A Simplified Technique for Detection of Paclobutrazol in Plant Sap Extracts, Using HPLC

The tree growth regulator paclobutrazol (PAC) was detected using a simplified high pressure liquid chromatography technique. PAC was detected in both xylem and phloem sap 6 months after trees were injected with the compound. PAC is believed to be exclusively xylem mobile in plants, thus its detection in the phloem during this experiment was unexpected. The technique presented is simple and effective and avoids the use of radioactive material and complicated purification processes before analysis. Received February 25, 1997; accepted July 18, 1997     

Inhibitor content of phloem and xylem sap obtained from willow (Salix viminalis L.) entering dormancy

Summary It is shown that there is one growth inhibitor in the phloem sap and two in the xylem sap of willow (S. viminalis L.). The concentration of the inhibitor in the phloem sap, (+)-abscisic acid, increases as the plants enter dormancy. This is also shown for (+)-abscisic acid in the xylem sap, but the concentration of the second inhibitor decreases in a reciprocal manner.     

Long-distance translocation of polyamines in phloem and xylem of Ricinus communis L. plants

Polyamine content and enzyme activities in the biosynthetic and degradative pathways of polyamine metabolism were investigated in sieve-tube sap, xylem sap and tissues of seedlings and adult plants of Ricinus communis L. Polyamines were present in tissues and translocation fluids of both seedlings and adult plants in relatively high amounts. Only free polyamines were translocated through the plant, as indicated by the finding that only the free form was detected in the phloem and the xylem sap. Removal of the endosperm increased the polyamine content in the sieve-tube exudate of seedlings. The level and pattern of polyamines in tissue of adult leaves changed during leaf age, but not, however, in the sieve-tube sap. Xylem sap was relatively poor in polyamines. Polyamine loading in the phloem was demonstrated by incubating cotyledons with [¹⁴O]putrescine and several unlabelled polyamines. Feeding cotyledons with cadaverine and spermidine led to a decrease in the level of putrescine in sieve-tube sap, indicating a competitive effect. Comparison of polyamine content in the tissue and export rate showed that the export would deplete the leaves of polyamines within 1–3 d, if they were not replenished by biosynthesis. Polyamine biosynthesis in Ricinus proceeds mostly via arginine decarboxylase, which in vitro is 100-fold more active than ornithine decarboxylase. The highest arginine decarboxylase, ornithine decarboxylase and diamine oxidase activities were detected in cotyledons, while in sieve-tube sap only a slight arginine decarboxylase activity was found. Received: 18 March 1997 / Accepted: 20 August 1997     

Cytokinins in the Phloem Sap of White Lupin (Lupinus albus L.)

Cytokinin-like activity in samples of xylem and phloem sap collected from field-grown plants of white lupin (Lupinus albus L.) over a period of 9 to 24 weeks after sowing was measured using the soybean hypocotyl callus bioassay following paper chromatographic separation. The phloem sap was collected from shallow incisions made at the base of the stem, the base of the inflorescence (e.g. stem top), the petioles, and the base and tip of the fruit. Xylem sap was collected as root exudate from the stump of plants severed a few centimeters above ground level. Concentration of cytokinin-like substances was highest in phloem sap collected from the base of the inflorescence and showed an increase over the entire sampling period (from week 10 [61 nanogram zeatin equivalents] to week 24 [407 nanogram zeatin equivalents]). Concentrations in the xylem sap and in the other phloem saps were generally lower. Relatively high concentrations of cytokinin-like substances in petiole phloem sap (70 to 130 nanogram zeatin equivalents per milliliter) coincided in time with high concentrations in sap from the base of the inflorescence (see above). Concentrations in sap (phloem or xylem) from the base of the stem were very much lower. This finding is consistent with movement of cytokinins from leaves into the developing inflorescence and fruit, rather than direct input to the fruit from xylem sap. However, an earlier movement of cytokinins from roots into leaves via the xylem cannot be ruled out. Sap collected at an 18-week harvest was additionally separated by sequential C₁₈ reversed-phase high performance liquid chromatography → NH₂ normal phase high performance liquid chromatography, bioassayed, and then analyzed by electron impact gas chromatography-mass spectrometry. Identification of zeatin riboside and dihydrozeatin as two of the major cytokinins in combined sap samples was accomplished by gas chromatography-mass spectrometry-selected ion monitoring.     

Identification of high levels of phytochelatins, glutathione and cadmium in the phloem sap of Brassica napus. A role for thiol-peptides in the long-distance transport of cadmium and the effect of cadmium on iron translocation

Phytochelatins (PCs) are glutathione-derived peptides that function in heavy metal detoxification in plants and certain fungi. Recent research in Arabidopsis has shown that PCs undergo long-distance transport between roots and shoots. However, it remains unknown which tissues or vascular systems, xylem or phloem, mediate PC translocation and whether PC transport contributes to physiologically relevant long-distance transport of cadmium (Cd) between shoots and roots. To address these questions, xylem and phloem sap were obtained from Brassica napus to quantitatively analyze which thiol species are present in response to Cd exposure. High levels of PCs were identified in the phloem sap within 24 h of Cd exposure using combined mass spectrometry and fluorescence HPLC analyses. Unexpectedly, the concentration of Cd was more than four-fold higher in phloem sap compared to xylem sap. Cadmium exposure dramatically decreased iron levels in xylem and phloem sap whereas other essential heavy metals such as zinc and manganese remained unchanged. Data suggest that Cd inhibits vascular loading of iron but not nicotianamine. The high ratios [PCs]/[Cd] and [glutathione]/[Cd] in the phloem sap suggest that PCs and glutathione (GSH) can function as long-distance carriers of Cd. In contrast, only traces of PCs were detected in xylem sap. Our results suggest that, in addition to directional xylem Cd transport, the phloem is a major vascular system for long-distance source to sink transport of Cd as PC–Cd and glutathione–Cd complexes.     

Presence and identification of polyamines in xylem and Phloem exudates of plants

Polyamines were identified by high performance liquid chromatography (benzoylation) and by thin layer chromatography (dansylation) in xylem exudates from stems of sunflowers (Helianthus annuus [L.]), mung bean (Vigna radiata [L.] Wilczek), grapevine (Vitis vinifera [L.] cv Grenache), and orange (Citrus sinensis [L.] Osbeck, cv Valencia), as well as in phloem sap (using elution into EDTA) of sunflower and mung bean plants. Putrescine was the major polyamine detected, ranging in concentrations of 150 to 9200 picomoles per milliliter exudate, whereas only trace amounts of spermine were detected. High amounts of putrescine and spermidine were found in EDTA eluates (possibly phloem sap) as compared with elution into water. Concentrations of putrescine and spermidine in xylem exudates were related to the physiological conditions of the plants prior to exudate collection. More putrescine was found in exudates of older than in younger sunflower plants, and salt stress applied to sunflower plants resulted in a higher concentration of putrescine and spermidine in the exudate. The presence and abundance of putrescine and spermidine in xylem and phloem exudates indicate that polyamines may be translocated in plants. This long-distance translocation further supports the hypothesis that polyamines have a regulatory role in plant growth and response to stress.     

Solute balance of a maize (Zea mays L.) source leaf as affected by salt treatment with special emphasis on phloem retranslocation and ion leaching

Strategies for avoiding ion accumulation in leaves of plants grown at high concentration of NaCl (100 mol m(-3)) in the rooting media, i.e. retranslocation via the phloem and leaching from the leaf surface, were quantified for fully developed leaves of maize plants cultivated hydroponically with or without salt, and with or without sprinkling (to induce leaching). Phloem sap, apoplastic fluid, xylem sap, solutes from leaf and root tissues, and the leachate were analysed for carbohydrates, amino acids, malate, and inorganic ions. In spite of a reduced growth rate Na(+) and Cl(-) concentrations in the leaf apoplast remained relatively low (about 4-5 mol m(-3)) under salt treatment. Concentrations of Na(+) and Cl(-) in the phloem sap of salt-treated maize did not exceed 12 and 32 mol m(-3), respectively, and thus remained lower than described for other species. However, phloem transport rates of these ions were higher than reported for other species. The relatively high translocation rate of ions found in maize may be due to the higher carbon translocation rate observed for C(4) plants as opposed to C(3) plants. Approximately 13-36% of the Na(+) and Cl(-) imported into the leaves through the xylem were exported by the phloem. It is concluded that phloem transport plays an important role in controlling the NaCl content of the leaf in maize. Surprisingly, leaching by artificial rain did not affect plant growth. Ion concentrations in the leachate were lower than reported for other plants but increased with NaCl treatment.     

The origin and composition of cucurbit "phloem" exudate

Cucurbits exude profusely when stems or petioles are cut. We conducted studies on pumpkin (Cucurbita maxima) and cucumber (Cucumis sativus) to determine the origin and composition of the exudate. Morphometric analysis indicated that the exudate is too voluminous to derive exclusively from the phloem. Cold, which inhibits phloem transport, did not interfere with exudation. However, ice water applied to the roots, which reduces root pressure, rapidly diminished exudation rate. Sap was seen by microscopic examination to flow primarily from the fascicular phloem in cucumber, and several other cucurbit species, but primarily from the extrafascicular phloem in pumpkin. Following exposure of leaves to 14CO2, radiolabeled stachyose and other sugars were detected in the exudate in proportions expected of authentic phloem sap. Most of this radiolabel was released during the first 20 s. Sugars in exudate were dilute. The sugar composition of exudate from extrafascicular phloem near the edge of the stem differed from that of other sources in that it was high in hexose and low in stachyose. We conclude that sap is released from cucurbit phloem upon wounding but contributes negligibly to total exudate volume. The sap is diluted by water from cut cells, the apoplast, and the xylem. Small amounts of dilute, mobile sap from sieve elements can be obtained, although there is evidence that it is contaminated by the contents of other cell types. The function of P-proteins may be to prevent water loss from the xylem as well as nutrient loss from the phloem.     

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.     

Sodium fluxes in sweet pepper exposed to varying sodium concentrations

The sodium transport and distribution of sweet pepper (Capsicum annuum L.) under saline conditions were studied after transferring the plants to a sodium-free nutrient solution. Sodium stress up to 60 mM did not affect the growth of sweet pepper, as it appears able to counteract the unfavourable physiological effects of sodium efficiently. Sodium was particularly accumulated in the basal pith cells of the stem and in the root cells, while almost no sodium was directed to the leaves or the fruits. The sodium concentration in the pith cells and xylem sap gradually decreased towards the shoot tip. Removal of sodium from the medium resulted in a 50% release of sodium from the plant after 1 week without affecting the gradient in the pith cells. In contrast, the concentration profile in the xylem sap was completely changed: the sodium concentration in the xylem sap at the stem base was similar to that at the top.Phloem transport was studied in a split root experiment, in which both portions of the roots were exposed to 15 mM NaCl and one part was fed with additional ²²NaCl. During continuous exposure to 15 mM NaCl no label was detected in unlabelled root parts. However, after transferring the plants to a sodium-free solution, ²²Na was rapidly released from the unlabelled roots, indicating a downward phloem transport.It was concluded that pith cells, the intermediates between the xylem and phloem, play a decisive role in the recirculation of sodium throughout the plant. Release of sodium from the plants following transfer to a sodium-free solution may be explained by changes in the diffusion resistance for passive sodium efflux from the cells.Key words: Xylem, phloem, sodium, fluxes, sweet pepper      

Root-shoot interactions in mineral nutrition

In this paper four classes of co-operative root-shoot interations are addressed. (I) Nitrogen concentrations in the xylem sap originating from the root and in the phloem sap as exported from source leaves are much lower than those required for growth by apices and developing organs. Enrichment of xylem sap N is achieved by xylem to xylem (X-X) transfer, by which reduced N, but not nitrate, is abstracted from the xylem of leaf traces and loaded into xylem vessels serving the shoot apex. Nitrogen enrichment of phloem sap from source leaves is enacted by transfer of reduced N from xylem to phloem (X-P transfer). Quantitative data for the extent of the contribution of X-X and X-P transfer to the nutrition of young organs of Ricinus communis L. and for their change with time are presented. (II) Shoot and root cooperate in nitrate reduction and assimilation. The partitioning of this process between shoot and root is shifted towards the root under conditions of nitrate- and K-deficiency and under salt stress, while P deficiency shifts nitrate reduction almost totally to the shoot. All four changes in partitioning can be attributed to the need for cation-anion balance during xylem transport and the change in electrical charge occurring with nitrate reduction. (III) Even maintenance of the specificity of ion uptake by the root may – in addition to its need for energy – require a shoot-root interaction. This is shown to be needed in the case of the maintenance of K/Na selectivity under the highly adverse condition of salt stress and absence of K supply from the soil. (IV) Hormonal root to shoot interactions are required in the whole plant for sensing mineral imbalances in the soil. This is shown and addressed for conditions of salt stress and of P deficiency, both of which lead to a strong ABA signalling from root to shoot but result in different patterns of response in the shoot.     

PHLOEM OF SPHENOPHYLLUM

The phloem of most fossil plants, including that of Sphenophyllum, is very poorly known. Sphenophyllum was a relatively small type of fossil arthrophyte with jointed stems bearing whorls of leaves ranging in form from wedge or fan-shaped to bifid, to linear. The aerial stem systems of the plant exhibited determinate growth involving progressive reduction in the dimensions of the stem primary bodies, fewer leaves per whorl, and smaller and simpler leaves distally. The primary phloem occurs in three areas alternating in position with the arms of the triarch centrally placed primary xylem. Cells of the primary phloem, presumably sieve elements, are axially elongate with horizontal to slightly tapered end walls. In larger stems with abundant secondary xylem and secondary cortex or periderm, a zone of secondary phloem occurs whose structure varies in the three areas opposite the arms of the primary xylem, as opposed to the three areas lying opposite the concave sides of the primary xylem. The axial system of the secondary phloem consists of vertical series of sieve elements with horizontal end walls. In the areas opposite the protoxylem the parenchyma is present as a prominent ray system showing dilation peripherally. Sieve elements in the areas opposite the protoxylem arms have relatively small diameters. In the areas between the protoxylem poles the secondary phloem sieve elements have large diameters and are less obviously in radial files, while the parenchyma resembles that of the secondary xylem in these areas in that it consists of strands of cells extending both radially and tangentially. An actively meristematic vascular cambium has not been found, indicating that this layer changed histologically after the cessation of growth in the determinate aerial stem systems and was replaced by a post-meristematic parenchyma sheath made up of axially elongate parenchyma lacking cells indicative of being either fusiform or ray initials. A phellogen arose early in development in a tissue believed to represent pericycle and produced tissue comparable to phellem externally. Normally, derivatives of the phellogen underwent one division prior to the maturation of the cells. Concentric bands of cells with dark contents apparently represent secretory tissue in the periderm and cell arrangements indicate that a single persistent phellogen was present. Sphenophyllum is compared with other arthrophytes as to phloem structure and is at present the best documented example of a plant with a functionally bifacial vascular cambium in any exclusively non-seed group of vascular plants.     

Detection and characterization of nitrogen circulation through the sieve tubes and xylem vessels of rice plants

Nitrogen movement through the xylem vessels and sieve tubes in rice plants was studied using xylem and phloem sap analysis in combination with stable and radioactive nitrogen isotope techniques.More than 90% of nitrogen was translocated in the sieve tubes of rice plants as amino acids. When 15N (99.6 atom%) was applied as a nitrate to the root, 15N first appeared in phloem sap of the leaf sheath within 10 minutes and increased to 37 atom% excess 5 hours after the experiment had started. In long-term experiments, 63% of nitrogen in the phloem sap of the leaf sheath and 15% in that of the uppermost internode came from nitrogen absorbed within the last 24 hours and 50 hours, respectively.To obtain information about the more rapid circulation of nitrogen in the plant, radioactive 13N was used as a tracer. A positron-emitting tracer imaging system was used to show that 13N was transferred to the leaf sheath within 8 minutes of its application to the roots. Analysis of the xylem sap of the leaf sheath showed that when the nitrate was applied to the roots, most of the nitrogen in the xylem was transported as a nitrate.These data showed that phloem and xylem sap analysis together with the stable and radioactive nitrogen techniques provide a good method for the detection of nitrogen cycles in plants.     

Identification of three wheat globulin genes by screening a Triticum aestivum BAC genomic library with cDNA from a diabetes-associated globulin

Background

Plant systemic signaling characterized by the long distance transport of molecules across plant organs involves the xylem and phloem conduits. Root-microbe interactions generate systemic signals that are transported to aerial organs via the xylem sap. We analyzed the xylem sap proteome of soybean seedlings in response to pathogenic and symbiotic interactions to identify systemic signaling proteins and other differentially expressed proteins.

Results

We observed the increase of a serine protease and peroxidase in the xylem sap in response to Phytophthora sojae elicitor treatment. The high molecular weight fraction of soybean xylem sap was found to promote the growth of Neurospora crassa in vitro at lower concentrations and inhibit growth at higher concentrations. Sap from soybean plants treated with a P. sojae elicitor had a significantly higher inhibitory effect than sap from control soybean plants. When soybean seedlings were inoculated with the symbiont Bradyrhizobium japonicum, the abundance of a xyloglucan transendoglycosyl transferase protein increased in the xylem sap. However, RNAi-mediated silencing of the corresponding gene did not significantly affect nodulation in soybean hairy root composite plants.

Conclusion

Our study identified a number of sap proteins from soybean that are differentially induced in response to B. japonicum and P. sojae elicitor treatments and a majority of them were secreted proteins.     

Spring Sap of Trees

In the spring, sap in perennial plants becomes available after winter dormancy. As is well known, substances including minerals and organic nutrients are transported from storage compartments in roots and stems to growth regions. For this solute distribution, xylem as well as phloem are used and probably also the cambial region and the ray system. In the present study, a puncture method has been used to record sap availability in six north European tree species during the spring. Attempts were made to withdraw sap from the trees at points 0.5 m, 1.3 m and 3.0 m above ground with “phloem needles” (constructed according to Hammel 1968), and a syringe. Sap could be withdrawn for the longest period for the sycamore (71 days). Birch (47 days) gave an intermediate period, beech (12 days), oak (12 days) and alder (13 days) gave short periods, and sap could not be withdrawn from ash. Effects of two environmental factors, temperature and light intensity on sap availability were examined. Temperature dependence of sap availability, already observed in the genus Acer, was confirmed in sycamore maple (Acer pseudoplatanus). There also seemed to be a correlation between sap availability and light intensity. Analysis of sugar in the tree sap revealed that sucrose was the only sugar in the sap of sycamore but hexoses (glucose and fructose) were preponderant in the sap of the other trees. The pH of sycamore sap (pH 6.9–5.4) fell as the season advanced. However, the pH-values of sap from the other trees did not vary significantly.     

Direct and indirect measurements of Phloem turgor pressure in white ash

Direct determinations and indirect calculations of phloem turgor pressure were compared in white ash (Fraxinus americana L.). Direct measurements of trunk phloem turgor were made using a modified Hammel-type phloem needle connected to a pressure transducer. Turgor at the site of the direct measurements was calculated from the osmotic potential of the phloem sap and from the water potential of the xylem. It was assumed that the water potentials of the phloem and xylem were close to equilibrium at any one trunk location, at least under certain conditions. The water potential of the xylem was determined from the osmotic potential of xylem sap and from the xylem tension of previously bagged leaves, measured with a pressure chamber. The xylem tension of bagged leaves on a branch adjacent to the site of the direct measurements was considered equivalent to the xylem tension of the trunk at that point. While both the direct and indirect measurements of phloem turgor showed clear diurnal changes, the directly measured pressures were consistently lower than the calculated values. It is not clear at present whether the discrepancy between the two values lies primarily in the calculated or in the measured pressures, and thus, the results from both methods as described here must be regarded as estimates of true phloem turgor.     

Phloem loading in peach: Symplastic or apoplastic?

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ³H-inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of ¹⁴C-labelled source leaves with 2 mMp-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.     

Is coordination of leaf and root growth mediated by abscisic acid? Opinion

Leaf growth is more inhibited than root growth when the soil is nitrogen-deficient, dry, saline, compacted, or of restricted volume. Similar differential responses in leaf and root growth occur when ABA is applied to plants in well-watered and well-fertilised conditions, and opposite responses are often found in ABA-deficient mutants. ABA levels increase in plants in dry or saline soils, suggesting a regulating role in leaf and root growth in soils of low water potential. In nitrogen-deficient or compacted soils, or soils of restricted volume, ABA only sometimes increases, and in these situations its accumulation may be of secondary importance. Use of ABA-deficient mutants has so far indicated that ABA influences leaf and root growth in unstressed plants, and plants in dry soils, but not in soils that are compacted, of restricted volume, or are nitrogen-deficient.For ABA to determine the relationship between the rate of leaf growth and the rate of root growth, there must be long-distance transport of either ABA itself or a compound that controls ABA synthesis in the growing cells of leaves and roots. ABA invariably increases in xylem sap as the soil becomes dry or saline, and sometimes when it becomes nitrogen-deficient or compacted, however the ABA is of too low a concentration to affect leaf growth. There may be a compound in xylem sap that controls the synthesis of ABA in the leaf, but no such compound has been identified. ABA accumulates in phloem sap of plants in dry or saline soil, but its function in controlling root or leaf growth is unknown.We conclude that ABA affects the ratio of root growth to leaf growth via its independent effects on root and leaf growth, and may regulate the ratio of root to leaf growth via feedforward signals in xylem or phloem, but there is no satisfactory explanation of its mechanism of control.     

A phloem-sap feeder mixes phloem and xylem sap to regulate osmotic potential

Phloem-sap feeders (Hemiptera) occasionally consume the dilute sap of xylem, a behaviour that has previously been associated with replenishing water balance following dehydration. However, a recent study reported that non-dehydrated aphids ingested xylem sap. Here, we tested the hypothesis that the consumption of xylem sap, which has a low osmolality, is a general response to osmotic stresses other than dehydration. Alate aphids were subjected to different treatments and subsequently transferred onto a plant, where electrical penetration graph (EPG) was used to estimate durations of passive phloem sap consumption and active sucking of xylem sap. The proportion of time aphids fed on xylem sap (i.e., time spent feeding on xylem sap/total time spent feeding on phloem plus xylem sap) was used as a proxy of the solute concentration of the uptake. The proportion of time alate aphids fed on xylem sap increased: (1) with the time spent imbibing an artificial diet containing a solution of sucrose, which is highly concentrated in phloem sap and is mainly responsible for the high osmotic potential of phloem sap; (2) with the osmotic potential of the artificial diet, when osmotic potential excess was not related to sucrose concentration; and (3) when aphids were deprived of primary symbionts, a condition previously shown to lead to a higher haemolymph osmotic potential. All our results converge to support the hypothesis that xylem sap consumption contributes to the regulation of the osmotic potential in phloem-sap feeders.