflection coefficient from 0.91 to 0.76, whereas gramicidin D lowered the reflection coefficient from 0.86 to 0.81. The results support the hypothesis that one or more lipoprotein membranes in the root system function in regulation of ion movement from the ambient solution to the stele during high transpiration rates.

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PubMed1979

Effect of temperature on water and ion transport in soybean and broccoli systems.

Steady-state flow rates and exudate osmotic potentials were measured from complete root systems from warm- (28/23 C) or cold-(17/11 C) grown soybean or broccoli (Brassica oleracea) plants at various pressures or different temperatures.In warm-grown soybean roots systems, a break occurred at 14.7 C in the Arrhenius plot of total flow at constant pressure. When plants were grown at lower temperatures, the break point shifted to 8 C. Broccoli, a chilling-resistant species, showed no break for the temperature range used, but cooler growth temperatures decreased the activation energy for water flow through the root system from 18 kilocalories per mole to 9 kilocalories per mole. In both broccoli and soybean, cold-grown plants had lower exudate potentials and greater flow rates at low hydrostatic pressures than the warm-grown plants.These observations indicate that the rate-limiting site for passive water transport is a membrane which may be modified as the plant acclimates to varying growth temperatures. An additional part of the acclimation process is an increase in activity of root ion pumps.

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PubMed1979

Effect of abscisic Acid on root hydraulic conductivity.

Reports of the effects of abscisic acid (ABA) on ion and water fluxes have been contradictory. Some of the confusion seems due to the interaction of ion and water transport across membranes. In these experiments root systems were subjected to hydrostatic pressures up to 5.0 bars to enable measurement of root conductance that was independent of measurement of osmotic potentials or ion fluxes.ABA between 5 x 10(-5) molar and 2 x 10(-4) molar resulted in a decrease in the conductance of the soybean root systems as compared with the controls. ABA treatment also eliminated the discontinuity in the Arrhenius plot of total flow versus reciprocal temperature at constant pressure. The results suggest that ABA acts at the membrane that is rate-limiting to water flow directly, or by altering metabolism that in turn affects the membrane.

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