Manganese absorption by excised barley roots

Short-term absorption studies with 5-day-old excised barley roots revealed that the basic aspects of Mn absorption were similar to those of other metabolically absorbed cations. Following an initial non-metabolic equilibration with the root, Mn was absorbed for several hours at a slower steady-state rate comparable to that of other inorganic cations. Complete or nearly complete inhibition of the steady-state phase by low temperature, dinitrophenol, and azide provides strong evidence that Mn transport into this tissue was metabolically mediated. Within limits, the rate of transport was strongly dependent upon the concentrations of Mn and the hydrogen ions in the ambient solution. Absorption increased rapidly with increasing concentrations of Mn up to 1 meq per liter. Above this concentration, the rate leveled off, apparently due to a saturation of the transport mechanism. Within the physiological pH range in which Mn is soluble (below pH 7), absorption increased greatly with decreasing hydrogen-ion concentration.     

Effect of calcium on the absorption and translocation of heavy metals in excised barley roots: Multi-compartment transport box experiment

Summary The effect of Ca on the absorption and translocation of Mn, Zn and Cd in excised barley roots was studied using a multi-compartment transport box technique. A radioisotope (⁵⁴Mn,⁶⁵Zn or^115mCd)-labelled test solution was supplied to the apexes of excised roots and the distribution pattern in the roots was examined in the absence or presence of Ca. Results obtained were as follows. Addition of Ca to the test solution reduced the absorption of Mn and inhibited drastically its translocation in excised roots. With increasing concentrations of Ca in test solutions, its inhibitory effects on the absorption and translocation of Mn became severe. Similar results were observed for the absorption and translocation of Zn. Ca in the test solution decreased the absorption and inhibited drastically the translocation of Zn; as in the case of Mn, higher concentrations of Ca had severe effects on these functions. It was also evident that the addition of Ca to the test solution reduced the absorption of Cd at all levels of Cd concentration (1, 10, and 100 μM). Cd absorption decreased with increasing concentrations of Ca in the test solution. However, Ca accelerated the translocation of Cd in excised roots supplied with test solutions containing up to 10μM Cd. At 100μM Cd, addition of Ca caused a negligibly small acceleration of Cd translocation. The accelerating effect of Ca on Cd translocation, especially “xylem exudation”, decreased markedly with the addition of 2,4-dinitrophenol, but not with the addition of chloramphenicol or p-chloromercuribenzene sulphonic acid. When barley plants were supplied with only CaSO₄ during the entire growing period, that is, plants were not supplied with nutrient solution on the last day of this period, Ca had no accelerating effect on Cd translocation in excised roots.     

The regulation of potassium absorption in barley roots

The dynamics of changes in K(+) influx across the plasmalemma and of internal K(+) concentrations [K(+)](1) of intact barley (Hordeum vulgare) roots were examined as the roots were converted from ;high-salt' to ;low-salt' roots. Following the transfer of plants grown in 0.5 mm CaSO(4) solutions plus various concentrations of KCl to 0.5 mm CaSO(4) solutions, influx rates increased and internal K(+) concentrations declined as a function of time and the initial K(+) status of the tissue. The relationship between plasmalemma influx and [K(+)](1) was examined over a wide range of [K(+)](1) values by growing intact plants in various concentrations of KCl. Plasmalemma influx was inversely correlated with the square of [K(+)](1). A model for the regulation of plasmalemma influx by [K(+)](1) is considered.     

Regulation of potassium absorption in barley roots: an allosteric model

Plasmalemma influx isotherms for K(+) were measured in the system I concentration range (0.01-0.32 mm), for barley (Hordeum vulgare L.) roots of varying internal K(+) concentration, and Km values for influx calculated. In plants grown for several days in CaSO(4) or in CaSO(4) plus KCl solutions, as well as in plants grown in CaSO(4) for several days and then rapidly loaded with KCl during a pretreatment period, Michaelis constant values were positively correlated with internal K(+) concentrations. Influx of K(+) is shown to be sigmoidally related to internal K(+) concentration and Hill plots of influx data give linear transformations with n = 4. This information is taken as support for an allosteric model for the regulation of K(+) influx in which the "carrier" is envisaged as possessing a single external binding site for K(+) as well as four internal sites for allosteric control of influx.     

Response-velocity of chloride absorption by barley roots to oligomycin

Chloride absorption by excised barley roots is significantlyinhibited within the first 2 min of incubation in oligomycin.A steady inhibited absorption rate is attained in the presenceof oligomycin after 10 min (Received February 18, 1970; )     

Kinetics of iron absorption by excised rice roots

Summary Studies on the rate of iron absorption by excised rice roots from solutions of different concentrations of FeSO₄ showed the presence of two patterns, one in the low (0.005–0.5 mM) and the other in the high (1–30 mM) concentration range. The presence of CaSO₄ or MnSO₄ at 0.5 mM enhanced Fe⁺⁺ absorption in the low concentration range, while CaSO₄ at 10 mM inhibited Fe absorption in the high concentration range in a competitive manner. Fe⁺⁺ absorption at both low and high concentrations was sensitive to metabolic inhibitors. The isotherm for Fe⁺⁺ absorption at O° exhibited an initial absorption shoulder in both low and high concentrations and was suggestive of a latent ion-transport capacity for Fe⁺⁺ in rice roots.     

The absorption and translocation of diclofop-methyl and amitrole in wheat and oat roots

The absorption and translocation of diclofop-methyl (methyl 2-[4(2',4'-dichlorophenoxy)phenoxy]propanoate) was examined by using a specially designed treatment apparatus that separated excised roots or roots of seedlings into four zones. [¹⁴C]-Diclofop-methyl was absorbed along the entire root length of both wheat ( Triticum aestivum L.) and oat ( Avena sativa L.). In both species, absorption was greatest in the apical region of the root. Absorption by the apical region of wheat roots was more than three times greater than the basal portions, and more than twice as great as the apical region of oat roots. Less than 5% of the absorbed diclofop-methyl was translocated in both wheat and oat roots. Diclofop-methyl and diclofop(2-[4(2',4'-dichlorophenoxy)phenoxy]propanoic acid) were the predominant translocated forms. The absorption and translocation of amitrole (3-amino-1,2,4-triazole) were also examined. Amitrole was absorbed along the entire length of wheat roots and translocated primatily in the basipetal direction. The usefulness of the specially designed apparatus for biochemical and physiological studies is discussed.     

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.     

The absorption of potassium and several organic compounds by barley roots: Effect of siduron

Absorption of ⁴²K by excised roots of barley (Hordeum vulgareL.) grown in 0 or 5 ppm siduron (l-(2-methylcyclohexyl)-3-phenylurea)was a linear function of time for at least 60 minutes with transportbeing unidirectional. Absorption of siduron was a function ofthe external concentration to the limits of its solubility (0.09mM). However, the siduron- ¹⁴C absorbed by roots grown in either0 or 5 ppm siduron was in a readily exchangeable form and desorptionfor 4 hr exchanged 80 % of the label. Glucose-¹⁴C, adenine-8¹⁴Cand leucine¹⁴C were actively absorbed with 70 to 85 % of thelabel being absorbed in 24 hr. Although roots grown in iduronabsorbed less ⁴²K, glucose-¹⁴C, adenine-¹⁴C and siduron-¹⁴C,and more leucine-¹⁴C than similar roots grown in water culture,it is probable that these differences were not large enoughto account for the noted reduction (60%) in root growth. (Received January 9, 1968; )     

Simultaneous uptake and translocation of magnesium and calcium in barley (Hordeum vulgare L.) roots

Summary The patterns of uptake and translocation of magnesium in different regions of the root are very similar to those of calcium. Once the endodermis has become suberized translocation of either ion to the shoot is greatly reduced and it is concluded that magnesium, like calcium, appears to move across the root cortex largely in the free space.     

A specific transporter for iron(III)-phytosiderophore in barley roots

Iron acquisition of graminaceous plants is characterized by the synthesis and secretion of the iron-chelating phytosiderophore, mugineic acid (MA), and by a specific uptake system for iron(III)-phytosiderophore complexes. We identified a gene specifically encoding an iron-phytosiderophore transporter (HvYS1) in barley, which is the most tolerant species to iron deficiency among graminaceous plants. HvYS1 was predicted to encode a polypeptide of 678 amino acids and to have 72.7% identity with ZmYS1, a first protein identified as an iron(III)-phytosiderophore transporter in maize. Real-time RT-PCR analysis showed that the HvYS1 gene was mainly expressed in the roots, and its expression was enhanced under iron deficiency. In situ hybridization analysis of iron-deficient barley roots revealed that the mRNA of HvYS1 was localized in epidermal root cells. Furthermore, immunohistological staining with anti-HvYS1 polyclonal antibody showed the same localization as the mRNA. HvYS1 functionally complemented yeast strains defective in iron uptake on media containing iron(III)-MA, but not iron-nicotianamine (NA). Expression of HvYS1 in Xenopus oocytes showed strict specificity for both metals and ligands: HvYS1 transports only iron(III) chelated with phytosiderophore. The localization and substrate specificity of HvYS1 is different from those of ZmYS1, indicating that HvYS1 is a specific transporter for iron(III)-phytosiderophore involved in primary iron acquisition from soil in barley roots.     

Bicarbonate blocks iron translocation from cotyledons inducing iron stress responses in Citrus roots

The effect of bicarbonate ion (HCO₃⁻) on the mobilization of iron (Fe) reserves from cotyledons to roots during early growth of citrus seedlings and its influence on the components of the iron acquisition system were studied. Monoembryonic seeds of Citrus limon (L.) were germinated “in vitro” on two iron-deprived media, supplemented or not with 10 mM HCO₃⁻ (−Fe+Bic and −Fe, respectively). After 21 d of culture, Fe concentration in seedling organs was measured, as well as gene expression and enzymatic activities. Finally, the effect of Fe resupply on the above responses was tested in the presence and absence of HCO₃⁻ (+Fe+Bic or +Fe, respectively). −Fe+Bic seedlings exhibited lower Fe concentration in shoots and roots than −Fe ones but higher in cotyledons, associated to a significative inhibition of NRAMP3 expression. HCO₃⁻ upregulated Strategy I related genes (FRO1, FRO2, HA1 and IRT1) and FC-R and H⁺-ATPase activities in roots of Fe-starved seedlings. PEPC1 expression and PEPCase activity were also increased. When −Fe+Bic pre-treated seedlings were transferred to Fe-containing media for 15 d, Fe content in shoots and roots increased, although to a lower extent in the +Fe+Bic medium. Consequently, the above-described root responses became markedly repressed, however, this effect was less pronounced in +Fe+Bic seedlings. In conclusion, it appears that HCO₃⁻ prevents Fe translocation from cotyledons to shoot and root, therefore reducing their Fe levels. This triggers Fe-stress responses in the root, enhancing the expression of genes related with Fe uptake and the corresponding enzymatic activities.     

Sodium absorption by barley roots: role of the dual mechanisms of alkali cation transport

Radioactively labeled Na(+) absorbed by barley roots was sequestered in an intracellular compartment or compartments ("inner" spaces) in which it was only very slowly exchangeable with exogenous Na(+). Absorption of this fraction proceeded at a constant rate for at least 1 hour.When the rate of Na(+) absorption was examined over the range of concentrations, 0.005 to 50 mm, the isotherm depicting the relation showed dual kinetics as follows. Over the range, 0.005 to 0.2 mm, a single Michaelis-Menten term describes the relation between the concentration of Na(+) and the rate of its absorption. The mechanism of Na(+) absorption operating over this range of concentrations, mechanism 1 of alkali cation transport, is severely inhibited in the presence of Ca(2+) and virtually rendered inoperative for Na(+) transport by the combined presence of Ca(2+) and K(+). The mechanism is equally effective in Na(+) transport whether Cl(-) or F(-) is the anion, but is somewhat inhibited when the anion is SO(4) (2-).Over the high range of concentrations, 0.5 to 50 mm Na(+), a second, low-affinity mechanism of Na(+) absorption comes into play. In the presence of Ca(2+) and K(+), this mechanism 2 is the only one to transport Na(+) effectively, since Na(+) absorption via mechanism 1 is virtually abolished under these conditions.Anaerobic conditions, low temperature, and the uncoupler, 2,4-dinitrophenol, inhibit Na(+) absorption both at low and high Na(+) concentrations.     

Biosynthesis of limonoids in Citrus: Sites and translocation

Sites of limonoid biosynthesis were located in Citrus limon. The stem was found to be the major site of nomilin biosynthesis from acetate. Epicotyl, hypocotyl and root tissues were also capable of biosynthesizing nomilin from acetate, but leaves, fruits and seeds did not show this capacity under the conditions used. All the tissues tested were capable of biosynthesizing other limonoids starting from nomilin. C. limon was capable of translocating nomilin from the stem to other sites.     

Sodium absorption by barley roots: its mediation by mechanism 2 of alkali cation transport

Radioactively labeled Na⁺ absorbed by barley roots was sequestered in an intracellular compartment or compartments (“inner” spaces) in which it was only very slowly exchangeable with exogenous Na⁺. Absorption of this fraction proceeded at a constant rate for at least 1 hour.