Inhibition of ion uptake to barley roots by cycloheximide

In Petunia pollen tubes growing in the style there appear to be two ways of callose deposition. The first one is callose deposition outside the plasma membrane as a distinct layer closely appressed to the cell wall. The second one is callose deposition within the cytoplasm as distinct callose grains, leading to the formation of callose plugs. This second way is accompanied by a characteristic ultrastructure of the cytoplasm, namely strong electron-density of the plasma matrix, partial absence of the plasma membrane and the absence of plastids and dictyosomes. For both ways of callose deposition a mechanism is proposed and the function of callose plugs is discussed.Abbreviation RER rough endoplasmic reticulum     

Ultrastructure of xylem parenchyma cells of barley roots in relation to ion transport to the xylem

Summary The structure of xylem parenchyma cells is examined in relation to transport of ions through the root. Measurement of uptake of ⁸⁶Rb⁺ and its transport through the root at different distances from the apex show that this is a general activity along the length of the root and not confined to a limited region. Thus transport through the root is not stopped by removal of that part of the root tip containing metaxylem vessels with living contents. The structure of xylem parenchyma appears to be suitable for involvement in ion transport from the stele to the xylem. At 1 cm behind the tip, where metaxylem vessels have no living contents but ion uptake and transport are going on at high rates, xylem parenchyma cells are rich in cytoplasm with extensive rough endoplasmic reticulum and well-developed mitochondria. Their cell walls contain numerous plasmodesmata, establishing the possibility of a symplastic pathway across the stele up to the vessels. The results are discussed in relation to regulation of ion transport to the xylem vessels in roots.Dedicated to Professor O. Stocker on the occasion of his 85th birthday.     

Effects of organic acids on ion uptake and retention in barley roots

Effects of several organic acids on ion uptake and retention and on respiration in barley roots having low and high KCl contents were assayed by measurements of K⁺, Na⁺, Ca²⁺, Cl⁻, and oxygen uptake. Organic acids with high pK_a values increase the permeability of roots to ions and decrease respiration when present in sufficient concentrations at pH 5 but have no inhibitory effects at pH 7. Absence of respiratory inhibition in short times and at lower organic acid concentrations, under conditions that immediately produce a permeability increase, indicate that the permeability change is not a result of respiratory inhibition. Effects of formate, acetate, propionate, and glutarate are attributed to entry of undissociated acid molecules into the effective membranes. Lack of a permeability increase with succinate, which has lower distribution coefficients to lipid solvents than do the aliphatic acids, can be explained by failure of sufficient amounts of the hydrophilic succinic acid molecules to penetrate the membranes involved. These experiments suggest that undissociated acid in root membranes can increase permeability of the roots.     

Effect of azetidine 2-carboxylic Acid on ion uptake and ion release to the xylem of excised barley roots

Azetidine 2-carboxylic acid (AZ) was used as an analog of proline to investigate further the relationship between protein synthesis and ion transport. AZ does not inhibit protein assembly, but the proteins formed are ineffective as enzymes. At relatively low concentrations (50 μM) AZ was a potent inhibitor of release of ions to the xylem of excised roots of barley (Hordeum vulgare L.) and intact plants. Uptake to the root was also inhibited but to a lesser degree. A procedure was introduced for estimating unidirectional fluxes from measurements of net tracer uptake, net transport to the xylem, and net efflux from the roots. It was shown that inhibition of release to the xylem was not caused by reduction in influx at the plasmalemma or to stimulation of influx to the vacuoles. It was suggested that AZ was acting on the process of release from symplast to the xylem. The action of AZ is compared with similar effects on ion transport produced by p-fluorophenylalanine, cycloheximide, and abscisic acid.     

Effects of low water potential on ion uptake and loss for excised roots

Summary In excised roots of barley and tomato plants, lowering the water potential of nutrient solutions to-10.4 and-20.4 atm decreased the uptake of bromide and phosphorus while increasing the loss of these ions to the external solutions.Lowering the water potential greatly increased the rate of loss of potassium and bromide from the cytoplasm, but the increases in loss from the vacuoles were much smaller. The results suggest that the mechanisms of ion uptake are not affected by low water potential and that the decrease in ion accumulation is caused by the increased leakage from the cells.     

Exodermal Casparian bands: their significance for ion uptake by roots

The roots of many angiosperm species possess two Casparian bands, one in the endodermis and one in the outermost layer of the cortex. Over most of the root surface in these species, ions are taken up by the epidermis and may be transported symplastically to the xylem.     

Oligomycin effect on ion absorption by excised barley roots and on their ATP content

Summary Chloride absorption by excised barley roots from dilute solutions is more oligomycin-sensitive than its absorption from more concentrated solutions and than K⁺ and Na⁺ absorption from dilute as well as concentrated solutions. Oligomycin decreased the ATP content of excised barley roots. The mode of oligomycin interference with ion absorption by plant cells is discussed.     

Water uptake by seminal and adventitious roots in relation to whole-plant water flow in barley (Hordeum vulgare L.)

Prior to an assessment of the role of aquaporins in root water uptake, the main path of water movement in different types of root and driving forces during day and night need to be known. In the present study on hydroponically grown barley (Hordeum vulgare L.) the two main root types of 14- to 17-d-old plants were analysed for hydraulic conductivity in dependence of the main driving force (hydrostatic, osmotic). Seminal roots contributed 92% and adventitious roots 8% to plant water uptake. The lower contribution of adventitious compared with seminal roots was associated with a smaller surface area and number of roots per plant and a lower axial hydraulic conductance, and occurred despite a less-developed endodermis. The radial hydraulic conductivity of the two types of root was similar and depended little on the prevailing driving force, suggesting that water uptake occurred along a pathway that involved crossing of membrane(s). Exudation experiments showed that osmotic forces were sufficient to support night-time transpiration, yet transpiration experiments and cuticle permeance data questioned the significance of osmotic forces. During the day, 90% of water uptake was driven by a tension of about -0.15 MPa.     

Water uptake by barley roots as affected by the osmotic and matric potential in the rhizosphere

Summary The water uptake rates of roots in saline soils are depressed by the simultaneously decreasing matric and osmotic water potentials in the soil surrounding the roots (rhizospheric soil). Unfortunately there are no reliable tools available for direct measurements of the effect of decreasing water potentials in the rhizospheric soil on the uptake rate of soil water by roots. This paper presents some results of a vegetation technique for studying the effect of different combinations of osmotic and matric water potentials in the rhizospheric soil on the water uptake rates of barley roots. Water uptake rates were reduced to a greater extent by decreasing soil matric water potentials than by decreasing soil osmotic water potentials. According to the results of this experiment, there was no relationship between the total soil water potential of a sandy soil and the water uptake rates when the roots were exposed to different combinations of and .     

Effect of calcium on sulfate uptake by barley roots

Abstract Sulfate uptake by excised roots of barley (Hordeum vulgare L.) was maximal in the presence of about 3x10^-3M CaCl₂. Kinetic studies contraindicate a stoichiometric binding of calcium to the carrier for sulfate, in contrast to findings of Cuppoletti and Segel (Biochemistry 14: 471–4718, 1975) for the filamentous fungus Penicillium notatum. In barley, calcium affects the K_m but not the V_max for sulfate uptake, presumably by altering the conformation and, thereby, the affinity of the carrier. Calcium also affects the transition site for sulfate uptake.     

Temperature dependent membrane potential changes in snail neurons and their relation to active ion transport.

The mechanisms underlying the temperature response of the resting membrane potential (RMP) were investigated in 3 identified neurons of the buccal ganglion of Helix pomatia. Lowering the temperature evoked a decrease of the RMP and an increase in membrane resistance, and vice versa. The temperature response of the RMP had an equilibrium potential of ca, -60 mV. It is essentially evoked by changes in the potassium conductance. Indications of an electrogenic sodium transport were not detected.     

Nature of the calcium effect in phosphate uptake by barley roots

Summary The rates of phosphate uptake of young intact plants were measured in solution at concentrations between 3.2×10^–7 M and 3.2×10^–5 M KH₂PO₄. The addition of 0.005 to 0.01M CaCl₂ increased the uptake rates to values of up to 4 times those of controls. The increases in uptake relative to the controls were most marked at the lowest phosphate concentrations. The addition of KCl also increased the uptake rates, but to a much smaller extent. The results are analyzed by the enzyme kinetic theory, and it is concluded that the uptake of phosphate at low ionic strengths is impeded by negative potentials at the root surface.The experimental results in this letter have appeared in a thesis.     

Organic acids promote the uptake of lanthanum by barley roots

Organic acids play an important role in metal uptake by, and accumulation in, plants. However, the relevant mechanisms remain obscure. Acetic, malic and citric acids increased the uptake of lanthanum (La) by barley (Hordeum vulgare) roots and enhanced La content in shoots under hydroponic conditions. Concentration-dependent net La influx in the absence and presence of organic acids yielded nonsaturating kinetic curves that could be resolved into linear and saturable components. The saturable component followed Michaelis-Menten kinetics. The K(m) values were similar; however, the V(max) values in the presence of acetic, malic and citric acids were 4.3, 2.8, 1.5-times that of the control, respectively. Enhanced uptake of La by organic acids was mediated mainly, but not solely, by Ca(2+) channels. X-ray absorption spectroscopic techniques provided evidence of La-oxygen environment and established that La(III) was coordinated to 11 oxygen atoms that are likely to be involved in the binding of La(III) to barley roots via carboxylate groups and hydration of La(III).     

Carbon import into barley roots: effects of sugars and relation to cell expansion

The import of photosynthate labelled with ¹¹C from a sourceleaf into the two halves of a split root system of an intactbarley plant was studied. When applied to one half of a splitroot system sugars that are absorbed and metabolized reducesubsequent import of ¹¹C into that root half. The non-metabolizedsugar analogue 3-O-methyl glucose has no effect on import, whilstmannose and 2–deoxyglucose inhibit both root elongationand import of ¹¹C. EDTA, PCMBS, and apoplastic pH in the range4–7, have little effect on partitioning. These resultsare interpreted in terms of a suggestion that phloem unloadsdirectly into expanding cells in the elongation zone of roottips. Key words: Carbon partitioning, roots, ¹¹C, sucrose, phloem, sugars, cell expansion     

Aquaporin-facilitated water uptake in barley (Hordeum vulgare L.) roots

It is not known to what degree aquaporin-facilitated water uptake differs between root developmental regions and types of root. The aim of this study was to measure aquaporin-dependent water flow in the main types of root and root developmental regions of 14- to 17-d-old barley plants and to identify candidate aquaporins which mediate this flow. Water flow at root level was related to flow at cell and plant level. Plants were grown hydroponically. Hydraulic conductivity of cells and roots was determined with a pressure probe and through exudation, respectively, and whole-plant water flow (transpiration) determined gravimetrically in response to the commonly used aquaporin inhibitor HgCl(2). Expression of aquaporins was analysed by real-time PCR and in situ hybridization. Hydraulic conductivity of cortical cells in seminal roots was largest in lateral roots; it was smallest in the fully mature zone and intermediate in the not fully mature 'transition' zone along the main root axis. Adventitious roots displayed an even higher (3- to 4-fold) cortical cell hydraulic conductivity in the transition zone. This coincided with 3- to 4-fold higher expression of three aquaporins (HvPIP2;2, HvPIP2;5, HvTIP1:1). These were expressed (also) in cortical tissue. The largest inhibition of water flow (83-95%) in response to HgCl(2) was observed in cortical cells. Water flow through roots and plants was reduced less (40-74%). It is concluded that aquaporins contribute substantially to root water uptake in 14- to 17-d-old barley plants. Most water uptake occurs through lateral roots. HvPIP2;5, HvPIP2;2, and HvTIP1;1 are prime candidates to mediate water flow in cortical tissue.     

Localization of phytosiderophore release and of iron uptake along intact barley roots

Under iron deficiency the release of so-called phytosiderophores by roots of barley plants ( Hordeum vulgare L. cv. Europa) was greater by a factor of 10 to 50 compared to iron-sufficient plants. This enhanced release occurred particularly in apical zones of the seminal roots and in the lateral root zones. Under iron deficiency, uptake rates for iron, supplied as FeIII phytosiderophore, increased by a factor of ca 5 as compared to iron-sufficient plants. This enhanced uptake rate for iron was also much more pronounced in apical than in basal root zones. In contrast, with supply of the synthetic iron chelate, FelII EDDHA (ferric diaminoethane-N, N-di- o -hydroxyphenyl acetic acid), the Fe deficiency-enhanced uptake rates for iron were only small and similar along the roots, except for the lateral root zones. The high selectivity of barley roots for uptake and translocation of FeIII phytosiderophores compared with FeIII EDDHA is reflected by the fact that, at the same external concentration (2 μ M ), rates of uptake and translocation of iron from FeIII phytosiderophores were between 100 (Fe-sufficient) and 1 000 times higher (Fe-deficient plants) than from FeIII EDDHA. The relatively high rates of uptake and particularly of translocation of iron supplied as FeIII EDDHA in the zone of lateral root formation strongly suggest an apoplastic pathway of radial transport of the synthetic iron chelate into the stele in this root zone.
The results demonstrate that apical root zones are the main sites both for Fe deficiency-enhanced release of phytosiderophores and for uptake and translocation of iron supplied as FeIII phytosiderophores.