Spatial and temporal distribution of solutes in the developing carrot taproot measured at single-cell resolution

The time-course and spatial distribution of sugars and ions in carrot (Daucus carota L.) was studied at fine resolution using single cell (SiCSA) and tissue analysis. Four phases of osmolyte accumulation in the taproot were identified: an amino acid (germination) phase, when internal sources of amino acids provide seedlings with osmotica; an ion phase, when inorganic and organic ions were the main solutes; a hexose phase, when concentrations of glucose and fructose sharply increased and reached their maximum; and a sucrose phase, when sucrose became the major solute. Spatial distribution of sugar in taproot cells showed a general trend of highest concentration on both sides of the vascular cambium (some 200 mM sucrose, 150 mM glucose) and a minimum in the pith (some 100 mM sucrose, 60 mM glucose) and in periderm. Electrolytes (e.g. potassium) followed a distribution generally reciprocal to that of sugars; minimum in the tissue adjacent to the cambium (some 10 mM) and maximum in the pith and periderm (some 60-100 mM). The cambial cells contained unexpectedly low concentrations of sugars and potassium. These spatial and temporal patterns indicate that amino acids, other electrolytes and sugars are interchangeable in the tissue osmotic balance. The nature of the solute is developmentally determined both temporally and spatially. During the accumulation of electrolytes following the initial amino acid phase, osmotic pressure to 420 mosmol kg-1 rises and then remains constant despite large changes in the concentration of individual solutes. This indicates that osmotic pressure is regulated independently of the individual concentrations of solutes.     

The Relationship between Cyanide-Resistant Root Respiration and the Storage of Sugars in the Taproot in Daucus carota L.

Dry weight increase, root respiration, photosynthesis, and shootdark respiration were measured prior to and during the developmentof the taproot of carrot (Daucus carota L., var. AmsterdamseBak) which stores sucrose and reducing sugars. Before storageof sugars in the taproot started, root respiration was highand up to 50% inhibited by salicyl hydroxamic acid (SHAM), indicatinga high activity of the alternative pathway. The onset of sugar storage in the taproot coincided with a sharpdecrease in alternative pathway activity. When sugars were storedin the taproot, the alternative pathway in the taproot was notoperative, although present. It was concluded that the alternative pathway only consumessugars that are not used for energy production in growth ormaintenance processes, for carbon skeletons, osmoregulationor storage, in accordance with the energy overflow model. Apositive quantitative relationship exists between the storageof sugars in the taproot and the efficiency of root respiration.After day 32 the amount of daily produced carbohydrates transportedto the root system increased and these sugars were stored inthe taproot. In this period less sugars were invested in structuralshoot growth. In the present investigation no positive influence from thedeveloping sink is found on photosynthetic rate or net assimilationrate. Both rates decrease with increasing age of the plants,although increasing amounts of sugars were stored in the taproot.     

Effects of Sucrose on Water Relations of Cut, Senescing, Carnation Flowers

Carnation flower stems were stood in water or sucrose solutionand changes in water content, water and osmotic potential, turgorpressure and solutes (sugars, nitrogen, phosphorus, potassium)of petals were measured throughout the flower life. In bothtreatments the petals had a higher specific water content atincipient wilting than when the flowers were first cut. In water,turgor pressure decreased rapidly after the seventh day becauseof a decrease in tissue solute content. In sucrose solution,loss, of solutes was delayed probably because the sugar provideda respiratory substrate to maintain cell membrane integrity.In these cells, sugars and water accumulated causing decreasesin water potential and osmotic potential. Solutes and waterwere lost at about day 15 and turgor pressure decreased. Therewas some evidence that from about day 11 cells were so gorgedwith sugars that they burst when they were placed in water duringthe adjustment of water content prior to water potential measurements. Most of the initial petal osmotic energy content could be accountedfor by sugar, potassium, and anions associated with potassium,but in water, as the petals aged and sugar content decreased,so the potassium ions contributed a larger proportion of theosmotic energy; with stems in sucrose, the endogenous sugarcontent (reducing sugars plus sucrose) contributed an increasingproportion of the total osmotic energy. Dianthus caryophyllus, carnation, flowers, water relations, senescence     

Differential effects of aluminium on osmotic potential and sugar accumulation in the root cells of Al-resistant and Al-sensitive wheat

The changes in osmotic potential and the concentration of osmotic solutes in the cell sap of the root tips exposed to Al were examined in two cultivars of wheat ( Triticum aestivum ) differing in Al resistance. Root elongation was less influenced by an 8-h exposure to 20 μ M or 50 μ M Al in Al-resistant cv. Atlas 66 than in Al-sensitive cv. Scout 66. After Al treatment the osmotic potential of the root cells was decreased in Atlas 66 but increased in Scout 66 indicating that the Al treatment osmotically stimulated the driving force for water uptake in Atlas 66 but suppressed it in Scout 66. Al increased the concentration of soluble sugars, the major osmotic solute in the root cells in Atlas 66, but decreased it in Scout 66. Al at both low (5 μ M ) and high (50 μ M ) concentrations, also increased the concentration of soluble sugars in the Al-resistant genotype ET8 but a high Al concentration decreased it in Al-sensitive genotype ES8. Enzymatic analyses and thin-layer chromatography revealed that soluble sugars in the root cells of both Atlas 66 and Scout 66 mainly consisted of monosaccharides such as glucose, fructose and a small amount of sucrose. These results suggest that the accumulation of soluble sugars in Al-resistant wheat Atlas 66 keeps the osmotic potential in the root cells low and thus, enables the root cells to take up water and to elongate against the pressure produced by cell wall rigidification under Al stress.     

Sample preservation for determination of organic compounds: microwave versus freeze-drying

In search of a reliable drying method, which might be used evenunder field conditions, microwave drying was compared to freeze-dryingof plant material. Leaves of Ananas comosus and Avicennia germinansas well as buds and phloem of Acer pseudoplatanus were usedand checked for one or more of the following substances: sugars,sugar alcohols, organic and amino acids, total nitrogen, andglycinebetaine. With most samples good agreement was achieved between the twodrying methods. Only in the case of the Ananas comosus leaves,which exhibited low pH and high water content, did appreciabledifferences occur in organic and amino acids. Besides that,sucrose was the compound most susceptible to alterations, whichwas especially evident when leaves of Sambucus nigra were driedin the two different compartments (condenser compartment, dryingbell jar) of the freeze-dryer in use. For Ananas comosus leaf samples it was shown that microwavingcan also be used prior to extraction of tissue sap. Key words: Microwave, freeze-drying, drying method, tissue sap, organic solutes     

The Role of Nitrate in the Osmoregulation of Lettuce (Lactuca sativa L.) Grown at Different Light Intensities

Blom-Zandstra, M. and Lampe, J. E. M., 1985. The role of nitratein the osmoregulation of lettuce (Lactuca sativa L.) grown atdifferent light intensities.—J. exp. Bot. 36: 1043–1052. The effect of different light intensities on the nitrate accumulationvis-à-vis the concentration of other solutes in plantsap expressed from lettuce leaves was studied. After growinglettuce plants under constant environmental conditions for 52d, they were transferred to different light intensities andharvested periodically. A quantitative analysis of componentsin solution in the expressed plant sap showed a decrease innitrate concentration and an increase in the organic acids (mainlymalate) and sugars (mainly glucose) with increasing light intensity.The light intensity only slightly increased the osmolarity ofthe expressed plant sap. The measured osmolarity correspondedvery well with the value estimated from the quantitative analysesimplying that all osmotically active compounds had been accountedfor. The decrease in nitrate concentration in the expressedplant sap was fully compensated for by an increase in the dissociatedorganic acids that partly dissociate twofold to sustain electroneutralityand by an increase in both organic acids and sugars to maintainthe osmolarity. The suggestion is supported that nitrate mayserve as osmoticum at low light conditions to compensate forthe shortage of carbohydrates resulting from suboptimal photosynthesis. Key words: Nitrate accumulation, osmoregulation, Lactuca saliva L.     

Influence of phloem transport, N-fertilization and ion accumulation on sucrose storage in the taproots of fodder beet and sugar beet

To investigate the factors governing the accumulation of sucroseand amino acids in the taproots of sugar beet, their contentswere measured in the leaves, phloem sap and the taproots ofsugar beet, fodder beet and a hybrid between both, grown oneither 3.0 or 0.5 mM nitrate. In the taproots the contents ofmalate, citrate and inorganic ions were also determined. Forthe high sucrose accumulation in sugar beet as compared to theother varieties three factors were found. (a) In sugar beet,less amino acids and more sucrose are taken up into the phloemthan in fodder beet. (b) In sugar beet, the sucrose and aminoacid syntheses are less sensitive to the nitrate concentrationsthat are required for optimal plant growth than in other varieties.In fodder beet, upon raising the nitrate concentration from0.5 mM to 3 mM, the synthesis and storage of sucrose is decreasedand that of amino acids increased. The corresponding valuesin sugar beet (0.5 mM) are similar to those in fodder beet andare not much affected by an increase of nitrate. (c) The sucroseaccumulation is limited by the accumulation of inorganic ionsin the taproots. The sucrose content in the taproots is negativelycorrelated to the total ion content. Whereas sucrose representstwo-third of all solutes in the taproots of sugar beet, it amountsto only one-third of the solutes in fodder beet taproots. Key words: Amino acids, Beta vulgans L, phloem sap, potassium, sucrose storage, sugar beet, taproots, transport     

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     

Effects of drought on water relations and nonstructural carbohydrates in cladodes of Opuntia ficus-indica

Nocturnal acid accumulation, water content, osmotic pressure (π), and nonstructural carbohydrates were determined in the chlorenchyma and the water-storage parenchyma of Opuntia ficus-indica (L.) Miller for well-watered plants and those subjected to drought for 15 weeks. During the 15-week drought, total cladode water content decreased by 57%, the water-storage parenchyma losing a greater fraction of water than the chlorenchyma, which most likely helped maintain nocturnal acid accumulation in the latter tissue. Despite the preferential water loss from the water-storage parenchyma, it had a lower π than the chlorenchyma over the 15 weeks of drought, suggesting a substantial decrease in osmotically active solutes in the water-storage parenchyma. Also, the measured π increases of both tissues were much less than those predicted based on the loss of water during drought and the initial content of osmotically active solutes under well-watered conditions. A decrease in the amount of soluble sugars (glucose. fructose and sucrose) occurred in plants subjected to drought. accounting for 46% and 81% of the difference between the measured and the predicted increases in π of the chlorenchyma and the water-storage parenchyma. respectively. The decrease in soluble sugars was associated with an equivalenl increase in polysaccharides, presumably starch, in the water-storage parenchyma. but not in the chlorenchyma.     

The Effect of Salinity on Growth of Phaseolus vulgaris L.: II. Effect on Internal Solute Concentration

This paper reports the changes which occurred in the concentrationof ions, organic acids, sugars and polyhydric alcohols whenplants of Phaseolus vulgaris were grown in culture solutioncontaining 48 m mol l^–1 sodium chloride. Attention wasfocused particularly on the changes in the primary and firsttrifoliate leaves. In the latter leaves of salt-treated plantsthere were initially very high concentrations of sodium andmuch higher concentrations of glucose and inositol than in controlleaves. Subsequently concentrations of these solutes declined,the decline of sodium being due to retranslocation out of theleaf. There were no striking differences between the concentrationof organic solutes of the primary leaves of salt-treated plantsand those of control plants. There were greater concentrationsof ions in the salt-treated primary leaves, particularly a greaterconcentration of potassium (though that in the medium was thesame in both treatments). While the calcium concentrations inboth sets of primary leaves were very similar, it appeared thatin the leaves of salt-treated plants much of the ion is solublewithin the cells while in the leaves of the control plants,most of the ion is thought to be associated with the cell wall.There was a greater concentration of organic acids in the leavesof the control plants but in both treatments, primary and trifoliateleaves each had a similar composition of acids. Sodium ionswere restrained from entering the leaves by selection againstthe ion in favour of potassium in the root and possibly by retentionof sodium in the stem. The data on solute concentrations togetherwith calculated values of cellular osmotic potentials are discussedin relation to the development of the first trifoliate leaves.     

Turgor regulation of sucrose transport in sugar beet taproot tissue

Sink tissues that store osmotically active compounds must osmoregulate to prevent excessively high turgor. The ability to regulate turgor may be related to membrane transport of solutes and thus sink strength. To study this possibility, the kinetics of sugar uptake were determined in sugar beet (Beta vulgaris L.) taproot tissue discs over a range of cell turgors. Sucrose uptake followed biphasic kinetics with a high affinity saturating component below 20 millimolar and a low affinity linear component at higher concentrations. Glucose uptake exhibited only simple saturation type kinetics. The high affinity saturating component of sucrose and glucose uptake was inhibited by increasing cell turgor (decreasing external mannitol concentrations). The inhibition was evident as a decrease in V_max but no effect on K_m. Sucrose uptake by tissue equilibrated in dilute buffer exhibited no saturating component. Ethylene glycol, a permeant osmoticum, had no effect on uptake kinetics, suggesting that the effect was due to changes in cell turgor and not due to decreased water potential per se. p-(Chloromercuri)benzene sulfonic acid (PCMBS) inhibited sucrose uptake at low but not high cell turgor. High cell turgor caused the tissue to become generally leaky to potassium, sucrose, amino acids, and reducing sugars. PCMBS had no effect on sucrose leakage, an indication that the turgor-induced leakage of sucrose was not via back flow through the carrier. The ability of the tissue to acidify the external media was turgor dependent with an optimum at 300 kilopascals. Acidification was sharply reduced at cell turgors above or below the optimum. The results suggest that the secondary transport of sucrose is reduced at high turgor as a result of inhibition of the plasma membrane ATPase. This inhibition of ATPase activity would explain the reduced V_max and leakiness to low molecular weight solutes. Cell turgor is an important regulator of sucrose uptake in this tissue and thus may be an important determinant of sink strength in tissues that store sucrose.     

Isolation of Vacuoles from Immature Apple Fruit Flesh and Compartmentation of Sugars, Organic Acids, Phenolic Compounds and Amino Acids

Vacuoles of immature apple fruit (Malus pumila Mill. var. domesticaSchneid.) were obtained by purification using Ficoll densitygradient centrifugation after lysis of the protoplasts by bothmild osmotic shock and the addition of EDTA and BSA. The recoverywas about 35% of the protoplasts. The isolated vacuoles hada mean diameter of about 100 µm. The distribution of sugars, organic acids, phenolic compoundsand amino acids in the vacuole, the cytoplasm and the free spacewas determined. Almost all of the fructose and glucose, themajor sugars of the tissue, were found in the vacuole. Sorbitolwas mainly located in the free space and the vacuole, and sucrosein the free space and the cytoplasm. More than 90% of the malicacid, the main organic acid, was located in the vacuole. Almostall of the phenolic compounds were also deposited in the vacuole. The volumes of the vacuole, the cytoplasm and the free spacein the whole tissue were calculated from the cell numbers ofthe whole tissue, the volume of the isolated protoplasts, andthe volume of the vacuoles present in the protoplast. The soluteconcentration in each compartment was estimated: vacuoles, 888mM; cytoplasm, 37 mM; free space, 57 mM. How these compartmentationsof solutes affected the translocation of sugars into the fruitand the cell expansion is discussed. ¹This paper is contribution A-159 of the Fruit Tree ResearchStation. (Received July 7, 1983; Accepted November 14, 1983)     

Influence of arbuscular mycorrhiza on organic solutes in maize leaves under salt stress

A pot experiment was conducted to examine the effect of the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, on plant biomass and organic solute accumulation in maize leaves. Maize plants were grown in sand and soil mixture with three NaCl levels (0, 0.5, and 1.0 g kg⁻¹ dry substrate) for 55 days, after 15 days of establishment under non-saline conditions. At all salinity levels, mycorrhizal plants had higher biomass and higher accumulation of organic solutes in leaves, which were dominated by soluble sugars, reducing sugars, soluble protein, and organic acids in both mycorrhizal and non-mycorrhizal plants. The relative abundance of free amino acids and proline in total organic solutes was lower in mycorrhizal than in non-mycorrhizal plants, while that of reducing sugars was higher. In addition, the AM symbiosis raised the concentrations of soluble sugars, reducing sugars, soluble protein, total organic acids, oxalic acid, fumaric acid, acetic acid, malic acid, and citric acid and decreased the concentrations of total free amino acids, proline, formic acid, and succinic acid in maize leaves. In mycorrhizal plants, the dominant organic acid was oxalic acid, while in non-mycorrhizal plants, the dominant organic acid was succinic acid. All the results presented here indicate that the accumulation of organic solutes in leaves is a specific physiological response of maize plants to the AM symbiosis, which could mitigate the negative impact of soil salinity on plant productivity.     

Transport of organic solutes in Phloem and xylem of a nodulated legume

Collections of xylem exudate of root stumps or detached nodules, and of phloem bleeding sap from stems, petioles, and fruits were made from variously aged plants of Lupinus albus L. relying on nodules for their N supply. Sucrose was the major organic solute of phloem, asparagine, glutamine, serine, aspartic acid, valine, lysine, isoleucine, and leucine, the principal N solutes of both xylem and phloem. Xylem sap exhibited higher relative proportions of asparagine, glutamine and aspartic acid than phloem sap, but lower proportions of other amino acids. Phloem sap of petioles was less concentrated in asparagine and glutamine but richer in sucrose than was phloem sap of stem and fruit, suggesting that sucrose was unloaded from phloem and amides added to phloem as translocate passed through stems to sinks of the plant. Evidence was obtained of loading of histidine, lysine, threonine, serine, leucine and valine onto phloem of stems but the amounts involved were small compared with amides. Analyses of petiole phloem sap from different age groups of leaves indicated ontogenetic changes and effects of position on a shoot on relative rates of export of sucrose and N solutes. Diurnal fluctuations were demonstrated in relative rates of loading of sucrose and N solutes onto phloem of leaves. Daily variations in the ability of stem tissue to load N onto phloem streams were of lesser amplitude than, or out of phase with fluctuations in translocation of N from leaves. Data were related to recent information on C and N transport in the species.     

On the osmoregulation in Atriplex hymenelytra (Torr.) Wats. (Chenopodiaceae)

Summary Aspects of osmoregulation were studied in leaves of irrigated and nonirrigated plants of Atriplex hymenelytra (Torr.) Wats. (Chenopodiaceae) from their natural habitat in Death Valley, California. Using a set of several data concentrations of inorganic electrolytes (Na⁺, K⁺, Cl^-) and of oxalate in the mesophyll of this salt secreting species were calculated. The osmotic potential resulting from these solutes (under consideration of an empirically estimated osmotic coefficient) is in good agreement with field measurements of the overall osmotic potential in the leaf mesophyll as determined by pressure-volume curves. This indicates that these 4 electrolytes are the main osmotically active solutes. Oxalate is present in comparably high concentrations and is used to achieve ion balance.Organic solutes analyzed include soluble carbohydrates (mono-, di- and oligosaccharides), amino- and organic acids as well as glycinebetaine. Of these, organic- and amino acids (including proline) contribute only little to osmoregulation. Soluble carbohydrates and especially glycinebetaine exhibit concentrations high enough for generating considerable osmotic potentials, at least if these compounds are regarded to be restricted to the cytoplasm acting as compatible solutes.     

The role of organic and inorganic solutes in the osmotic adjustment of drought-stressed Jatropha curcas plants

This study aimed to assess the accumulation of organic and inorganic solutes and their relative contribution to osmotic adjustment in roots and leaves of Jatropha curcas subjected to different water deficit intensity. Plants were grown in vermiculite 50% (control), 40%, 30%, 20% and 10% expressed in gravimetric water content. The water potential, osmotic potential and turgor potential of leaves decreased progressively in parallel to CO₂ photosynthetic assimilation, transpiration and stomatal conductance, as the water deficit increased. However, the relative water content, succulence and water content in the leaves did not show differences between the control and stressed plants, indicating osmotic adjustment associated with an efficient mechanisms to prevent water loss by transpiration through stomatal closure. The K⁺ ions had greater quantitative participation in the osmotic adjustment in both leaves and roots followed by Na⁺ and Cl⁻, while the NO₃⁻ ion only showed minor involvement. Of the organic solutes studied, the total soluble sugars showed the highest relative contribution to the osmotic adjustment in both organs and its concentration positively increased with more severe water deficit. The free amino acids and glycinebetaine also effectively contributed to the osmotic potential reduction of both the root and leaves. The role of proline was quantitatively insignificant in terms of osmotic adjustment, in both the control and stressed roots and leaves. Our data reveal that roots and leaves of J. curcas young plants display osmotic adjustment in response to drought stress linked with mechanisms to prevent water loss by transpiration by means of the participation of inorganic and organic solutes and stomatal closure. Of all the solutes studied, soluble sugars uniquely display a prominent drought-induced synthesis and/or accumulation in both roots and leaves.     

Solute Accumulation In Plant Cells IV. Effects of Ammonium lons on Growth and Solute Content

Procedures previously described were used to study growth andsolute content of aseptically cultured carrot explants as affectedby supplementary salts in the medium. The salts chosen (KC1,KNO₃, NH₄,Cl, and NH⁴,NO₃) contrasted, with appropriate controls,the effects due to nitrate and ammonium. Growth was measuredin terms of fresh weight, the number and average size of cells:solute concentrations were recorded for total solutes, sugars,soluble nitrogen compounds, and the electrolytes K⁺, Na⁺, C1^–,NO^–₃, and organic acids. The time-response curves of thecultures were traced at a fixed concentration of the added saltsand the effects due to the concentration of the supplementarysalts were tested after a fixed time period, For the same nitrogensource the concentrations of metabolites and solutes in cellswere very similar despite some clonal differences in their growth.When cells in a nitrate medium were small and dividing, thecultures had a low osmotic value, contained K⁺ as the principalcation balanced by organic acid, had relatively low sugar content,and their enriched total nitrogen content emphasized proteinrather than soluble nitrogen compounds. Later, as the cellsbecame older and larger, salts (K⁺, organic anions, Cl^–)contributed substantially to their increased osmotic value butthey accumulated sugar as their main, osmotically active solute,and the ratio of soluble to protein nitrogen declined as proteinsynthesis progressed. The extra nitrogen supplied by the additionalpotassium nitrate contributed more to protein and caused potassium,organic acids, and sugars to accumulate to higher levela. Supplementaryammonium salts required that more sugar be metabolized to organicnitrogen compounds (e.g. glutamine), contributed more to solublethan to protein nitrogen, and sharply reduced. both the osmoticvalue of the cells and the potassium linked to organic anions.The selectivity of the growing cells for K⁺ over Na⁺ and theirdiscrimination. between alkali cations (Ka⁺+Na⁺) and halides(C1^–) were relaxed in the presence of ammonia. Attentionis drawn to the implications of these results for the accumulationof solutes, organic and inorganic, by dividing and enlargingcells.     

Mineral uptake and transport in xylem and phloem of the proteaceous tree,Banksia prionotes

Xylem sap of sinker (tap) root, cluster feeding roots, lateral roots and from an age series of main stem extensions of 6-year trees of Banksia prionotes was collected and analyzed for principal organic and inorganic solutes. During the phase of root uptake activity in winter and spring, cluster roots were principal xylem donors of malate, phosphate, chloride, sodium, potassium and amino acid N whereas other parts of the root served as major sources to the shoot of other cations, nitrate and sulphate. Sinker root xylem sap was at all times less concentrated in solutes than that of lateral roots into which cluster roots were voiding exported solutes. Phosphate was abstracted from xylem by stem tissue during winter and it and a range of other solutes released back to xylem immediately prior to extension growth of the shoot in summer. Phloem sap collected from mid regions of stems was unusually low in potassium and phosphate relative to chloride and sulphate in comparison with phloem sap of other species, and its low potassium: sodium ratio relative to xylem indicated poor discrimination against sodium during phloem loading. Data are discussed in relation to the asynchronous seasonal cycles of nutrient uptake and shoot growth.     

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.     

Further Studies on Growth and Osmoregulation of Sugar Beet Leaves under Low Salinity Conditions

In previous work, Nunes and Dias (1980) demonstrated that lowsodium concentrations in the root medium of intact or decapitatedyoung sugar beet plants grown under controlled conditions modifiedleaf water relations and increased leaf area and dry weight.The present study confirms these findings and presents furtherresults concerning the effect of salt on the concentrationsof the main osmotic substrates and on the structural and chemicalfractions of the cell dry weight. Increases of water and turgor potentials (0.25 MPa and 0.4 MPa,respectively) and a small decrease in osmotic potential (0.16MPa) were found in the leaves of salt treated plants. In theseplants, osmotic potentials estimated from the concentrationof ions and organic solutes in the leaf sap agree with thosemeasured showing that the observed increase in sodium concentrationmay account for the small decrease in the osmotic potential.No changes were detected in the concentration of orthophosphateor malic acid but total acidity of the leaf sap from salt treatedplants was significantly lower. It was found that all the main components of cell dry matter(total protein, soluble sugars, pigments and crude cell wall)contributed to the dry weight increase in the salt treated plants.Among the polysaccharide fractions of the cell wall, pectinsincreased significantly relative to hemicellulose and cellulose. Key words: Sugar beet, Sodium chloride, Growth, Osmoregulation