Sugar Uptake and Translocation in the Castor Bean Seedling I. Characteristics of Transfer in Intact and Excised Seedlings

Slices of corn scutellum were used to study amino acid uptake, a natural function of this tissue. The uptake of glutamine was found to be inhibited by several monovalent cations. The accompanying anion did not affect the inhibition. Divalent cations stimulated glutamine uptake, particularly at high glutamine concentrations. The inhibition by monovalent cations was reversed by divalent cations.There was a broad pH optimum (4.3-5.2), and a wide range of pH values over which substantial rates of glutamine uptake were observed (3-7).Seventeen protein amino acids were tested for their effects on glutamine uptake. L-Alanine, I.-cysteine, Lglutamic acid, glycine, L -serine, and L -methionine were the only inhibitory ano acids. The inhibition by alanine and methionine was overcome at high glutamine concentrations whereas the inhibition by monovalent cations was not. The inhibition by amino acids was not reversed by divalent cations. D-Alanine and D-methionine had no effect on glutamine uptake.High concentrations of sucrose (0.5 M) did not affect glutamine uptake. Thus, osmotic shock did not cause a dissociation of a transport protein from a membrane. Mannose inhibited glutamine uptake but glucose, galactose, and fructose did not. In addition to respiratory inhibitors, N-ethylmaleimide inhibited glutamine uptake. Ouabain did not affect glutamine uptake.During the germination of corn seeds, reserve proteins and starch in the endosperm are digested and the hydrolytic products, amino acids and sugars, are transported to the growing embryonic axis. The corn scutellum is a specialized absorptive organ which is wholly responsible for the uptake and transfer to the embryonic axis of amino acids, sugars, and other compounds required by the heterotrophic seedling. Thus, the scutellum is a logical tissue for the study of amino acid uptake.Glutamine was chosen for uptake studies because of its importance as a transport compound in plants. It is one of the most abundant free amino acids in partially digested corn endosperm and if radioactive glutamine is applied to the haustorial surface of the scutellum, radioactive glutamine is recovered in the growing points of the embryonic axis after a few hours (unpublished data). Thus, glutamine is available for uptake by the scutellum and can be transported in corn seedlings. Some plant tissues have been shown to accumulate amino acids against a concentration gradient (1), to require metabolic energy for amino acid uptake (2, 10), and to possess a mechanism of uptake of certain amino acids which is somewhat selective. Also, there is evidence (3) that there is competition among amino acids for uptake. This paper describes some features of the uptake of glutamine by scutellum slices. MATERIALS AND METHODSCorn seeds (Zea mays L. var. "Pioneer 3206") were germinated for 4 days between moist paper towels at 30 C. The scutella were excised and cut transversely into slices 0.5 mm or less in thickness. The slices were washed in distilled water until the wash water remained clear. The washed ...

Section: Resultsmentioning

confidence: 58%

Some Characteristics of the Uptake of Glutamine by Corn Scutellum

Stewart

1971

“…It appeared that removing the endosperm improved the water status of the hypocotyl by reducing the hydraulic resistance ofthe cotyledons. It is well known from other transport studies that the cuticle ofyoung cotyledons ofRicinus is fairly permeable for water and hydrophilic solutes such as sucrose (13,15). No significant differences of turgor of the inner cortex cell layers were found along the axes of the seedlings, i.e.…”

Section: Hydraulic Parameters Of Cortex Cells In the Intact Seedlingmentioning

confidence: 85%

Gradients of Turgor, Osmotic Pressure, and Water Potential in the Cortex of the Hypocotyl of Growing Ricinus Seedlings

Meshcheryakov

Steudle²,

Komor³

1992

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe.Because the osmotic pressure of individual cells (ir') was also determined, the water potential (4/) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of wellwatered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor = wr and #~0 across the cortex, there were also no gradients of i' across the tissue. Gradients of cell turgor and iri increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cufting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCI2 solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.

“…Studies of translocationi up to the p)resent time, have been focused mainly on downwarcd tranislocationi (1,2,3,4,5). Verv little work has been done on upward traluslocationi in comparison witlh downward trainslocation.…”

mentioning

confidence: 99%

Comparison of Upward and Downward Translocation of ¹⁴C From a Single Leaf of Sunflower

Shiroya

1968

Abstract. When single leaves attached at a given node were allowed to carry on photosynthesis in 14CO2 for 30 min, younger plants showed a higher proportion of upward translocation than did older plants. Downward translocation of 14C-photosynthate was stimulated by ATP pre-treatment of the translocating leaf, while upward translocation was not affected by ATP. A similar phenomenon was observed in the translocation of 14C-sucrose infiltrated into a leaf with or without ATP. Downward translocation of photosynthate was inhibited by DNP pre-treatment of a fed leaf. Upward translocation, however, was not affected by DNP. Thirty min after infiltration of 14C-glucose into a leaf, almost all the 14C translocated upwards was found to be in the form of glucose, whi'le a great part of the 14C translocated downwards was in the form of sucrose. In the case of translocation of infiltrated 14C-suerose, 14C found both above and below the fed leaf was mainly in the form of sucrose.In our previous study (7) 2) whether the mechanisms of upward and downward translocation were different: 3) whether the translocation pattern of infiltrated a C-sugars corresponded to that of 14 C-photosynthates. Materials and MethodsSunflower (Helianthts annuus) plants were grown at 25°for 4 to 9 weeks in a growth cabinet programmed to a 13 hr photoperiod of 30,000 lux at leaf level and an 11 hr dark period. Almost all experiments were carried out with 5-week-old plants about 20 cm tall. The first or second leaf from the base of the plant at the above-mentioned stage was mainly used as the fed leaf in all the experiments. Each experiment was done with 3 plants selected both for uniformity of height and leaf size. Every datum in the tables and figures is the average value for 3 plants.Feeding. To feed 14CO2, a leaf was enclosed in a transparent photosynthetic chamber. Seventy ,uc of 14CO.) were generated and introduced into the chamber as previously described (7). The whole planit was placed under illumination of 30,000 lux for 30 min to allowv the fed leaf to fix 14CO. and at the samiie time translocate 14C-photosynthate to parts both above and below the fed leaf. 14C-sucrose or '4C-glucose was introduced into leaves by vacuum infiltration. After infiltration for 2 min, the leaf was removed from the vessel, washed with distilled water, and kept under illumination for 30 min to translocate the infiltrated 14C-sugars.Treatmnent With A TP and DNP. The ATP solution (di-potassium salt, 2 X 10-2 M in phosphate buffer, pH 6.8, 5 X 10-2 M) or DNP solution (10-4 M in phosphate buffer, pH 6.8, 5 X 10-2 M) was introduced into the fed leaf by means of vacuum infiltration. The phosphate buffer above was infiltrated into a corresponding leaf of the control plant.