Transport of copper and manganese to the xylem exudate of sunflower

Abstract Absorption of copper and manganese by sunflower roots from solution cultures of varying composition was followed by measuring the concentrations of the metals appearing in whole roots, root cell sap and xylem exudate. Total copper in the fibrous roots was linearly related to the concentration of copper in the external solution but the concentration of copper released to the xylem exudate was buffered somewhat against the changes made externally. No such buffering was observed for managenese. A copper-sensitive electrode, responsive only to free cupric ions was used in conjunction with total copper analysis by atomic absorption spectrophotometry to show that little of the copper (usually < 1%) existed as a free ion in any phase of the system. Copper in the xylem exudate may be strongly complexed. An electron paramagnetic resonance spectrum of the xylem exudate indicated that manganese probably was a free divalent ion. Calculation of the electrochemical potential gradient for free cupric ions showed that no special metabolically-linked mechanism need be postulated to account for absorption of copper (or manganese) other than that necessary to maintain the transmembrane potential.     

Cation and anion balance in the xylem exudate of tobacco roots

Summary The sum of Na, K, Ca, Mg in the exudate of tobacco generally exceeded the sum of mineral anions. Insufficient organic acids were present to account for the differences and bicarbonate appeared to be the other anion involved. Amino acids were present in very low concentrations relative to mineral cations. When nitrate salts only were in the external solutions, the anions were mostly, but not entirely, nitrate. When chloride salts only were in the external solutions, the cations far exceeded the level of mineral anions in the exudate. It is postulated that nitrate is actively transported when nitrate salts are in the external solution regardless of the cation, but when anions other than nitrate are in the external solution, the cations are actively transported with the anions passively following. Nitrate transport was via a symplasm, but that of the other anions seemed to be different. When bicarbonate is the only anion in the external solution and when present at relatively high concentrations (5 × 10⁻³ M or higher), the volume of exudate is decreased. It appears that the organic acids which were synthesized as a result of the bicarbonate absorption were not transferred to the xylem vessels.     

Effect of calcium on transport of cations to the xylem exudate of tobacco

Summary Roots of detopped tobacco plants (Nicotiana tabacum var. Virginia Gold) were exposed to Na, K, and Ca salts or to water, and cation transfer to xylem vessels was measured. In some cases plants had been exposed to Na in addition to regular nutrient solutions before detopping. Calcium in the external medium greatly depressed the transport of Na from the external medium to the xylem vessels and it often stimulated the transfer of K from the external medium to the xylem vessels. The K/Na ratio in the exudate thus was dependent upon the Ca content of the external medium under these conditions. In contrast, externally applied Ca or Ca deficiency had very little effect on the transfer of preaccumulated K and Na from compartments within roots to the xylem vessels. The K/Na ratio in the exudate under these conditions was not related to Ca levels nor to mild Ca deficiency. The ratios decreased with time after detopping regardless of Ca level. Intact plants accumulated more Na than did root systems of detopped plants in a 6-day period.Riverside University of CaliforniaSoil Science and Agricultural Engineering     

Effect of temperature on potassium and calcium transport from within the root to the xylem exudate of tobacco

Summary Detopped root systems of tobacco plants (Nicotiana tabacum L. var. Virginia Gold) were exposed to air at different temperatures (14 to 23°C). The volume of xylem exudate was related to temperature and decreasing the temperature resulted in a lag in the appearance of exudate. For 4 and 8 days of exudation the amounts of cations transported in the exudate were temperature dependent. The Q₁₀ values were higher than usually encountered in biological reactions; the mean value was around 6. This work was supported in part by National Science Foundation Grant GB8170. University of California     

Translocation of nickel in xylem exudate of plants

Topped plants of tomato (Lycopersicon esculentum), cucumber (Cucumis sativus), corn (Zea mays), carrot (Daucus carota), and peanut (Arachis hypogaea) were treated with 0.5 to 50 micromolar Ni (containing ⁶³Ni) in nutrient solutions. Xylem exudate was collected for 10 hours or, in the case of corn, for 20 hours at 5-hour intervals. Electrophoresis of nutrient solution distributed all Ni cathodically as inorganic Ni²⁺. Low concentrations of Ni in tomato exudate migrated anodically, presumably bound to organic anion (carrier). However, this carrier became saturated at about 2 micromolar Ni in exudate, and excess Ni ran cathodically. Most of the Ni in cucumber, corn, carrot, and peanut exudate ran anodically, and its migration rate was identical for all exudates. Peanut root sap contained 14 to 735 micromolar Ni. The anodic Ni carriers in root sap and exudate appear identical. The carrier in root sap became saturated near 100 micromolar Ni, as shown by cathodic streaking of Ni exceeding that concentration. It appears that all five species translocate low concentrations of Ni in the same anionic form.     

Radial transport of sodium and chloride into tomato root xylem

Transport of Na and Cl across exuding tomato (Lycopersicon esculentum Mill.) roots was determined as a function of ambient NaCl concentrations in the ranges of both systems 1 and 2. Kinetics of radial transport under steady-state conditions and the effect of dinitrophenol indicate that Na and Cl were transported by two different mechanisms. Sodium was neither accumulated against a concentration gradient nor directly inhibited by dinitrophenol from diffusing into the xylem. Chloride was accumulated in the xylem and its transport was nearly completely blocked by dinitrophenol. A comparison of the radial transport isotherms for Na and Cl for intact and decapitated plants indicates that the separate mechanisms were not unique to excised roots. It is concluded that radial Na transport in tomatoes was facilitated by a passive convective type process with the rate-limiting barrier located at the outer cortical plasmalemma. Chloride transport in both concentration ranges involved, either directly or indirectly, a metabolic mechanism. Absorption and retention of Na in the root tissue was negligible. Chloride was accumulated by the tissue but was unaffected by dinitrophenol.     

Peroxidase-mediated integration of tyramine into xylem cell walls of tobacco leaves

When [2-¹⁴C]tyramine was fed in vivo by petiolar uptake to Nicotiana tabacum Xanthi n.c. leaves partially inoculated with tobacco mosaic virus, radioactivity accumulated in inoculated areas bearing necrotic lesions, mainly in the veins and around the lesions. Light-microscopic autoradiography showed that integration of radioactivity was especially evident in xylem cell walls. This was confirmed in sections of petiole by electron-microscopic autoradiography. Study of the mechanism of insolubilisation of tyramine showed that the amine was integrated in regions in which peroxidase activity could be located cytochemically using 3,3-diaminobenzidine and H₂O₂ as substrates. When sections of petiole were incubated with labelled tyramine and H₂O₂ after fixation in glutaraldehyde, a distribution of radioactivity similar to that obtained after feeding tyramine by petiolar uptake was observed. It is concluded that simple phenols such as tyramine can be integrated in vivo into cell walls because they are oxidised by peroxidases. This result illustrates the difficulty of studying the metabolism of exogenous phenols in plants, especially in lignifying tissues which contain active wall-bound peroxidases.Abbreviations DAB 3,3-diamino-benzidine - TMV tobacco mosaic virus     

Translocation of iron citrate and phosphorus in xylem exudate of soybean

Soybean plants, Glycine max (L.) Merrill, in standard solution received 2.5 μm ferric ethylenediamine di(o-hydroxyphenylacetate (FeEDDHA) and 0 to 128 μm phosphorus. Their stem exudates contained: 32 to 52 μm Fe, 120 to 5000 μm P, and 120 to 165 μm citrate. Electrophoresis of exudates with high P caused Fe trailing that precluded identification of any major form of Fe. Exudate with low P gave an anodic band of Fe citrate as the major Fe compound. Phosphate added to exudate in vitro depressed the Fe citrate peak and cause Fe trailing. EDDHA added to exudate in vitro pulled Fe from Fe citrate; citrate then migrated as a slower form and Fe migrated as FeEDDHA. A modified preculture system, involving 2-day renewals of 0.2 μm FeEDDHA with 3.2, 9.6, or 16 μm P and low levels of other ions, controlled pH depression and produced considerable change in citrate and P levels. The exudates contained: 45 to 57 μm Fe, 200 to 925 μm P, and 340 to 1025 μm citrate. The high citrate was from plants grown with low P. The major form of Fe in the exudates was Fe citrate. This is probably the form translocated in the plants.     

Movement to the xylem exudate of rubidium accumulated in the apex of corn roots

Apical segments of roots of corn (Zea mays, L.) were excised and mounted in experimental salt solutions containing ⁸⁶RbCl and CaCl₂. Xylem exudates were collected hourly beginning at the 21st hour. At the 24th hour, experimental solutions were replaced with solutions of other chlorides (nonlabeled) of Ca²⁺ ± some monovalent cation.     

Transport of nitrogen in the xylem of soybean plants

Experiments were conducted to characterize the distribution of N compounds in the xylem sap of nodulated and nonnodulated soybean plants through development and to determine the effects of exogenous N on the distribution of N compounds in the xylem. Xylem sap was collected from nodulated and nonnodulated greenhouse-grown soybean plants (Glycine max [L.] Merr. “Ransom”) from the vegetative phase to the pod-filling phase. The sum of the nitrogen in the amino acid, nitrate, ureide (allantoic acid and allantoin), and ammonium fractions of the sap from both types of plants agreed closely with total N as assayed by a Kjeldahl technique. Sap from nodulated plants supplied with N-free nutrient solution contained seasonal averages of 78 and 20% of the total N as ureide-N and amino acid-N, respectively. Sap from nonnodulated plants supplied with a 20 millimolar KNO₃ nutrient solution contained seasonal averages of 6, 36, and 58% of total N as ureide-N, amino acid-N, and nitrate-N, respectively. Allantoic acid was the predominant ureide in the xylem sap and asparagine was the predominant amino acid. When well nodulated plants were supplied with 20 millimolar KNO₃, beginning at 65 days, C₂H₂ reduction (N₂ fixation) decreased relative to nontreated plants and there was a concomitant decrease in the ureide content of the sap. A positive correlation (r = 0.89) was found between the ureide levels in xylem sap and nodule dry weights when either exogenous nitrate-N or urea-N was supplied at 10 and 20 millimolar concentrations to inoculated plants. The results demonstrate that ureides play a dominant role in N transport in nodulated soybeans and that the synthesis of ureides is largely dependent upon nodulation and N₂ fixation.     

Effect of EDTA on transport forms of manganese in maize xylem exudate

Xylem exudates were collected from 20-day-old maize seedlings grown for 24 h in labelled manganese chloride solution (10 ppm) with or without EDTA (18 mgl^-1). The distribution of radioactivity on paper ohromatograms of exudates did not show the existence of free Mn. Fractionation of exudate constituents by gel filtration on Sephadex G-26 showed that⁵⁴Mn is associated with three UV (254 nm) -absorbing fractions. Two fractions contain amino acids (M. wt.:≧ 10 000 and ∼ 900). The third fraction (M. wt. ∼ 600) however, appears to be a carrier of a different type, which is practically amino acid free, although it contains 10.23% N (by elementary analysis). The properties of this carrier are discussed. Application of EDTA has favoured the; transport of Mn in xylem sap with fractions of relatively lower molecular weights.     

Diurnal and temporal changes in the chemical profile of xylem exudate from Vitis rotundifolia

Positive root pressure in Vitis rotundifolia Michx. cv. Noble was employed to quantify diurnal and temporal changes in the chemical profile of xylem exudate. Xylem fluid osmolarity (7.2 to 16.8 m M ), water flux (8.2 to 18.5 ml h⁻¹) and solute flux (0.7 to 2.2 mmol h⁻¹) from a cut spur exhibited a diurnal pattern with maxima during midday and minima at night. Total osmolarity was similar to the sum of all organic and inorganic entitites quantified, indicating that the major solutes have been identified. Total amino acid and organic acid concentration were about equal (2 to 7 m M ), and sugars accounted for a minor fraction of the total profile (<0.2 m M ). Glutamine represented ca 80% of the organic N and 70% of the total N transported in the xylem fluid. A circadian rhythm in water flux and net flux of most organic and inorganic entities was observed with maxima during midday and minima at night. The increase in xylem fluid osmolarity occurring during midday was primarily a consequence of increased organic acid (oxalic, citric, tartaric, malic and succinic acids) and ion (NH₄⁺, No₃⁻, P and Ca) concentration. A diurnal cycle in amino acid concentration was less clear. The concentration of individual organic and inorganic entities varied asynchronously with time. Xylem solute was comprised of 80% organic and 20% inorganic components when collected 5 min to 2 h after the commencement of bleeding, but the ratio of organic to inorganic components fell to about 50% after 7 days.     

The effect of manganese and EDTA on the balance of nitrogen in maize xylem exudate

The effect of manganese chloride (10 mg Mn l^-1), EDTA (18 mg l^-1) and a mixture of these compounds on the nitrogen balance in maize xylem exudate was investigated. The compounds were applied as experimental solutions to the roots of 20 day old plants 24 h before excision. Application of Mn resulting in a lowered nitrogen level in the xylem exudate increased the relative content of the organic N-compounds in the exudate, particularly that of free amino acids. EDTA appreciably enhanced the content of total nitrogen in xylem exudate, however no significant changes were found in the proportion of inorganic and organic N-compounds in comparison with the water control. The significant features of the free amino acid exudate fraction of all experimental variants were, among others, the relatively high lysine content and the absence of proline arid sulphur containing amino acids. By gel filtration on Sephadex G-25 of xylem exudates UV (254 nm) absorbing fractions, four in the water control, five in Mn and six in EDTA variants were isolated. The UV absorbing fractions with the exception of one in each experimental variant (K_av 0.73 in water control, K_av 0.61 in Mn-variant, K_av 0.75 in EDTA and Mn + EDTA variant) were of peptide character, proline and sulphur-containing amino acids were missing in them. In the exceptional UV absorbing fractions (K_av 0.61–0.75) in spite of their high N-content (10.23%) after hydrolysis practically no amino acid could be detected.     

Studies of the site and mode of biosynthesis of tobacco trichome exudate components

Detached glandular trichome head preparations and epidermal strips with and without trichome heads were used to identify glandular trichome heads as the site of sucrose ester biosynthesis in tobacco. Carbon dioxide in solution as well as sucrose, glucose, and acetate were shown to serve as precursors to both sucrose esters and duvatrienediol diterpenes in detached trichome heads or epidermal strips, and gaseous CO2 was also efficiently utilized by epidermal strips. Thus, glandular heads can biosynthesize these principal exudate components from a molecule as simple as CO2. While formation of duvatriendiols from all precursors tested and conversion of sucrose and glucose to sucrose esters was light dependent, utilization of acetate to label the 6-O-acetyl group of the glucose moiety of sucrose esters occurred equally well in light and dark. The data suggest that CO2 and/or monosaccharides produced in trichome head cells and perhaps that supplied by other epidermal cells can act as carbon sources for sucrose ester and duvatrienediol biosynthesis which occurs in the glandular trichome head.     

Effects of external cations and respiratory inhibitors on electrical potential of the xylem exudate of excised corn roots

Glass capillary microelectrodes were used to study the electrical potential difference (PD) between the xylem exudate of excised corn roots, Zea mays L. Golden Bantam hybrid, and the external solution. A survey of the effects of various ions on the PD was made. With 1 mm single salt solutions, the PD was between 25 and 50 mv, exudate negative. The PD responded to concentration differences in single salt solutions of K⁺, Na⁺, and Ca²⁺ in a manner suggestive of cation selectivity and cation diffusion potentials. With Ca²⁺ present, the PD was insensitive to concentration changes of other cations. Substitution of NO₃⁻ for Cl⁻ in K⁺ solutions increased the PD by 2 to 5 mv, although in general the PD showed little response to anion concentration changes. The PD was partially abolished by cyanide. The remaining fraction of the PD was sensitive to concentration changes in external K⁺, and we postulate that the PD is the result of both a diffusion potential and an electrogenic pump.     

Inter-varietal differences in xylem exudate composition and growth under contrasting forms of N supply in cucumber

Variations in the inorganic and organic composition of xylem exudate, growth and N content under contrasting forms of N supply in three cucumber cultivars (Hyclos, Medusa and Victory) were studied in glasshouse conditions. The plants were grown hydroponically with two NO₃ ^-:NH₄ ⁺ ratios (100:0 and 60:40).The xylem sap of Medusa grown with both N sources displayed an increase of organic N and carboxylate concentrations and a decrease of cations, inorganic anions and carbohydrates compared with that of those grown with NO₃ ^- alone, showing a higher growth and N content in tissues and thus better utilization of N supplied as NO₃ ^- and NH₄ ⁺. Mixed N nutrition in Hyclos caused the greatest amounts of NO₃ ^- and NH₄ ⁺ in xylem sap, lower root weight and N levels in the leaves, while its root was unable to generate an adequate supply of organic N compounds. Despite the levels of cations, inorganic and organic anions were reduced by the NH₄ ⁺ supplied to Victory, the ionic balance in the xylem sap, growth and N content remained similar to that of those supplied with NO₃ ^- alone. Finally, the cucumber cultivars studied here, responded differently to the form of N supplied, it may partly be due to their ability of assimilating N in the roots and partly to the form in which the N is translocated to the shoot.     

Changes in the xylem exudate composition of poplar Populus tremula x P. alba)-dependent on the nitrogen

The capacity of poplar (P. tremula x P. alba, clone INRA 717 1/B4) to respond to changes in the nutrient supply with modifications in the xylem exudate collected after decapitation was investigated with special respect to N-compounds. The composition (inorganic ions and amino-N) was analysed with respect to (a) the time after decapitation at different times of day, (b) a change in the nitrogen concentration from NO^-3 to N-free medium, a change in the nitrogen source from NO^-3 to NH⁺4, (d) an increase in NO^-3 supply from 1 mM to 8 mM, and (e) the withdrawal of K⁺ supply. (a) The ion concentration in the xylem exudate was not affected up to 15 min after decapitation of the plants. Later a continuous increase in the concentration was observed. This increase was large if decapitation was performed in the middle of the light period and small at night-time. In both types of experiments (b,c) the NO^-3 concentration dropped immediately after the transfer, indicating the close connection between NO^-3 uptake and xylem loading. (b) After transfer to N-free medium poplar did not balance the charge in the xylem by increasing the concentration of other inorganic anions or decreased xylem loading of cations within 3 d of treatment. The N-status of the xylem exudate was reduced within 15 min. After transfer of the NO^-3-grown plants to NH⁺4, as the sole N-source, the charge compensation in the xylem exudate was maintained by reducing the loading of cations, and 3 d later by an enhanced xylem loading of mainly SO²4-. The N-status in the xylem exudate was maintained by an immediate increase in glutamine concentration (2-fold in 15 min). (d) Increasing the NO^-3 supply to 8 mM had no effect on the ionic composition or the N-status of the xylem exudate. (e) The withdrawal of K⁺ from the medium for 11 d resulted in a limitation of the S- and N-supply of the plant, causing a decrease in the N-status of the xylem sap. The data are discussed with respect to charge compensation after changes in the nutrient supply and to the maintenance of the nitrogen status in the xylem sap.     

Factors which affect the amount of inorganic phosphate, phosphorylcholine, and phosphorylethanolamine in xylem exudate of tomato plants

Phosphate in the xylem exudate of tomato (Lycopersicon esculentum) plants was 70 to 98% inorganic phosphate (Pi), 2 to 30% P-choline, and less than 1% P-ethanolamine. Upon adding ³²Pi to the nutrient, Pi in xylem exudate had the same specific activity within 4 hours. P-choline and P-ethanolamine reached the same specific activity only after 96 hours. The amount of Pi in xylem exudate was dependent on Pi concentration in the nutrient and decreased from 1700 to 170 micromolar when Pi in the nutrient decreased from 50 to 2 micromolar. The flux of 0.4 nmoles organic phosphate per minute per gram fresh weight root into the xylem exudate was not affected by the Pi concentration in the nutrient solution unless it was below 1 micromolar. During 7 days of Pi starvation, Pi in the xylem exudate decreased from 1400 to 130 micromolar while concentrations of the two phosphate esters remained unchanged.

The concentration of phosphate esters in the xylem exudate was increased by addition of choline or ethanolamine to the nutrient solution, but Pi remained unchanged. Upon adding [¹⁴C]choline to the nutrient, 10 times more [¹⁴C]P-choline than [¹⁴C]choline was in the xylem exudate and 85 to 90% of the ester phosphate was P-choline. When [¹⁴C]ethanolamine was added, [¹⁴C]P-ethanolamine and [¹⁴C]ethanolamine in the xylem sap were equal in amount. P-choline and P-ethanolamine accumulated in leaves of whole plants at the same time and the same proportion as observed for their flux into the xylem exudate. No relationship between the transport of P-choline and Pi in the xylem was established. Rather, the amount of choline in xylem exudate and its incorporation into phosphatidylcholine in the leaf suggest that the root is a site of synthesis of P-choline and P-ethanolamine for phospholipid synthesis in tomato leaves.