Beryllium uptake by excised barley roots

The uptake of beryllium by excised barley roots from a solutioncontaining 111 µM beryllium and 500 µM calcium wasstudied. The exchange-adsorbed portion was 15% of the totalBe uptake after 30 min of exposure. The total Be content couldbe reduced to 55% of the control in 1 hr with demineralizedwater. At pH levels above 6, Be uptake was increased dramatically.A Q₁₀ of 1.2 was found to exist between 1 and 23°C. Mg (111µM) did not influence Be absorption while Zn and Al atthe same concentration reduced it, possibly competitively, 27and 62% of the control, respectively. Dinitro-phenol (500 µM),cyanide (500µM), azide (500 µM) and m-chlorocarbonylcyanidephenylhydrazone (100 µM) at least doubled it indicatingthe existence of a possible metabolic barrier to beryllium entrywhich was disrupted by these compounds. Absorption kineticsover an external beryllium concentration range of 11 to 444µM suggested a number of binding sites. Above 444 µMa scattering effect was noted due possibly to its toxic effect. ¹Pesticide Programs, Environmental Protection Agency (P.S. -769),Washington, D.C. 20460, U.S.A. (Received August 13, 1979; )     

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.     

Regulation of nitrate reductase in excised barley roots

When excised barley roots (Hordeum distichum L.) are appropriately pretreated, the level of nitrate reductase in the roots increases upon exposure to nitrate. Relatively low levels of nitrate (10 mum) gave maximum induction of nitrate reductase. This increase was inhibited by inhibitors of protein and RNA synthesis, indicating that de novo protein synthesis is probably involved. Induction of nitrate reductase by nitrate is partially prevented by the inclusion of ammonium, an eventual product of nitrate reduction, in the incubation medium. Under the experimental conditions used, ammonium did not inhibit the uptake of nitrate by excised barley roots. It is concluded, therefore, that ammonium, or a product of ammonium metabolism, has a direct effect on the synthesis of nitrate reductase in this tissue.     

Absorption of cobalt by excised barley roots

Co absorption by excised roots of Hordeum vulgare L. appearsto be an active process exhibiting a Q₁₀ of 2.2. The effectsof poisons also indicate active absorption. After 1 hr difluorodinitrobenzeneat a concentration of 5000 µM inhibited it to 20% of thecontrol, while dinitrophenol (500 µM) and carbonyl cyanidephenylhydrazone (100 µM) inhibited it to 43% and 38% respectively.A study of absorption kinetics over a concentration range of1 through 100 µM CoCl₂ indicated that a number of carriersites were available for absorption depending upon the concentration.Results which were consistent with this interpretation wereobtained when the concentration of a competing ion, Ni, wasvaried. An exchangeable, absorbed fraction amounting to about25% of the total cobalt present in the tissue after 1 hr ofabsorption was found. (Received September 28, 1973; )     

In vivo visualization of the water-refilling process in xylem vessels using X-ray micro-imaging

BACKGROUND AND AIMS: Xylem vessels containing gases (embolized) must be refilled with water if they are to resume transport of water through the plant, so refilling is of great importance for the maintenance of water balance in plants. However, the refilling process is poorly understood because of inadequate examination methods. Simultaneous measurements of plant anatomy and vessel refilling are essential to elucidate the mechanisms involved. In the present work, a new technique based on phase-contrast X-ray imaging is presented that visualizes, in vivo and in real time, both xylem anatomy and refilling of embolized vessels. METHODS: With the synchrotron X-ray micro-imaging technique, the refilling of xylem vessels of leaves and a stem of Phyllostachys bambusoides with water is demonstrated under different conditions. The technique employs phase contrast imaging of X-ray beams, which are transformed into visible light and are photographed by a charge coupled device camera. X-ray images were captured consecutively at every 0.5 s with an exposure time of 10 ms. KEY RESULTS: The interface (meniscus) between the water and gas phases in refilling the xylem vessels is displayed. During refilling, the rising menisci in embolized vessels showed repetitive flow, i.e. they temporarily stopped at the end walls of the vessel elements while gas bubbles were removed. The meniscus then passed through the end wall at a faster rate than the speed of flow in the main vessels. In the light, the speed of refilling in a specific vessel was slower than that in the dark, but this rate increased again after repeated periods in darkness. CONCLUSIONS: Real-time, non-destructive X-ray micro-imaging is an important, useful and novel technique to study the relationship between xylem structure and the refilling of embolized vessels in intact plants. It provides new insight into understanding the mechanisms of water transport and the refilling of embolized vessels, which are not understood well.     

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.     

Mechanism of sulfate uptake by excised maize roots

The mechanism of uptake of sulfate by excised maize roots (Zea mays L. cv. Ganga-5) was investigated. It was found that in the concentration range l × 10^?9 M ? 5 × 10^?2 M, the sulfate uptake could be represented by a single multiphasic isotherm having four phases. Each phase covered a limited concentration range and obeyed Michaelis-Menten kinetics, which indicates mediation in sulfate uptake by a structure which changes characteristics (kinetic constants) at certain discrete salts concentrations (transition points).     

Thioredoxin targets in Arabidopsis roots

The importance of redox‐regulation in Arabidopsis thaliana roots has been investigated through the identification of the proteins interacting with thioredoxin (TRX), an ubiquitous thiol‐disulfide reductase. We have applied a proteomic approach based on affinity chromatography on a monocysteinic mutant of plastidial y‐type TRX used as a bait to trap putative partners in a crude extract of root proteins. Seventy‐two proteins have been identified, functioning mainly in metabolism, detoxification and response to stress, protein processing and signal transduction. This study allowed us to isolate 24 putative new targets and to propose the mevalonic acid‐dependent biosynthesis of isoprenoids as a new redox‐mediated process. The redox‐regulation of phenylpropanoid biosynthesis is also suggested, three enzymes of this pathway being retained on the column. We also provided experimental evidence that phenylammonia‐lyase was enzymatically more active when reduced by TRXy in root crude extract. Among the high number of partners involved in defense against stress we isolated from the column, we focused on plastidial monodehydroascorbate reductase and showed that its activity was dramatically increased in vitro in the presence of DTT‐reduced TRXy1 in root crude extracts. Our data strongly suggest that TRXy1 could be the physiological regulator of monodehydroascorbate reductase in root plastids.     

pH control in nitrate uptake studies with excised roots

Summary In the investigation of NO ₃ ^– uptake by excised roots, pH of salt solutions can be effectively controlled by periodic additions of a cation exchange resin, Amberlite IR-120, in the H⁺ form. This method of pH control provides for the addition of H⁺ to systems without an accompanying anion which might influence the absorption of NO ₃ ^– . The resin appeared to provide a favorable buffering capacity during an extended period and the method was especially useful for NO ₃ ^– uptake studies conducted at high root-to-solution ratios.Contribution from the Plant Nutrition Laboratory, Department of Botany, The University of Michigan, paper number 39. This work was supported by research grant GM-07339-02 from the National Institutes of Health, U. S. Public Health Service.     

The regulation of K+ influx in excised barley roots

The electrochemical potential differences for potassium, between excised barley (Hordeum vulgare L.) roots and external media containing 0.05 mM KCl+0.5 mM CaSO₄, were determined over a 4-h period during which initially low-K⁺ roots accumulated K⁺ by pretreatment in 50 mM KCl plus 0.5 mM CaCl₂. This pretreatment resulted in increased internal [K⁺], decreased K⁺ influx (as measured from 0.05 mM KCl+0.5 mM CaSO₄) and decreased values of . These observations indicate that the decline of K⁺ influx associated with increased internal K⁺ concentration cannot be accounted for by passive adjustment to the electrochemical gradient for this ion.     

Chemical composition of the exudate from excised maize roots

Summary The concentrations of inorganic ions in the exudate from excised roots of Zea mays have been measured for roots grown in both chloride and sulphate media. The results indicated that in roots exuded from sulphate media there was a significant inorganic anion deficit. Using the methods of thin layer chromatography and high voltage electrophoresis some organic constituents of the exudate have been identified. Exudate from sulphate-grown roots was found to contain 3 organic acids and a complex mixture of amino acids. Exudate from chloride-grown roots contained similar organic solutes, but at much lower concentrations than in the sulphate-grown root exudate.     

Na+ and K+ transport in excised soybean roots

Uptake, accumulation and xylem transport of K⁺ and Na⁺ in excised roots of soybean were investigated by use of a perfusion technique. This technique permitted independent quantification of, on the one hand, entry of ions into the roots and their transport through the cortex to the xylem vessels, and on the other hand reabsorption from the xylem vessels to the neighbouring cells and the external medium. Data are consistent with a low degree of selective uptake of K⁺ over Na⁺. However, Na⁺ depletion of the xylem stream by reabsorption limits, although weakly, its translocation to the shoots. Na⁺ reabsorbed is for a great part reexcreted into the external medium. The low efficiency of these processes is discussed in relation to the Na⁺ sensitivity of soybean.