The Physiology of the Sieve Tube: PRELIMINARY EXPERIMENTS USING APHID MOUTH PARTS

The willow aphid Tuberolachnus salignus feeds by inserting itsstylets into a single sieve element. If the insect is severedfrom its proboscis whilst feeding, sap from the sieve tube exudesfrom the cut end of the stylet canal and may continue for manyhours. By attaching a capillary tube the rate of exudation canbe measured and the sap analysed. Normally the rate of exudationwas about 1 mm.³ per h. This means a movement of water and sucroseof 100 cm./h. or 100 sieve elements per min. The sap was foundto contain between 5 and 15 per cent, sucrose, up to 0.4 percent, raffinose, no reducing sugars, and about 0.5 per cent,amino-acids. Girdles some distance above and below exuding stylets failedto stop exudation (Tables I and II). This indicates a rapidsealing of the cut ends of the sieve tube and a switch-overin source of supply from the leaves to storage cells in thestem. The use of isolated stem segments and even irrigated stripsof bark followed. Using these, the following points have beenestablished, (a) There is no polarity of movement (Table IV);(b) a minimal length of about 16 cm. of stem or 800–1,000sieve elements are needed to give the full exudation rate (Fig.5); (c) when the D.P.D. on the inside of the bark is raised(Figs. 6, 7, and 8), the rate of exudation declines, but theconcentration of the exudate rises and exudation continues,indicating a maintenance of turgor, even in face of a D.P.D.of 20 atm. or more. Secretion of sugar into the sieve tube continuesalbeit slowly even against a sieve tube concentration of 50per cent. These findings are discussed in relation to current theoriesof translocation and are considered to favour the mass flowhypothesis.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.     

Accumulation,selection and covariation of amino acids in sieve tube sap of tansy (Tanacetum vulgare) and castor bean (Ricinus communis): evidence for the function of a basic amino acid transporter and the absence of a γ‐amino butyric acid transporter

Sieve tube sap was obtained from Tanacetum by aphid stylectomy and from Ricinus after apical bud decapitation. The amino acids in sieve tube sap were analyzed and compared with those from leaves. Arginine and lysine accumulated in the sieve tube sap of Tanacetum more than 10‐fold compared to the leaf extracts and they were, together with asparagine and serine, preferably selected into the sieve tube sap, whereas glycine, methionine/tryptophan and γ‐amino butyric acid were partially or completely excluded. The two basic amino acids also showed a close covariation in sieve tube sap. The acidic amino acids also grouped together, but antagonistic to the other amino acids. The accumulation ratios between sieve tube sap and leaf extracts were smaller in Ricinus than in Tanacetum. Arginine, histidine, lysine and glutamine were enriched and preferentially loaded into the phloem, together with isoleucine and valine. In contrast, glycine and methionine/tryptophan were partially and γ‐amino butyric acid almost completely excluded from sieve tube sap. The covariation analysis grouped arginine together with several neutral amino acids. The acidic amino acids were loaded under competition with neutral amino acids. It is concluded from comparison with the substrate specificities of already characterized plant amino acid transporters, that an AtCAT1‐like transporter functions in phloem loading of basic amino acids, whereas a transporter like AtGAT1 is absent in phloem. Although Tanacetum and Ricinus have different minor vein architecture, their phloem loading specificities for amino acids are relatively similar.     

A Reanalysis of Particle Motion in Sieve Tubes of Heracleum

From an analysis of the Brownian motion of particles in sieveelements of Heracleum mantegazzianum and Heracleum sphondylium,Barclay and Johnson have suggested that the in situ viscosityof sieve tube sap is four to six times higher than has previouslybeen assumed. In particular, they obtained a value for the sapviscosity of about 10^–2 Pa s, which compares with a valueof 2 x 10^–3 Pa s for a 20 per cent (w/v) sucrose solution.The present paper describes a reanalysis of their data. It isargued that Barclay & Johnson underestimated the Brownianmotion of sieve element particles and so overestimated the sapviscosity. An exact correction was not possible, but it is concludedthat the in situ viscosity of Heracleum sieve tube sap mustbe less than 3 x 10^–3 Pa s, which corresponds to a sucroseconcentration of less than 29 per cent. Hence it may not beunreasonable to suppose that the viscosity of sieve tube sapis determined primarily by the concentration of sucrose, ashas been assumed in theoretical analyses of the Munch hypothesis.It is also concluded that the sieve tubes studied by Barclayand Johnson were not functional, in the sense that they didnot exhibit an axial bulk flow of sap. Heracleum, sieve tubes, Brownian motion, viscosity, Munch hypothesis     

Changes in the amino acid composition and protein modification in phloem sap of rice

Pure phloem sap was collected from insects feeding on rice (Oryza sativa L.) leaves by a laser technique similar to the aphid stylet technique. Rapid circulation of nitrogen in the sieve tubes was demonstrated directly using ¹⁵N as a tracer. Application to the roots of the metabolic inhibitors of amino acids, aminooxyacetate and methioninesulfoximine, changed the amino acid composition in the sieve tubes. Feeding methionine to leaf tips resulted in its bulk transfer into the sieve tubes. In vitro experiments confirmed the existence of protein kinases in the pure rice phloem sap. The phosphorylation status of the sieve tube sap proteins was affected by the light regime. The possibility that changes in chemical composition or protein modification such as phosphorylation in the sieve tubes might affect plant growth are discussed.Analysis of pure phloem sap collected from rice plants by insect laser technique has shown dynamic changes in the chemical composition and the quality of proteins in the sap.     

Distribution of Nitrogen during Growth of Sunflower (Helianthus annuus L.)

The accumulation, distribution and redistribution of dry matterand nitrogen is described for Helianthus annuus L. cv. Hysun21 grown on 6 mM urea in glasshouse culture. Seed dry matterand nitrogen were transferred to seedlings with net efficienciesof 40 and 86 per cent respectively. At flowering, the stem hadmost of the plant's dry matter and the leaves most of its nitrogen.About 35 per cent of the plant's nitrogen accumulated afterthree-row anthesis. The amount of protein in vegetative parts,especially leaves, declined after flowering. Concentrationsof free amino compounds also decreased during growth. Matureseeds had 38 per cent of the total plant dry weight and 68 percent of the total nitrogen. Seeds acquired 33 per cent of theirdry matter and nitrogen from redistribution from above-groundplant parts. The stem was most important for storage of carbohydrate,leaves the most important for nitrogen. Over 50 per cent ofthe nitrogen in the stem and leaves was redistributed. Plantsthat received 6 mM nitrate accumulated more dry matter thanurea-grown plants. Seeds from nitrate-grown plants were heavier(58 mg) than those of urea-grown plants (46 mg), and their percentageoil was greater (50 and 41 respectively). The amount of nitrogenper seed was the same. Little or no urea was detected in xylem sap of plants suppliedwith 5 mM urea, but it was detected in sap of plants which received25 mM. Concentrations of urea and amino compounds in the sapdecreased up the stem. Plants supplied with nitrate had mostof the nitrogen in xylem sap as NO₂^–, suggesting littlenitrate reduction in roots. Plants grown on 6 mM nitrate andchanged to high levels of urea-nitrogen for 14 days still hadhigh levels of nitrate; little nitrate remained in plants receivinglow levels of urea. When urea is applied in irrigation waterto field-grown sunflower, the nitrogen is subsequently takenup as nitrate due to rapid nitrogen transformations in the soil. Helianthus annuus L., sunflower, urea, nitrate, nitrogen transport, xylem sap, nitrogen accumulation nitrogen distribution     

Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues3: I. INTERFERENCE BETWEEN SOLUTES AND RATE OF TRANSLOCATION MEASUREMENTS

Certain aspects of the secretion of solutes into, and removalfrom, the sieve tubes of isolated stem segments and rooted cuttingsof Salix viminalis have been studied. Sieve-tube sap was obtainedeither as honeydew from whole individuals or via the severedstylets of the aphid Tuberolachnus salignus (Gmelin). It was shown that interference occurred between the chemicallyunrelated solutes, sucrose and the cations potassium and rubidium.On raising the potassium concentration in the sieve-tube sapby passing a solution of this ion through the xylem, the sucroseconcentration declined. When the sucrose concentration fellover a period of days due to respiratory loss of carbohydratesfrom an isolated stem segment, a concomitant rise in eitherthe potassium or rubidium level in the sap occurred. When a solution of sodium was passed through the xylem, theconcentration of this ion in the sieve-tube sap rose, whilstthat of potassium fell at first, but later rose higher thanits initial value, indicating that both antagonism and synergycan occur between these ions. On introducing both these cationsinto the xylem simultaneously, more sodium than potassium wastaken up by the segment, though the increase in the sodium concentrationin the sieve-tube sap was less than that of the potassium. Perfusingthe xylem with a calcium solution had no effect upon the concentrationof potassium in the sieve tube. It has been shown that the rate of translocation of a solutealong the sieve tube, as measured by the two colony technique,depends upon the rate of removal of this solute from the sievetube. The amount of such lateral loss from the sieve tube isrelated to the potential gradient for a solute between the sievetube and surrounding cells.     

Stimulation of sugar loading into sieve elements of willow by potassium and sodium salts

Potassium as the chloride, nitrate or sulphate or sodium as the chloride, were applied at a concentration of 50 mM either to the xylem of stem segments or to the cambial surface of bark strips of willow. Potassium chloride increased the concentration of sucrose in sieve tube exudate collected via severed aphid stylets, without significantly affecting the volume flow rate, or the concentration of potassium in the exudate. The increase in the sucrose level in the sieve tube sap was shown to be due to a stimulation of loading, rather than to an enhancement of longitudinal transport. Potassium nitrate and sulphate or sodium chloride, were not as effective as potassium chloride in stimulating the loading of sucrose. It is suggested that uptake of the cation into cells supplying sugars to the sieve tube is linked to the rate of release of sugars by the supplying cells.     

Auxins or Sugars: What Makes the Difference in the Adventitious Rooting of Stored Carnation Cuttings?

Cold storage of cuttings is frequently applied in the vegetative propagation of ornamental plants. Dianthus caryophyllus was used to study the limiting influences of auxin and sugars on adventitious root formation (ARF) in cuttings stored at 5°C. Carbohydrate levels during storage were modulated by exposing cuttings to low light or darkness. The resulting cuttings were treated (or not) with auxin and planted, and then ARF was evaluated. Carbohydrate levels in the cuttings were monitored and the influence of light treatment on indole-3-acetic acid (IAA) and zeatin (Z) in the basal stem was investigated. Dark storage for up to 4 weeks increased the percentage of early rooted cuttings and the final number and length of adventitious roots, despite decreased sugar levels in the stem base. Light during cold storage greatly enhanced sugar levels, particularly in the stem base where the Z/IAA ratio was higher and ARF was lower than observed in the corresponding dark-stored cuttings. Sugar levels in nonstored and dark-stored cuttings increased during the rooting period, and auxin application enhanced the accumulation of sugars in the stem base of nonstored cuttings. Auxin stimulated ARF most strongly in nonstored, less so in light-stored, and only marginally in dark-stored cuttings. A model of auxin-sugar interactions in ARF in carnation is proposed: cold storage brings forward root induction and sink establishment, both of which are promoted by the accumulation of auxin but not of sugars, whereas high levels of sugars and probably also of cytokinins act as inhibitors. Subsequent root differentiation and growth depend on current photosynthesis.     

Viruspartikel im Siebröhrensaft von Cucumis sativus L. nach Infektion durch das Cucumisvirus 2 A

Summary Sieve tube sap obtained from cucumber plants infected by Cucumis Virus 2 A induced the typical mosaic disease when it was inoculated into healthy plants of the same species. The infectious factor could not be removed by dialysis or by treatment with phosphodiesterase. Therefore it is improbable that the virus is transported in the sieve tubes as low molecular units or as an unprotected RNA. Rod shaped particles (345×23 m) were found in the infectious sieve tube sap when it was investigated by electron microscopy. The same particles could be found in the sap extracted from infected leaves, but never in sieve tube sap obtained from healthy plants. There is reason to suppose that the Cucumis Virus 2 A is transported in the sieve tubes as complete particles.

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Einige Ergebnisse dieser Arbeit sind Teil einer Dissertation der Technischen Hochschule Darmstadt (D 17, 1964).     

Isolation and characterization of a cDNA clone encoding an auxin influx carrier in carnation cuttings. Expression in different organs and cultivars and its relationship with cold storage

Polar auxin transport (PAT) is necessary for the formation of adventitious roots in the base of leafy stem cuttings, as has been demonstrated in several studies in which the application of PAT inhibitors strongly inhibited the rooting of cuttings. However, unlike in the case of lateral roots, there is almost no information on the molecular mechanism that controls PAT in the formation of adventitious roots. A novel cDNA encoding an auxin influx carrier has been isolated and characterized from carnation (Dianthus caryophyllus) cuttings. The full length of DcAUX1 was obtained and the deduced aminoacid sequence revealed a high degree of identity with the corresponding auxin carrier proteins from several species. The expression of this gene depended on the organ, the carnation cultivar and the length of time cuttings had been stored in a cold chamber. As a rule, expression was higher in stem than in leaves, in the basal than in the first internode and in mature than in young leaves irrespective of the cultivar and the duration of the storage. This pattern of expression agrees with the results of a previous study showing that auxin from mature leaves was essential for rooting, while exogenous auxin applied to mature leaves was polarly transported in the stem and accumulated in the basal internode (the rooting zone). Variations in the expression observed during storage (depending of the cultivar) might be related to the variation in PAT and rooting reported in previous studies.     

Amino Acid and sucrose content determined in the cytosolic, chloroplastic, and vacuolar compartments and in the Phloem sap of spinach leaves

Amino acid and sucrose contents were analyzed in the chloroplastic, cytosolic, and vacuolar compartments and in the phloem sap of illuminated spinach leaves (Spinacia oleracea L.). The determination of subcellular metabolite distribution was carried out by nonaqueous fractionation of frozen and lyophilized leaf material using a novel three-compartment calculation method. The phloem sap was collected by aphid stylets which had been severed by a laser beam. Subcellular analysis revealed that the amino acids found in leaves are located mainly in the chloroplast stroma and in the cytosol, the sum of their concentrations amounting to 151 and 121 millimolar, respectively, whereas the amino acid concentrations in the vacuole are one order of magnitude lower. The amino acid concentrations in the phloem sap are found to be not very different from the cytosolic concentrations, whereas the sieve tube concentration of sucrose is found to be one order of magnitude higher than in the cytosol. It is concluded that the phloem loading results in a preferential extraction of sucrose from the source cells.     

The effect of low temperature on sucrose,ATP and potassium concentrations and fluxes in the sieve tubes of willow

Summary Experiments have been performed on the effect of localised low (0°C) temperature application on solute concentration and fluxes in the sieve elements of willow. Sieve tube exudate was obtained via the severed stylets of the aphid Tuberolachnus salignus (Gmelin). In stem segments, low temperature caused a fall in both the concentration and flux of sucrose. No recovery was observed during a 24 h cold application period. The concentrations of ATP and potassium were generally also reduced, though the effect on the fluxes of these solutes was not as marked. Both ATP and potassium appear to be translocated along the sieve tubes of stem segments as evidenced by girdling experiments. In leafy cuttings low temperature consistently reduced the concentration of sucrose in the sieve tube exudate. These data are discussed in relation to previous work on low temperature effects on the phloem transport system of willow.     

Redistribution of Potassium, Calcium, Magnesium, and Manganese in the Plant

The extent of redistribution in apple was described by calculating the fruit/leaf ratio of the cation content. It was found that the redistribution diminishes in the sequence K > Mg > Ca ～ Mn. These results are more or less in agreement with those found elsewhere. Investigations were made to see whether it was possible to account for the differences in redistribution by the phloem by means of the solubility of these cations in the sieve tube sap. As model plants Yucca flaccida and Ricinus communis were used, plants from which it was possible to obtain phloem sap in a rather pure state. It was found that the addition of potassium and magnesium as a chloride in the usual investigated range of concentrations did not give precipitation. With calcium and manganese, however, a precipitate soon occurred. Manganese was demonstrated to be less soluble than calcium. For Ricinus the maximum amount of calcium and manganese the sieve tube sap could contain before precipitation set in was higher than for Yucca sap. The results confirm the possibility that the redistribution of the different cations in the plant can be related inter alia to their solubility in the sieve tube sap. It was also found that the calcium in the phloem sap is present in ionic condition. Thus the normal laws of solubility should be applicable.     

Relation between Nitrogen Status, Carbohydrate Distribution and Subsequent Rooting of Chrysanthemum Cuttings as Affected by Pre-harvest Nitrogen Supply and Cold-storage

This study investigated the relationship between internal nitrogenand carbohydrate distribution in chrysanthemum cuttings of twocultivars (‘Puma’, ‘Cassa’) when affectedby nitrogen supply to stock plants (0.6, 1.5, or 4.0 g N m^-2week^-1)and different periods (2, 3, or 4 weeks) of dark cold-storage(0.5 or 5°C), and adventitious rooting. Concentrations oftotal nitrogen (N_t) and nitrate in cuttings and the levels ofsugars, starch and fructan in different cutting parts (leaves,upper stem, and basal stem) were studied in relation to subsequentadventitious rooting at natural radiation in a greenhouse. Increasingnitrogen supply resulted in substantially lower starch levelsand higher sucrose concentrations in leaves when cuttings wereexcised. Fructan concentrations were low and decreased withincreasing nitrogen levels. Starch completely disappeared fromleaves and to a large extent from stems within the shorteststorage period. A less pronounced decrease in sugar concentrationwas observed, particularly in low-nitrogen cuttings and thecuttings of ‘Puma’. The number and length of adventitiousroots subsequently formed by unstored and stored cuttings waspositively correlated with initial N_t, and to a lesser extentwith initial nitrate concentrations in cuttings. Whereas rootingwas not limited by pre-rooting concentrations of carbohydratesin the different cutting parts, the generally higher rootingcapability of nitrogen-rich cuttings, a stronger nitrogen responseof ‘Cassa’, and increased rooting at a particularharvest date, were associated with higher sucrose:starch ratiosin leaves at harvest. This reflected an increased assimilateexport. By using this characteristic in a linear regressionmodel, total variability of root numbers, ranging from three–35per cutting, could be predicted to 57% for the unstored andto 40% for all cuttings. Increased basipetal transport of carbohydrates,of nitrogen compounds, and of auxins may be causally involvedin these associations. Copyright 2000 Annals of Botany Company Adventitious rooting, nitrogen, sugars, carbohydrates, source-sink, partitioning, quality, storage, cuttings, stock plants, chrysanthemum, Dendranthema grandiflorum     

Aphid stylectomy reveals an osmotic step between sieve tube and cortical cells in barley roots

The aim of the present study was to quantify osmotic pressuresdirectly in the translocation pathway, from leaf to growingroot tip, in order to understand the forces driving solutesfrom a source to a sink. Solutes move through the translocationpathway down an osmotically derived turgor gradient. Accordinglyaphid stylectomy and single cell sampling techniques have beencombined to examine the osmotic pressure of root phloem andgrowing root cells. Sieve tube sap was obtained from shootsand, for the first time, roots of barley seedlings using aphidstylectomy. Vacuolar sap was also obtained from a variety ofcells in leaf and root tissues using single cell sampling methods.Osmotic pressure of sieve tube sap from roots and shoots wasmeasured at high temporal resolution (within min) and over longperiods of time (up to 24 h). Osmotic pressure did not changesignificantly in the minutes immediately following excision,suggesting that confidence can be placed in the assumption thatstylet exudate is representative of sieve tube sap in vivo.There were no differences in the osmotic pressure of sieve tubesap from shoots (1.240.26 MPa, n = 10) or roots (1.420.15MPa, n = 13). However, osmotic pressure of sap from root corticalcells (0.710.09, n = 12) was about 0.7 MPa lower than thatof the sieve elements from roots, this difference may be maintainedby consumption of incoming solutes at the root tip. Resultsare discussed in the context of pressure driven flow in thephloem and symplastic contact between root tip cells and sievetube. It is hoped that the approach described here will provideimportant insights into the nature of the relationship betweenroot cell extension and assimilate supply through the phloem. Key words: Phloem, sieve tube, aphid, root, barley, osmotic pressure, translocation     

Water relations in silver birch during springtime: How is sap pressurised?

• Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known.
• We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non‐structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period.
• The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations.
• Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

     

Mobilization of Minerals to Developing Seeds of Legumes

The mineral nutrition of fruiting plants of Pisum sativum L.,Lupinus albus L. and Lupinus angustifolius L. is examined insand cultures supplying adequate and balanced amounts of essentialnutrients. Changes in content of specific minerals in leaves,pods, seed coat, and embryo are described. P, N and Zn tendto increase precociously in an organ relative to dry matteraccumulation, other elements more or less parallel with (K,Mn, Cu, Mg and Fe) or significantly behind (Ca and Na) dry weightincrease. Some 60–90 per cent of the N, P and K is lostfrom the leaf, pod and seed coat during senescence, versus 20–60per cent of the Mg, Zn, Mn, Fe and Cu and less than 20 per centof the Na and Ca. Mobilization returns from pods are estimatedto provide 4–39 per cent of the seeds' accumulations ofspecific minerals, compared with 4–27 per cent for testatransfer to the embryo. Endosperm minerals are of only minorsignificance in embryo nutrition. Comparisons of the mineral balance of plant parts of Lupinusspp. with that of stem xylem sap and fruit tip phloem sap supportthe view that leaves and pod are principal recipients of xylem-borneminerals and that export from these organs via phloem is themajor source of minerals to the seeds. Endosperm and embryodiffer substantially in mineral compostition from phloem sap,suggesting that selective uptake occurs from the translocationstream during seed development. Considerable differences are observed between species in mineralcomposition of plant organs and in the effectiveness of transferof specific minerals to the seeds Differences between speciesrelate principally to Ca, Na and certain trace elements.     

Water relations link carbon and oxygen isotope discrimination to phloem sap sugar concentration in Eucalyptus globulus

A strong correlation was previously observed between carbon isotope discrimination (Delta(13)C) of phloem sap sugars and phloem sap sugar concentration in the phloem-bleeding tree Eucalyptus globulus Labill. (J. Pate, E. Shedley, D. Arthur, M. Adams [1998] Oecologia 117: 312-322). We hypothesized that correspondence between these two parameters results from covarying responses to plant water potential. We expected Delta(13)C to decrease with decreasing plant water potential and phloem sap sugar concentration to increase, thereby maintaining turgor within sieve tubes. The hypothesis was tested with analyses of E. globulus trees growing on opposite ends of a rainfall gradient in southwestern Australia. The Delta(13)C of phloem sap sugars was closely related to phloem sap sugar concentration (r = -0.90, P < 0.0001, n = 40). As predicted, daytime shoot water potential was positively related to Delta(13)C (r = 0.70, P < 0.0001, n = 40) and negatively related to phloem sap sugar concentration (r = -0.86, P < 0.0001, n = 40). Additional measurements showed a strong correspondence between predawn shoot water potential and phloem sap sugar concentration measured at midday (r = -0.87, P < 0.0001, n = 30). The Delta(13)C of phloem sap sugars collected from the stem agreed well with that predicted from instantaneous measurements of the ratio of intercellular to ambient carbon dioxide concentrations on subtending donor leaves. In accordance, instantaneous ratio of intercellular to ambient carbon dioxide concentrations correlated negatively with phloem sap sugar concentration (r = -0.91, P < 0.0001, n = 27). Oxygen isotope enrichment (Delta(18)O) in phloem sap sugars also varied with phloem sap sugar concentration (r = 0.91, P < 0.0001, n = 39), consistent with predictions from a theoretical model of Delta(18)O. We conclude that drought induces correlated variation in the concentration of phloem sap sugars and their isotopic composition in E. globulus.     

A Study of Stelar Ultrastructure in the Heterosporous Water Fern Marsilea quadrifolia L

Sieve tube elements occur in the rhizomes and petioles of Marsileaquadrifolia. These are either thick walled with compound sieveplates in oblique end walls or thin walled with simple sieveplates in transverse end walls. Vessels are restricted to themetaxylem in the roots where the phloem contains sieve cellsonly. The sieve pores are invariably callose lined and as inother pteridophytes, excepting the Lycopsida, refractive spherulesare ubiquitous in the sieve elements of Marsilea. The luminaof the protoxylem tracheary elements in the rhizomes and petiolesare occluded by tyloses but probably remain functional in theroots. Pericycle cells backing on to the root protoxylem armspossess wall ingrowths. Transfer cells are however absent fromthe vascular tissue of the rhizomes and leaves. It is suggestedthat their presence in the root pericycle is related to theretrieval of ions from the xylem sap which may be particularlycritical in water plants. The incidence of transfer cells incryptogams appears to be far more sporadic than in angiosperms.The root endodermis of Marsilea possesses a casparian stripand abundant vacuolar tannin deposits. Plasmalemmasomes arenumerous adjacent to the pericycle transfer cells. vascular ultrastructure, Marsilea quadrifolia L, transfer cells, sieve tube elements, tyloses