Identification of the silicon form in xylem sap of rice (Oryza sativa L.)

Rice (Oryza sativa L.) is a typical silicon (Si)-accumulating plant, but the mechanism responsible for the translocation from the root to the shoot is poorly understood. In this study, the form of Si in xylem sap was identified by (29)Si-nuclear magnetic resonance (NMR) spectroscopy. In rice (cv. Oochikara) cultured in a monosilicic acid solution containing 0.5 mM Si, the Si concentration in the xylem reached 6 mM within 30 min. In the (29)Si-NMR spectra of the xylem sap, only one signal was observed at a chemical shift of -72.6 ppm, which is consistent with that of monosilicic acid. A (1)H-NMR study of xylem sap did not show any significant difference between the wild-type rice and mutant rice defective in Si uptake, and the components of the xylem sap were not affected by the Si supply. The Si concentration in the xylem sap in vitro decreased from an initial 18 mM to 2.6 mM with time. Addition of xylem sap to a solution containing 8 mM Si did not prevent the polymerization of silicic acid. All these results indicate that Si is translocated in the form of monosilicic acid through the xylem and that the concentration of monosilicic acid is high in the xylem only transiently.     

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.

     

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.     

Phosphorus nutritional effects on root hydraulic conductance,xylem water flow and flux of magnesium and calcium in squash plants

Previous studies have found that P nutrition of plants is an important factor in the uptake and translocation of Mg and Ca, and increasing root osmotic hydraulic conductance (L_o) and osmotically driven xylem exudate flow (J_v). Experiments were designed to determine if the observed changes in Mg and Ca uptake and translocation, J_v, and L_o from altered P nutrition are related or are separate functions. When six-week old squash (Cucurbita pepo L.) plants grown in perlite were treated with P levels ranging from 50 to 400 μM P for seven days, J_v and L_o increased as P treatment level increased. Xylem exudate concentrations of Mg and Ca were maintained as J_v increased, resulting in an increase in total flux of these mineral elements. The increase in Mg and Ca flux in the xylem exudate correlated with increased shoot Mg and Ca levels as P nutritional level was raised. Further studies with greenhouse grown plants indicated that the increases in J_v, L_o, and Mg and Ca flux were more responsive to changes in P nutritional level than to similar changes in levels of other anions. In hydroponically grown squash plants, xylem exudate was collected for a 20 min period after 0, 2 and 4 h in treatments of 50 and 500 μM P or after P treatment was increased from 50 to 500 μM. Immediately after nutrient solution P was increased (time 0), there was a 33% increase in J_v and a 22% increase in L_o when compared to the 50 μM P treatment. The J_v and L_o of the 50–500 μM P treatment did not equal levels of the continuous 500 μM control at time 0, but were similar after 2 and 4 h. Flux of Mg and Ca did not increase as rapidly as J_v in the 50–500 treatment indicating that regulation of Mg and Ca uptake and xylem loading by P may lag behind that of water movement. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

[Benzilato(2−)-O,O]bis[1,3-diphenylpropane-1,3-dionato(1−)-O,O]silicon(IV): a neutral heteroleptic hexacoordinate silicon(IV) complex with an SiO6 skeleton

Reaction of tetramethoxysilane with 1,3-diphenylpropane-1,3-dione and benzilic acid (molar ratio 1:2:1) in tetrahydrofuran/n-pentane yielded the neutral heteroleptic hexacoordinate silicon(IV) complex [benzilato(2−)-O¹,O²]bis[1,3-diphenylpropane-1,3-dionato(1−)-O,O]silicon(IV) (5). Compound 5 was structurally characterized by single-crystal X-ray diffraction, solid-state VACP/MAS NMR spectroscopy (²⁹Si), and solution NMR spectroscopy (¹H, ¹³C, ²⁹Si). The chiral silicon(IV) complex, with its octahedral SiO₆ coordination polyhedron, is configurationally stable in solution on the NMR time scale (solvent: CDCl₃; maximum temperature studied: 58 °C).     

Electrophysiological factors in the influence of nitrate and ammonium ions on calcium uptake and translocation in tomato plants

Abstract Tomato plants (Lycopersicon esculentum Mill. cv. San Marzano), grown in dilute nutrient solutions containing (in meq ˙ 1^-1) 0.5 NaNO₃, 0.5 NH₄NO₃ or 0.25 (NH₄)₂ SO₄ as the nitrogen source, were detopped for collection of xylem sap and measurement of trans-root electrical potentials. The plant parts and the xylem exudate were subsequently analysed for mineral content. The commonly observed effects of NH₄⁺ were noted, including reduction of calcium concentration in the xylem sap, and of calcium content in stems and leaves, compared with NO₃^-fed plants. This effect was attributed principally to the less negative trans-root electrical potential measured in NH₄⁺-fed plants, and the resultant reduction of inward driving force on passively moving divalent cations.     

Silicon Isotopic Fractionation by Banana (Musa spp.) Grown in a Continuous Nutrient Flow Device

The determination of the plant-induced Si-isotopic fractionation is a promising tool to better quantify their role in the continental Si cycle. Si-isotopic signatures of the different banana plant parts and Si source were measured, providing the isotopic fractionation factor between plant and source. Banana plantlets (Musa acuminata Colla, cv Grande Naine) were grown in hydroponics at variable Si supplies (0.08, 0.42, 0.83 and 1.66 mM Si). Si-isotopic compositions were determined on a multicollector plasma source mass spectrometer (MC-ICP-MS) operating in dry plasma mode. Results are expressed as δ²⁹Si relative to the NBS28 standard, with an average precision of ± 0.08‰ (±2σ_D). The fractionation factor ²⁹ε between bulk banana plantlets and source solution is −0.40 ± 0.11‰. This confirms that plants fractionate Si isotopes by depleting the source solution in ²⁸Si. The intra-plant fractionation Δ²⁹Si between roots and shoots amounts to −0.21 ± 0.08‰. Si-isotopic compositions of the various plant parts indicate that heavy isotopes discrimination occurs at three levels in the plant (at the root epidermis, for xylem loading and for xylem unloading). At each step, preferential crossing of light isotopes leaves a heavier solution, and produces a lighter solution. Si-isotopic fractionation processes are further discussed in relation with Si uptake and transport in plants. These findings have important implications on the study of continental Si cycle.     

Binding of transition metals to monosilicic acid in aqueous and xylem (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) solutions: a low-T electron paramagnetic resonance study

The supplementation of monosilicic acid [Si(OH)₄] to the root growing medium is known to protect plants from toxic levels of iron (Fe), copper (Cu) and manganese (Mn), but also to mitigate deficiency of Fe and Mn. However, the physicochemical bases of these alleviating mechanisms are not fully understood. Here we applied low-T electron paramagnetic resonance (EPR) spectroscopy to examine the formation of complexes of Si(OH)₄ with Mn²⁺, Fe³⁺, and Cu²⁺ in water and in xylem sap of cucumber (Cucumis sativus L.) grown without or with supply of Si(OH)₄. EPR, which is also useful in establishing the redox state of these metals, was combined with measurements of total concentrations of metals in xylem sap by inductive coupled plasma. Our results show that Si(OH)₄ forms coordination bonds with all three metals. The strongest interactions of Si(OH)₄ appear to be with Cu²⁺ (1/1 stoichiometry) which might lead to Cu precipitation. In line with this in vitro findings, Si(OH)₄ supply to cucumber resulted in dramatically lower concentration of this metal in the xylem sap. Further, it was demonstrated that Si(OH)₄ supplementation causes pro-reductive changes that contribute to the maintenance of Fe and, in particular, Mn in the xylem sap in bioavailable 2+ form. Our results shed more light on the intertwined reactions between Si(OH)₄ and transition metals in plant fluids (e.g. xylem sap).     

KINETICS OF SILICON-LIMITED GROWTH IN THE FRESHWATER DIATOM ASTERIONELLA FORMOSA1,2

Growth rates of two clones of the freshwater planktonic diatom Asterionella formosa Hass. were measured under conditions in which external silicon concentrations controlled growth. Clone AfOH2 from Lake Ohrid, Yugoslavia, had a higher maximum growth rate (μ_max= 1.11 doublings/day) and apparent half-saturation constant (K_si] + Si_o= 1.93 μM Si) than clone L262 from Lake Windermere, England. (μ_max= 0.61 doublings/day; K_si+ Si_o= 1.09 μM Si). K_lim, the silicon concentration at μ= 0.9 μ_max, is 13.8 μM Si for clone AfOH2 and 6.5 μM Si for clone L262. These values agree well with published field observations showing A. formosa populations decreasing below 0.5 mg/l SiO₂ (= 8.4 μM Si). Calculations of yield gave a range of 0.5–1.5 μM Si/10⁶ cells for clone AfOH2 and 0.6–1.9 μM Si/10⁶ cells for clone L262.     

Silicon influences growth and mycorrhizal responsiveness in strawberry plants

Effect of silicon (Si) on the response of strawberry (Fragaria?×?ananassa var. Parus) plants to arbuscular mycorrhizal fungus (AMF) was studied under growth chamber conditions. Plants were grown in perlite irrigated with nutrient solution without (??Si) or with (+?Si) 3 mmol L^?1 Si (~?84 mg L^?1 Si as Na₂SiO₃) in the absence (??AMF) or presence (+?AMF) of fungus. Dry matter production, root colonization rate, photosynthesis rate and water relation parameters were all improved by both Si and AMF, and the highest amounts were achieved by +?Si?+?AMF treatment. Mycorrhizal effectiveness increased by Si treatment associated with higher Si concentration in the +?AMF plants. Leaf concentrations of total soluble and cell wall-bound phenolics were increased by Si accompanied by the enhanced activity of phenylalanine ammonia lyase, but not polyphenol oxidase. Profile of phenolics compound revealed that gallic acid, caffeic acid, epicatechin, chlorogenic acid, ellagic acid and kaempferol increased by both Si and AMF treatments, while p-coumaric acid decreased. In addition to vegetative growth, both treatments improved fruit yield and its quality parameters. Our results showed that Si and AMF acted in a synergistic manner and improved growth and biochemical parameters in strawberry plants. However, the mechanism for Si-mediated increase of mycorrhizal effectiveness is not known, thereby needing further elucidation.     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

Contrasting dynamics of water and mineral nutrients in stems shown by stable isotope tracers and cryo‐SIMS

Lateral exchange of water and nutrients between xylem and surrounding tissues helps to de‐couple uptake from utilization in all parts of a plant. We studied the dynamics of these exchanges, using stable isotope tracers for water (H₂¹⁸O), magnesium (²⁶Mg), potassium (⁴¹K) and calcium (⁴⁴Ca) delivered via a cut stem for various periods to the transpiration stream of bean shoots (Phaseolus vulgaris cv. Fardenlosa Shiny). Tracers were subsequently mapped in stem cross‐sections with cryo‐secondary ion mass spectrometry. The water tracer equilibrated within minutes across the entire cross‐section. In contrast, the nutrient tracers showed a very heterogeneous exchange between xylem vessels and the different stem tissues, even after 4 h. Dynamics of nutrients in the tissues revealed a fast and extensive exchange of nutrients in the xylem parenchyma, with, for example, calcium being completely replaced by tracer in less than 5 min. Dilution of potassium tracer during its 30 s transit in xylem sap through the stem showed that potassium concentration was up‐regulated over many hours, to the extent that some of it was probably supplied by phloem recirculation from the shoot.     

Synthesis and Olfactory Characterization of Silicon‐Containing Derivatives of the Acyclic Lily‐of‐the‐Valley Odorant 5,7,7‐Trimethyl‐4‐methylideneoctanal

5‐Methyl‐4‐methylidene‐6‐(trimethylsilyl)hexanal ( 1b ), a sila analog of the acyclic lily‐of‐the‐valley odorant 5,7,7‐trimethyl‐4‐methylideneoctanal ( 1a ), and the Si‐containing derivatives 2 – 6 were prepared in multistep syntheses, starting from Cl₃SiH and Cl₂SiMe₂, respectively. Compounds 1b, 2 – 6 , and their new precursors were characterized by elemental analyses (C, H, N) and NMR spectroscopic studies (¹H, ¹³C, ¹⁵N, and ²⁹Si). To gain more information about the structure? odor correlation in the family of lily‐of‐the‐valley or ‘muguet’ odorants, C/Si analogs 1a / 1b and derivatives 2 – 6 were evaluated for their olfactory properties.     

Radial salt transport in corn roots

Primary roots of solution-grown, 5-day-old or 6-day-old seedlings of corn (Zea mays L.) 10 to 14 cm in length were used to study radial salt transport. Measurements were made of the volume of root pressure exudation, salt concentration of the exudate, and rate of salt movement into the xylem exudate. The ³²P uptake, O₂ consumption, and dehydrogenase activity of the root cortex and stele also were studied.

These roots produced copious root pressure exudate containing 4 to 10 times the concentration of ³²P in the external solution. Freshly separated stele from 5-day-old roots accumulated ³²P as rapidly as the cortex from which it was separated and the stele of intact roots also accumulated ³²P. Separated stele has a higher oxygen uptake than cortex. It also shows strong dehydrogenase activity with the tetrazolium test. The high oxygen consumption, ³²P uptake and strong dehydrogenase activity indicate that the cells of the stele probably play a direct role in salt transport.

These data raise doubts concerning theories of radial salt transport into the xylem based on the assumption that the stele is unable to accumulate salt vigorously.

     

Restricted nitrate influx and reduction in corn seedlings exposed to ammonium

The effect of ambient ammonium (0.5 millimolar [¹⁴NH₄]₂SO₄) added to a nutrient solution containing 1.0 millimolar K¹⁵NO₃, 99 atom per cent ¹⁵N, upon [¹⁵N]nitrate assimilation and utilization of previously accumulated [¹⁴N]nitrate was investigated. Corn seedlings, 5-day-old dark-grown decapitated (experiment I) and 10-day-old light-grown intact (experiment II), which had previously been grown on K¹⁴NO₃ nutrient solution, were used. In both experiments, the presence of ambient ammonium decreased [¹⁵N]nitrate influx (20% after 6 hours) without significantly affecting the efflux of previously accumulated [¹⁴N]nitrate. In experiment I, relative reduction of [¹⁵N]nitrate (reduction as a percentage of influx) was inhibited more than was [¹⁵N]nitrate influx. Nevertheless, in experiment I, where all reduction could be assigned to the root system, the absolute inhibition of reduction during the 12 hours (13 micromoles/root) was less than the absolute inhibition in influx (24 micromoles/root). The data suggest that the influence of ammonium on [¹⁵N]nitrate influx could not be totally accounted for by the decrease in the potential driving force which resulted from restricted reduction; an additional impact on the influx process is indicated. Reduction of [¹⁵N]nitrate in experiment II after 6 hours accounted for 30 and 18% of the tissue excess ¹⁵N in the control and ammonium treatments, respectively. Relative distribution of ¹⁵N between roots and exudate (experiment I), or between roots and shoots (experiment II) was not affected by ammonium. On the other hand, the accumulation of [¹⁵N]nitrate in roots, shoots, and xylem exudate was enhanced by ammonium treatment compared to the control, whereas the accumulation of reduced ¹⁵N was inhibited.     

Biochemical aspects of bean resistance to anthracnose mediated by silicon

This study investigated the effect of silicon (Si) on resistance of bean plants (cv. ‘Peróla’) to anthracnose, caused by Colletotrichum lindemuthianum, grown in a nutrient solution containing 0 (?Si) or 2 mmol Si L^?1 (+Si). The concentration of Si in leaf tissue and the incubation period increased by 55.2% and 14.3%, respectively, in +Si plants in relation to ?Si plants. The area under anthracnose progress curve and the severity estimated by the software QUANT significantly decreased by 32.9% and 27%, respectively, for +Si plants. Si did not affect the concentration of total soluble phenolics. Chitinases activity was higher in the advanced stages of infection by C. lindemuthianum for leaves of ?Si plants. β‐1,3‐Glucanase activity increased after C. lindemuthianum infection, but it was not enhanced by Si. Peroxidase and polyphenoloxidase activities had no apparent effect on the resistance of bean plants to anthracnose, regardless of the presence of Si. The increase in lignin concentration as well as on the phenylalanine ammonia‐lyase and lipoxygenase activities were important for the resistance of +Si plants against anthracnose. The results of this study suggest that Si may increase resistance to anthracnose in bean plants by enhancing certain biochemical mechanisms of defence as opposed to just acting as a physical barrier to penetration by C. lindemuthianum.     

The velocities of ion transport into and through the xylem of roots: findings with a two-point application pulse-chase technique

Excised corn roots, Zea mays, had radioactively labeled solution applied at two points along their length, for 1 minute, and were then kept in dilute, unlabeled nutrient solution. During this “chase” period, exudate was collected at 1-minute intervals, and its content of radioions was determined. Two pulses of label appeared in succession, originating at the points of application near the cut end of the root and farther from the cut end, respectively. Calculation yields the velocity at which the ions moved radially across the root into the xylem (v_r) and the velocity at which they moved longitudinally within the xylem (v_l). For Rb⁺ labeled with ⁸⁶Rb, v_r was 1.8 and v_l, 35 cm/hr. For Br⁻ labeled with ⁸²Br, v_r was 1.4 and v_l, 103 cm/hr.     

Sucrose in the free space of translocating maize leaf bundles

Following exposure of portions of mature maize (Zea mays L.) leaf strips to ¹⁴CO₂, xylem exudate from the leaf strips contained [¹⁴C]sucrose. Sucrose was the only sugar in the xylem exudate which was obtained from the cut surface of the leaf strips by reducing the external pressure. The sucrose found in the xylem exudate apparently was obtained from the free space of the vascular bundles, its concentration amounting up to 0.25%. When [¹⁴C]glucose or [¹⁴C]fructose was supplied in the dark to one end of a maize leaf strip, each was taken up by the xylem, and transported to the opposite end. Xylem exudate from such leaf strips contained ¹⁴C-labeled sucrose in addition to the ¹⁴C-labeled hexose. The results of this study support the view that sucrose is loaded into the companion cell-sieve tube complexes from the apoplast of the vascular bundles in the maize leaf.     

29Si and 13C chemical shifts and coupling constants of tris(phenyldimethylsilyl)methyl silicon compounds

Silicon-29 (δ ²⁹Si) NMR chemical shifts are reported for the first time for a number of tris(phenyldimethylsilyl)methyl silicon compounds (disilylated derivatives), (PhMe₂Si^A)₃C_αL, where LSi^BR₁R₂R₃, where R varies widely in electronegativity. δ ²⁹Si^B for these series exhibited to some extent good correlations with the electronegativities of the groups bonded to silicon (particularly, δ ²⁹Si^BMe₂X, X H, OH, OCH₃, COCH₃ and Cl). Substitution with electronegative atoms shifts the chemical shift of silicon to high field.The ¹³C NMR spectra of these compounds have been recorded and assigned. The chemical shifts of the α-carbon (C_α) resonances are shown to depend on the type of substituent on the silicon-B, thus ¹³C_α exhibit downfield shifts when X=oxygen ligand. The ¹³C phenyl resonances have been measured and show the same order of o, m and p signals, viz. δ ortho (downfield)>δ para>δ meta.The variation of ²⁹Si-¹H coupling constants with the electronegativity of X was studied.