Multiphasic Absorption of Glucose and 3-O-Methyl Glucose by Aged Potato Slices

118

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 '4C-sucrose to approximately 20% of the control. Dinitrophenol at a concentration of 8 mM inhibited rather than promoted C02 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 energyrequiring process for both phloem loading and translocation in the source leaf.Application of '4C-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 14C-sucrose solutions. The first and possibly the second phases were attributed to active, carriermediated accumulation in the minor vein phloem. Autoradi The source leaf, a sugar-exporting region of a plant, seems to play a key role in providing the driving force for long distance transport of organic solutes throughout the plant. Few translocation studies have been focused on events in the source leaf, and some crucial questions remain unanswered. Is there, for example, an energy-requiring step in the transport of sugar from chloroplasts to sieve elements that is directly related to phloem loading and long distance transport. If an active step is demonstrated, do the kinetics of uptake indicate that this step involves a membrane carrier?In 1939, Curtis and Asai (3) suggested that the osmotic gradient from the mesophyll to the sink tissue was in the opposite direction required for a pressure flow mechanism. Roeckl (26) observed that the mesophyll cells, which produce mobile sugars, have a substantially lower osmotic potential than the phloem exudate. A gradient of increasing concentration toward the sieve tubes would require energy to drive transport in that direction, suggesting the need for an active step between sugar production and translocation out of source leaves. Barrier and Loomis (1) termed the active step "loading," implying that the rate-limiting step is transport into the veins, rather than a simple chemical transformation or synthesis of the translocate. There now seems to be general agreement that an active step does occur in source tissue (10,...

Section: Discussioncontrasting

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Evidence for Active Phloem Loading in the Minor Veins of Sugar Beet

Sovonick¹,

Geiger²,

Fellows³

1974

118

“…Multiphasic responses relating the rate of uptake to the concentration are familiar in ion uptake studies (18) and have also been shown for sugar uptake as a function of the external sugar solution in castor bean seedlings (19), tobacco cells (3), and in sugar beet leaves (27). These similar relationships may suggest common mechanisms but the cause of the multiphasic responses has yet to be established.…”

Section: General Features Of Carbon 1 1 Translocationmentioning

confidence: 75%

Relations between Light Level, Sucrose Concentration, and Translocation of Carbon 11 in Zea mays Leaves

Troughton¹,

Currie²,

Chang

1977

The mechanism of carbon transport in Zea mays leaves was investigated using carbon 11 which is a short lved (half-lfe 20.4 min) positronemitting isotope. The gamma radiation produced on annihilation allows in vivo or nondestructive measurement of the isotope and the short halflife allows many measurements of trandocation to be made on the same leaf within the same day.Carbon 11 produced by the 10B (d,n)"C nuclear reaction was converted to "CO2, fed to a leaf as a short pulse, and assimilated during photosynthesis. The progress of the radioactive pulse along the leaf in the phloem was monitored in severd positions simultaneously with counters. The counters were Nal crystals with photomaltipliers and the output was amplified, passed to single channel analyzers, and the counts accumulated for 20 seconds every 30 seconds. Corrections were made for the half-life and background radiation by computer, and the results were displayed on a high speed plotter. Information derived from the corrected data induded the speed of translocation, the shape of the radioactive carbon pulse, and the influence of light and distance along the leaf on these parmeters. The plants were kept under controlled environment conditions during all measurements.A speed was derived from the time displacement of the midpoint of the front of the pulse, measured at two positions along the leaf. This was an apparent mean speed of translocation because it averaged a variation in speed with distance, variation in speed between or within sieve tubes, and it averaged the mean speed of all of the particles in the pulse.A wide range of speeds of translocation from 0.25 to 11 cm min-' was observed but most of the variability was due to the variation in light available to the leaf. For example, the speed of translocation was proportional to the light level on either the whole plant or individual leaf. Shading of the leaf established that the light effect was not localized in either the feeding area or in the portion of the leaf on which the measurements were made. It was proposed that the speed was dependent on the proportion of the leaf in the light upstream from the last counter. The speed of translocation was relatively independent of the stage of growth of the plant, age of the leaf, and the time during the diurnal light cyde. Data Carbon translocation is an integral component of the growthand yield-determining processes in higher plants. To characterize long distance C transport, it will be necessary to measure the speed and sugar concentration of the solution and the crosssectional area of the sieve tubes active in translocation. Other data, such as the turgor pressure and the water, ion, and sugar exchange rates of the sieve tubes, are required to establish the mechanism of C transport and explain the cause of the variation in C translocation between species, or between plants, under different environmental conditions.Routine measurement of some of these translocation parameters has not been easy. Through the development of techniques to prod...

“…Uptake of 2-deG by maize seedling roots exhibited Michaelis-Menten kinetics in each of the two concentration ranges tested, 0.05 to 1 mm and 3 to 40 mm. Hexose uptake by potato slices exhibits a similar behavior, with four phases detected (9), and multiphasic absorption of amino acids has also been reported (5). Ophn A concentrations as low as 2 ,ug/ml (5 ,UM) inhibited 2-deG uptake significantly (Fig.…”

Section: Resultsmentioning

confidence: 62%

Effects of Ophiobolin A on Ion Leakage and Hexose Uptake by Maize Roots

Tipton¹,

Paulsen²,

Betts³

1977

Ophiobolin A, a sesterterpene metaboHte ofHelminthosporium maydis, Nisikado and Miyake, stimulates net leakage of electrolytes and glucose from maize (Zea mays L.) seedlng roots. Treatmnent of the roots with ophiobolin A at a concentration of 10 ag/nd (25 jAm) inhbits uptake of 10 mM 2-deoxyglucose by 50% and of 0.5 mM 2-deoxyglucose by 85%. Compartmental (15) showed that ophn A at concentrations as low as 30 ,g/ml inhibited the elongation of rice coleoptiles and seedling roots and of oat coleoptiles. Treatment of beet root slices with ophn A caused leakage of anthocyanin. In a later report (16), leakage of phosphate and organic compounds from corn seedling roots and from potato tuber discs was mentioned. The inhibition of rice seedling growth was confirmed by Ohkawa and Tamura (13) and it was shown by Oku (14) that ophn A could be isolated from infected rice leaves, suggesting that ophiobolin toxicity may play a role in disease development.In this paper, we report that ophn A is an effective inhibitor, at low concentrations, of hexose uptake in maize and carrot roots and stimulates ion leakage from these tissues. (10) with the addition of 0.02 g FeSO4 7H20 and 1 g yeast extract/l; pH of the medium was 5.6. Spore suspensions were used to inoculate 200-ml portions of medium in 2,800-ml Fernbach flasks, and the flasks were incubated at 25 ± 1 C in a growth chamber with approximately 1200 ft-c of continuous light.After 10 days, the growth medium was separated from the mycelial mats by straining through four layers of cheesecloth and filtering through Whatman No. 1 filter paper. The combined filtrate (12 liters) was transferred to a liquid-liquid extractor and extracted with diethyl ether for a total of 12 hr. The ether extract was dried with sodium sulfate and taken to an orange, nondrying oil by evaporation under vacuum at 45 C. The oil was dissolved in methylene chloride and crystallized by the addition of heptane. Washing with cold diethyl ether yielded 283 mg of white, needle-shaped crystals. Progress of the purification was monitored by TLC on Silica Gel GF254 plates developed with chloroform-methanol (100:2, v/v), and visualized by spraying with 1 % phosphomolybdic acid in methanol followed by heating at 100 C for 2 min. After repeated recrystallization, TLC revealed a major component with an RF (0.5) identical to that of authentic ophn A and a small amount of a second component with higher mobility. Nuclear magnetic resonance and low resolution mass spectra of this material corresponded to published data (6, 12).Plant Material. Corn seeds (Zea mays L., W64A, N or T cytoplasm) were washed with water and placed in 20% H202 for 30 min. The seeds were washed three times with sterile distilled H20 and transferred aseptically into sterile Petri dishes lined with two filter paper discs and moistened with 5 ml of 0.1 mm CaCl2. Ten seeds were placed in each dish. The seeds were incubated for 3 days at 30 C in the dark. The root tips (approximately 2.5 cm) of the germinated seeds were removed for use in 90...