The Effect of Phloem Ringing on Root Growth and Potassium Uptake by Helianthus annuus

The effect of phloem ringing on the uptake and transport ofpotassium by the roots of 4 week old sunflower plants has beeninvestigated. Ringing caused a rapid decline in both K⁺ uptakeand its transport (⁸⁶Rb tracer) to the shoot. The rate of rootelongation and the levels of sucrose in the root showed paralleldecreases after ringing. Measurement on isolated roots indicatedthat the effect of ringing the stem on K⁺ uptake by the rootswas confined to the apical 10 mm that included the extensionzone. It is postulated that the decline in potassium uptakeand transport, brought about by ringing, is due to the severanceof the sucrose supply which stops root growth. Key words: Roots, Growth, Salt uptake     

Sucrose Leakage from Isolated Parenchyma of Sugar Beet Roots

The kinetics of sugar efflux from slices of sugar beet rootswas investigated using washing solutions of different osmoticpressure and calcium concentration. The leakage of sucrose isstrongly reduced in solutions of high osmotic pressure (>0·8MPa) or high calcium concentration (10 mM). Turgor-dependentnecrosis of parenchyma cells (plasmoptysis) is the main causeof sucrose efflux from the tissue in hypotonic media with lowcalcium activity. This was shown by good correlation betweenthe percentage of leaked sucrose and the percentage of tissuewater, which was in the free space after the washing procedure.The kinetics of sugar leakage from beet root parenchyma is nobasis for the estimation of the sugar contents of the free spaceor the cytoplasm in situ.     

Exchange diffusion of alkali ions through the apoplast of the potato (Soianum tuberosum L.) storage parenchyma

Interdiffusion coefficients of K and Na for tangential transport through cell walls were found to be independent of ion strength and only moderately (c. 50%) lower than values known for diffusion in water. Although at low salt concentrations alkali ion exchange diffusion on the cell wall pathway is facilitated by accumulation of diffusing species in the Donnan space, at all concentrations the resistance of the whole tissue for apoplastic cation exchange diffusion remained significantly higher than that of unstirred liquid of equal thickness, as the area fraction of the cell wall pathway in the parenchyma is small.     

Culturing of stoneworts and submersed angiosperms with phosphate uptake exclusively from an artificial sediment

1. We aimed to demonstrate reproducible nutrition and growth of macrophytes in non‐axenic laboratory cultures preventing growth of phytoplankton and epiphytes. 2. Macrophyte shoot segments were planted in a mixture of commercial acid‐washed silica sand with crystalline tricalcium phosphate, and this artificial sediment was covered with a layer of pure silica sand. The liquid mineral media used did not contain phosphorus but were rich in all other nutrient elements. A CO₂ reservoir provided sustainable CO₂ supply to macrophyte cultures by gas diffusion through a polyethylene membrane. 3. Chara hispida, Chara tomentosa, Chara baltica, Elodea canadensis, Potamogeton pectinatus and Zanichellia palustris could be cultivated for long term without medium exchange and aeration. Microalgae growth was prevented by the absence of phosphate in the water column. Mobilisation of tricalcium phosphate and phosphate uptake by the rhizoids of C. hispida enabled sustainable rapid shoot growth and increased the concentration of inorganic phosphate in the shoot dry weight by five to six times in comparison with plants cultivated on pure silica sand. A significant growth support from tricalcium phosphate was also observed for E. canadensis, but the rate of phosphate uptake by the roots was not sufficient to maintain a storage pool of inorganic phosphate (P_i) in the growing shoots of this plant. 4. Membrane‐controlled CO₂ supply from a reservoir and artificial sediments like the one described provide attractive options for the laboratory culture of macrophytes.     

Gas and Liquids in Intercellular Spaces of Maize Roots

Oils are spontaneously absorbed by gas-filled intercellularspaces (IS) in maize root cortex. The network of these spacesin living root sections was imaged by confocal laser scanningmicroscopy using a fluorescent solution of Nile red in oil.The gas volume fraction (GVF) of root segments was quantifiedby the increase in weight (differentiated zones) or tissue density(2–3 mm root tips) due to complete vacuum infiltration.Cooling to 6 °C or inhibition of oxidative phosphorylationdiminished the GVF of root tips but did not significantly affectthe GVF of differentiated root zones. The threshold pressuredifference for measurable infiltration of isolated root segmentsis lower (10 to 15  kPa) than the threshold for infiltrationof comparable zones of attached roots or of detached roots withthe cut surface sealed (>60 kPa). In the absence of an opencut, pressure-driven infiltration of the root cortex is acceleratedby microscopic fissures within the epidermal/hypodermal barrier.The GVF of the root cortex was reduced after transferring rootsfrom sugar solutions (0.1 to 0.3M ) to water. This points toefficient water transport from the medium to sugar-containingcortical cell walls through epidermal and hypodermal protoplasts.When 2-cm-long primary roots were vacuum infiltrated in situand then allowed to grow on aerated mineral medium for a further5 d, cortical IS of the originally infiltrated root bases remainedfilled with liquid but the subsequently grown apical root zoneshad a normal GVF. Copyright 1999 Annals of Botany Company Apoplastic and protoplasmic route, maize, infiltration, intercellular spaces, oil absorption, confocal laser scanning microscope, water transport, Zea mays L.     

Specificity and Reversibility of the Bicarbonate Effect on Hexose Uptake by Maize Root Tips

The inhibition of hexose uptake by bicarbonate ions was investigatedin detail in order to test the specificity and reversibilityof the effect and to compare it with those of other electrolytes.The degree of inhibition was similar at pH 7.0 and pH 8.0. AtpH 4.5 no influence of a high concentration of CO₂ on 3-O-methylglucoseuptake was found. Therefore, the inhibition of hexose uptakeby bicarbonate cannot be explained by consequences of CO₂ influx.The inhibition of sugar absorption by calcium and potassiumions was similar to that exerted by bicarbonate in so far asit was observed at higher pH only. The inhibition exerted bysodium salts of different monovalent weak acids was limitedto lower pH and needed some time to become established or reversed.The bicarbonate effect was independent of time and reversiblewithout a lag phase. Sodium salts of strong mineral monovalentacids did not differ significantly in their effect on sugaruptake. Bicarbonate inhibited phosphate uptake in a similarmanner to hexose uptake but strongly stimulated the absorptionof potassium. The bicarbonate effect is assumed to result froma change in the degree of coupling of secondary active transportto the proton pump. Key words: Inhibition, Transport coupling, pH, Proton pump     

Radial transport of salt and water in roots of the common reed (Phragmites australis Trin. ex Steudel)

To understand the root function in salt tolerance, radial salt and water transport were studied using reed plants growing in brackish habitat water with an osmotic pressure (π_M) of 0.63 MPa. Roots bathed in this medium exuded a xylem sap with NaCl as the major osmolyte and did so even at higher salt concentration (π_M up to 1.3 MPa). Exudation was stopped after a small increase of π_M (0.26 MPa) using polyethylene glycol 600 as osmolyte. The endodermis of fine lateral roots was found to be the main barrier to radial solute diffusion on an apoplastic path. Apoplastic salt transfer was proven by rapid replacement of stelar Na⁺ by Li⁺ in an isomolar LiCl medium. Water fluxes did not exert a true solvent drag on NaCl. Xylem sap concentrations of NaCl in basal internodes of transpiring culms were more than five times higher than in medial and upper ones. It was concluded that the radial NaCl flux was mainly diffusion through the apoplast, and radial water transport, because of the resistance of the cell wall matrix to convective mass flow, was confined to the symplast. Radial salt permeation in roots reduced the water stress exerted by the brackish medium.     

Suitability of the Osmotic Shock Procedure for the Analysis of Membrane Transport in Root Tips of Zea mays L.

Root tips of Zea mays L. previously subjected to osmotic shockwere studied as to uptake and efflux of glucose and uptake ofphosphate, as well as release of protein and other Lowry-positivesubstances. The osmotic shock procedure applied was similarto that frequently used in order to release periplasmic proteinsfrom bacteria. After treatment with hypertonic solutions of sorbitol (>0?3M) the uptake of phosphate and glucose is reduced and proteinis released into the medium. All osmotic treatments leadingto a significant reduction of either glucose or phosphate uptakeare accompanied by a strongly increased release of glucose andLowry-positive substances. Damage of plasma membrane barrierfunction can be directly observed by the uptake of Evans Blue(Gaff and Okong'o-Ogola, 1971) into the cells involving mainlyolder vacuolated rhizodermal cells. The occurrence and extent of the alterations of membrane semipermeabilityare strongly dependent on the duration of exposure to and theconcentration of the sorbitol solution used. The velocity ofthe osmotic transitions (from hypotonic to hypertonic and viceversa) is of minor importance. Sensibility of cells to osmotic stress increases with differentiationand vacuolation. Conditions for a selective reduction of phosphateuptake seem to occur when only meristem and near tip regionssubjected to a mild osmotic shock are used.     

Solute Leakage from the Isolated Parenchyma of Allium cepa and Kalanchoe daigremontiana

Efflux of different solutes from leaf slices of Kalanchoëdaigremontiana and from slices of the onion bulb scale was reinvestigatedwith respect to (1) dependence on turgor, (2) selectivity, (3)integrity of protoplasts and cellular changes. In both materials efflux of solutes (electrolytes or sugars)is non-selective and strongly dependent on turgor. Treatmentof tissue slices with hypotonic solutions (below a criticalosmotic pressure) resulted in high leakage rates, an increasein free space and an increased number of damaged cells. Lowconcentrations of calcium did not prevent this loss of retentionand cell stability. Part of the surviving cells were found to have a strongly decreasedosmotic pressure of cell sap. Leakage did not occur simultaneouslyat all cells of the tissue slice. It can be concluded that effluxfrom parenchyma cells in hypotonic solutions results from irreversibleosmotic breakdown and reversible membrane defects both favouredby high turgor. Key words: Parenchyma cells, Allium cepa, Kalanchoé daigremontiana, Solute efflux, Viability, Permeability, Plasmoptysis