Interaction of xylem and phloem during exudation and wound occlusion in Cucurbita maxima

Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.     

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.     

Isoenzymes of acid phosphatase in germinating peas

Summary Phloem exudation from Ricinus has been examined in plants subjected to changes in water balance induced by a number of means. The results have provided a clear demonstration that the phloem system can operate osmotically. When the availability of water in the xylem is reduced by withholding water, the rate of exudation decreases sharply and this is accompanied by a rise in the sap concentration. On removing the water stress, the rate increases rapidly with a corresponding fall in sap concentration.Small variations in water availability do not give significant results and may be buffered by responses from the plant itself. This could also explain the insignificant changes in sap composition during exudation previously reported, where exudation rate, which should bear some relation to sieve tube turgor pressure, seems independent of sap concentration. Fluctuations in exudation rate are large in comparison with the changes in sap concentration when severe water stresses are applied. This result, coupled with the observation that exudation will occur from plants under considerable water stress suggests the operation of a sugar pump capable of maintaining a high turgor pressure at the source against a considerable water potential gradient. The main pump is probably located in the leaves.Thus interpreted, the results seem to accord with the Münch pressure flow hypothesis in all significant aspects.     

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.     

Analysis of the Exudation Process in Tomato Plants

We started from the supposition that the exudation is broughtabout by two processes: (1) an active transport of salt to thexylem, the ‘salt secretion’, a process that dependson metabolism and is sensitive to inhibitors, and (2) a watertransport effected by the osmotic suction of the sap in thexylem. It was demonstrated that by enhancing the concentrationof the medium the exudation rate rapidly decreases. This isfollowed by a slower rise to a new level which is the consequenceof the salt secretion that continues and enhances the osmoticvalue of the exudation sap. After some time secondary changesof the exudation rate appear. In lowering the concentrationof the medium identical changes occur, but now in the reversedirection. By tracing the influence of addition of substancesdifferent in mol. weight and lipoid solubility it could be establishedthat the gradual change was the result of salt secretion andnot of permeation of the added substance. From this simple suppositionabout the causes of the exudation process we derived formulaefor calculating the salt secretion, the osmotic value of theexudation sap, and the conductivity for water of the root system.Though most observations are in harmony with this concept, thereis a difficulty that too low a value is mostly found for theosmotic value of the exudation sap. The possible causes of thisdeviation are discussed. There is no necessity to accept anactive water secretion as well as active salt secretion sincesalt and water transport cannot be separated. Important factors in exudation are water conductivity of theprotoplasm and salt secretion into the xylem. Both factors areinfluenced by the osmotic value of the outer solution (D.P.D.of the medium) and by the presence of ions in the outer solution.The two factors must have an identical influence on the transportof water into the transpiration stream.     

Sucrose transport into the phloem of Ricinus communis L. seedlings as measured by the analysis of sieve-tube sap

Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2–3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes. The concentration which was achieved by loading and the time course could also be documented. This study concentrated on the loading of sucrose because it is the major naturally translocated sieve-tube compound. The sucrose concentration of sieve-tube sap was approx. 300 mM when the cotyledons were buried in the endosperm. When the cotyledons were excised from the endosperm and incubated in buffer, the sucrose concentration decreased gradually to 80–100 mM. This sucrose level was maintained for several hours by starch breakdown. Incubation of the excised cotyledons in sucrose caused the sucrose concentration in the sieve tubes to rise from 80 to 400 mM, depending on the sucrose concentration in the medium. Thus the sucrose concentration in the sieve tubes could be manipulated over a wide range. The transfer of labelled sucrose to the sieve-tube sap took 10 min; full isotope equilibration was finally reached after 2 h. An increase of K⁺ in the medium or in the sieve tubes did not change the sucrose concentration in the sievetube sap. Similarly the experimentally induced change of sucrose concentration in the sieve tubes did not affect the K⁺ concentration in the exudate. High concentrations of K⁺, however, strongly reduced the flow rate of exudation. Similar results were obtained with Na⁺ (data not shown). The minimum translocation speed in the sieve tubes in vivo was calculated from the growth increment of the seedling to be 1.03 m·h^-1, a value, which on average was also obtained for the exudation system with the endosperm attached. This comparison of the in-vivo rate of phloem transport and the exudation rate from cut hypocotyls indicates that sink control of phloem transport in the seedlings of that particular age was small, if there was any at all, and that the results from the experimental exudation system were probably not falsified by removal of the sink tissues.Abbreviations PTS 3-hydroxy-5,8, 10-pyrenetrisulfonate     

A combinatory approach for analysis of protein sets in barley sieve-tube samples using EDTA-facilitated exudation and aphid stylectomy

This study investigated advantages and drawbacks of two sieve-tube sap sampling methods for comparison of phloem proteins in powdery mildew-infested vs. non-infested Hordeum vulgare plants. In one approach, sieve tube sap was collected by stylectomy. Aphid stylets were cut and immediately covered with silicon oil to prevent any contamination or modification of exudates. In this way, a maximum of 1muL pure phloem sap could be obtained per hour. Interestingly, after pathogen infection exudation from microcauterized stylets was reduced to less than 40% of control plants, suggesting that powdery mildew induced sieve tube-occlusion mechanisms. In contrast to the laborious stylectomy, facilitated exudation using EDTA to prevent calcium-mediated callose formation is quick and easy with a large volume yield. After two-dimensional (2D) electrophoresis, a digital overlay of the protein sets extracted from EDTA solutions and stylet exudates showed that some major spots were the same with both sampling techniques. However, EDTA exudates also contained large amounts of contaminative proteins of unknown origin. A combinatory approach may be most favourable for studies in which the protein composition of phloem sap is compared between control and pathogen-infected plants. Facilitated exudation may be applied for subtractive identification of differentially expressed proteins by 2D/mass spectrometry, which requires large amounts of protein. A reference gel loaded with pure phloem sap from stylectomy may be useful for confirmation of phloem origin of candidate spots by digital overlay. The method provides a novel opportunity to study differential expression of phloem proteins in monocotyledonous plant species.     

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.     

Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms

Sap flow rates and sap pressure changes were measured in dormant sugar maple trees (Acer saccharum Marsh.). In the forest, sap flow rates and pressure changes were measured from tap holes drilled into tree trunks in mature trees and sap flow rates were measured from the base of excised branches. Excised branches were also brought into the laboratory where air temperature could be carefully controlled in a refrigerated box and sap flow rates and sap pressures were measured from the cut base of the branches.

Under both forest and laboratory conditions, sap uptake occurred as the wood temperature declined but much more rapid sap uptake correlated with the onset of the freezing exotherm. When sap pressures were measured under conditions of negligible volume displacement, the sap pressure rapidly fell to −60 to −80 kilopascals at the start of the freezing exotherm. The volume of water uptake and the rate of uptake depended on the rate of freezing. A slow freezing rate correlated with a large volume of water uptake, a fast freezing rate induced a smaller volume of water uptake. The volume of water uptake ranged from 0.02 to 0.055 grams water per gram dry weight of sapwood. The volume of water exuded after thawing was usually less than the volume of uptake so that after several freezing and thawing cycles the sapwood water content increased from 0.7 to 0.8 grams water per gram dry weight.

These results are discussed in terms of a physical model of the mechanism of maple sap uptake and exudation first proposed by P. E. R. O'Malley. The proposed mechanism of sap uptake is by vapor distillation in air filled wood fiber lumina during the freezing of minor branches. Gravity and pressurized air bubbles (compressed during freezing) cause sap flow from the canopy down the tree after the thaw.

     

Specific proteins in the sieve-tube exudate of Ricinus commuais L. seedlings: separation,characterization and in-vivo labelling

Ricinus communis L. seedlings exuded pure phloem sap from the cut hypocotyl for several hours. Throughout the entire exudation period proteins were present in the phloem exudate at a constant concentration ranging from 0.11 to 0.41 mg·ml^–1 depending on the culture conditions and the age of the seedlings. Manipulation of the nutrient supply at the cotyledons after removal of the endosperm did not change the protein concentration in the exudate. Comparison of sieve-tube exudate proteins (STEPs) with soluble proteins extracted from the hypocotyl and the cotyledons showed a unique abundance of small proteins in the exudate, with molecular weights ranging from 10 to 25 kDa. Bands at 18, 19 and 20 kDa were especially dominant. The proteins found transiently in the xylem exudate, which might represent proteins secreted at the wound surface, were different in pattern. Two-dimensional separation of STEPs revealed that more than 100 distinct polypeptides occurred in the sieve-tube exudate, most of them slightly acidic with isoelectric points ranging from 4 to 6 and a few basic ones around 8. [³⁵S]Methionine fed to the cotyledons led to labelling of STEPs, demonstrating their rapid synthesis. It is concluded that there is a continuous synthesis and translocation of specific sieve-tube proteins, whose function is unknown.Abbreviations IEF isoelectric focussing - pI isoelectric point - STEP sieve-tube exudate protein - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - TCA trichloroacetic acid We wish to thank Pia Großmann and Libuse Badewitz for technical help.     

Life history of the newly discovered Japanese tree sap mite, Hericia sanukiensis (Acari, Astigmata, Algophagidae)

Hericia sanukiensis (Acari, Algophagidae) is a species of tree sap mite which has been newly discovered in the sap flux of oak trees (Quercus acutissima). In this study, we have clarified the life history of H. sanukiensis at the onset of sap exudation in spring, overwintering deutonymphs molt to tritonymphs as their molting is exacerbated by the sugar contained in the tree sap, and develop to adulthood and reproduce. The mite colony develops as long as the sap continues to exude during the spring and summer. In this period, most of the protonymphs develop into tritonymphs directly bypassing the deutonymphal stage completely. Facultative deutonymphs which arise in the summer season are phoretic morphs. They attach to coleopteran insects, especially of the family Nitidulidae, as carriers by which to disperse. In autumn (when sap exudation terminates), most of the protonymphs molt to non-phoretic deutonymphs. In winter (without sap exudation), the mite colony is composed only of non-phoretic deutonymphs.     

Nitrate,ammonium and amino acids in the bleeding sap of tomato plants in relation to form and concentration of nitrogen in the medium

Summary When nitrate was raised from 6 to 12 meq/l of nutrient solution there was little change in the sap concentration, but the translocation rate of nitrate (exudation rate × concentration) increased considerably. Sap concentration and transport of glutamine, NH₄-ions, leucine and transport of asparagine also increased. Replacement of nitrate by ammonium immediately enhanced sap concentration and transport especially of glutamine. Initially sap concentration and transport of ammonium increased in a similar way, but a few hours later they decreased to that of plants with high nitrate. re]19750318Institute für Pflanzenernährung der T.U. Hannover     

Enhancement of Phloem Exudation from Fraxinus uhdei Wenz. (Evergreen Ash) using Ethylenediaminetetraacetic Acid

Ethylenediaminetetraacetic acid (EDTA) enhanced the exudation of ¹⁴C-labeled assimilates from excised leaflets and whole plant specimens of Fraxinus uhdei Wenz. A 2 millimolar EDTA concentration was found to be most effective in promoting exudation from excised leaflets, while 10 millimolar EDTA was most effective in whole plants experiments. Exudation rate reached a maximum after 24 hours in both experiments. The continuous presence of EDTA throughout the treatment period was required for maximum exudation from excised leaflets. Stachyose, raffinose, verbascose, and sucrose were the principal compounds found to occur in exudate samples. These compounds are typically transported in sieve elements of various Fraxinus species suggesting the exudate was of phloem origin. Electron microscope studies of petiolule sieve plate pores from excised leaflets showed substantially less callose appearing after treatment with EDTA than after H₂O treatment. It is suggested that EDTA enhances phloem exudation by inhibiting or reducing callose formation in sieve plate pores. The exudation enhancement technique described for whole plant specimens is suggested as a useful means of collecting phloem sap and studying translocation in woody plants.     

Measurement of sap flow in plant stems

Transpiration rates for whole plants, individual branches ortillers can be determined by techniques which measure the rateat which sap ascends stems. All of these methods use heat asa tracer for sap movement, but they are fundamentally differentin their operating principles. Two methods commonly employed,the stem heat balance and trunk sector heat balance methods,use the heat balance principle; the stem is heated electricallyand the heat balance is solved for the amount of heat takenup by the moving sap stream, which is then used to calculatethe mass flow of sap in the stem. In the heat-pulse method,rather than using continuous heating, short pulses of heat areapplied and the mass flow of sap is determined from the velocityof the heat pulses moving along the stem. In addition, ratesof sap flow can be determined empirically, using the thermaldissipation technique, from the temperature of sapwood neara continuously-powered heater implanted in the stem. Users mustunderstand the theory underlying each of these methods, so thatthey can select the method most appropriate to their applicationand take precautions against potential sources of error. Whenattempting to estimate transpiration by stands of vegetationfrom measurements of sap flow in individual plants, users mustalso select an appropriate sampling strategy and scaling method. Key words: Sap flow, transpiration, stem heat balance, heat pulse velocity, review     

The influence of temperature on the exudation of xylem sap from detached root systems of rye (Secale cereale) and barley (Hordeum vulgare)

Summary Roots of intact plants of rye and barley which had been growing at 20° were cooled for 12–72 h at 8–14° C while the shoots were kept at 20°. The roots were then excised and placed in solutions at temperatures ranging from 2.5–22.5° C. The rate of exudation of xylem sap and the chemical composition and osmotic potential of the sap were measured and compared with controls which had been kept at 20° C during the pretreatment period. Pre-cooling increased the fluxes of K⁺, Ca²⁺ and H₂PO ₄ ^- into the xylem sap of both species by factors of two to three; the total volume of exudate rose by larger factors. Thus the concentrations of these ions were lower in the sap exuding from cooled roots than in that from controls. Measurements of the osmotic potential of the sap from barley roots indicated that the osmotic driving force in cooled and control roots was similar even though flow in the former was much greater.The enhancement of exudation was shown to be dependent on the duration and the temperature experienced by the roots during pretreatment, and was lost rapidly when roots of intact plants were returned to 20°.Analysis of the temperature coefficients for exudation and Arrhenius plots revealed very distinct changes in the activation energy for exudation above and below a transition temperature. In control plants of barley and rye this temperature was around 10° C, but in cooled roots of rye there was a significant shift in the transition temperature to 5° C. Activation energies for exudation of control and cooled roots above or below the transition temperature were broadly similar, thus pre-cooling roots did not alter the temperature sensitivity of exudation but merely its rate at a given temperature.The results are discussed in relation to active ion transport, membrane fluidity and the resistance of the root to water flow.Abbreviation ABA abscisic acid     

Concentration and Delivery of Abscisic Acid in Xylem Sap are Greater at the Shoot Base than at a Target Leaf Nearer to the Shoot Apex

Abstract: Samples of xylem sap from 5-week-old Ricinus corn-munis L. were obtained after severing a lamina, or shoot, from plants pressurized at the roots with air to raise hydrostatic xylem water potentials to atmospheric. In situ sap flow gauges, and mass flow measurements, showed that removing the lamina approximately doubted sap flow rate through the petiole stub that remained attached to the plant. This was a consequence of flow out of the roots being diverted along this low-resistance pathway and away from leaves higher in the canopy. Leaf and whole shoot excision temporarily released extra solutes in to sap as it discharged from the cut petiole or from the hypo-cotyl stump. This contamination prevented the use of sap extracted from detached lamina by overpressurizing in a Scholan-der bomb. To minimise distortions to sap flow and wound-induced contamination, estimates of in planta concentration and delivery (concentration × sap flow rate) of ABA and osmolality in xylem sap were made using sap flow rates measured before excision and concentrations in flowing sap collected approximately 30 mm after excision. At this time, effects of excision on solute contamination had subsided. The approach revealed that withholding water from upper roots increased ABA delivery from roots into the shoot base 3-fold. However, approximately half this ABA was lost en route to the youngest fully open leaf. This loss of ABA may explain the slow stomatal response to drying of upper roots shown by R. communis .     

The effect of certain pre-sowing seed treatments and early phosphorus supplement on cell sap concentration and water fractions in leaves of maize (Zea mays L.) plants grown under soil moisture stress conditions

The exposure of maize plants to drought led in most cases to a decrease in both the total and free water percentages, but to an increase in cell sap concentration, bound water percentage and bound water/free water ratio, comparing with the case of normal water supply. The employment of any of the studied treatments at either the normal or low soil moisture levels led throughout the period of time preceding the milky stage to a decrease in the free water percentage in maize leaves, but to the reverse effect with respect to both the bound water percentage and the bound water/free water ratio. The determinations of water fractions in leaves (free, bound water % and the ratio between them) before or at the tasseling stage appeared to be more reliable, compared with osmotic pressure determinations, as indication for the effective use of a given treatment in increasing the drought resistance of maize plants.     

液流径向变化对尾巨桉单株日蒸腾量估算的影响

采用Granier热扩散方法(TDP)测定了尾巨桉2个径向深度(0～2和2～4 cm)的液流速率,探讨液流径向变化对树木蒸腾估算产生的影响、不同个体大小的估算误差以及估算误差与光合有效辐射(PAR)和水汽压亏缺(VPD)的关系.结果表明: 尾巨桉的日蒸腾量在单点径向深度0～2 cm条件下估算值最大,两点综合估算值次之,单点径向深度2～4 cm最小,单点径向0～2 cm和径向2～4 cm所得的日蒸腾量分别较两点综合估算值高估了32.9%和低估了58.7%;用单点径向0～2 cm估算蒸腾量时所产生的误差随胸径的递减而减少,胸径为17.7、12.9和9.8 cm的树木产生的估算误差分别为51.7%、33.0%、18.0%,而单点径向2～4 cm所产生的估算误差则没有明显的径级差异;PAR和VPD都与单点径向0～2 cm估算误差有较好的正相关性,相比较VPD而言,PAR与单点径向0～2 cm估算误差的相关性更好.相比传统的仅用单一深度液流速率估算单株蒸腾耗水量的方法,两点估算方法一定程度上提高了日蒸腾量计算的精度,为尾巨桉人工林森林耗水量估算提供了可靠依据.     

Effect of Potassium Supply on the Acropetal Transport of Water, Inorganic Ions and Amino Acids in Young Decapitated Sunflower Plants (Helianthus annuus)

The effect of potassium supply on the exudation rate and the content of ions and amino acids in exudation sap was studied using young decapitated sunflower plants (Helianthus annuus L.). Plants were grown originally in a complete nutrient solution. After decapitation one set of plants was transferred to a nutrient solution without potassium and another set to a solution with potassium, all other nutrients remaining the same for both treatments. During an experimental period of 3 days the plants supplied with potassium showed exudation rates which were more than twice as high as plants in the nutrient medium without potassium. The potassium supply affected the potassium and nitrate contents of the exudation sap. Both were lower in the treatment without potassium. The calcium and magnesium contents of the exudation sap were not significantly influenced by the potassium treatment, whereas the phosphate content was higher in the treatment without potassium. At the beginning of the exudation period the potassium supply did not affect the content of amino acids in the exudate, but later the plants supplied with potassium showed higher contents of amino acids in the exudation sap. As the absence of potassium reduced the exudation rate considerably the acropetal transport of all exudation sap constituents analysed in this experiment was markedly reduced.     

Phloem sap exudation in Ricinus communis: elastic responses and anatomical implications

Abstract. Ricinus communis plants have an unusually high capacity to exude considerable quantities of phloem sap from bark incisions. We have used Ricinus as an experimental system to study different aspects of sap exudation. Dimensional changes in the bark, monitored by a displacement transducer, showed that pressure release in the sieve tubes was accompanied by elastic shrinkage. The rate of exudation was also controlled by the degree of pressurization and elastic properties of the sieve tubes. A displacement transducer was used to measure the elastic modulus (ɛ) of phloem samples by immersing them in a range of different osmotica. The cells had a low elastic modulus (ɛ= 1.62 ± 0.41 MPa at full turgor). ɛ of phloem tissue in massage pretreated bark, from which exudation was enhanced, was not significantly different from that of unmassaged bark in contrast with the suggestion of Lee (1981). However, anatomical studies showed that massage pretreatment has a stimulating influence on the cambial cell division, which increased the phloem tissue cross-section up to 160%. The newly-formed sieve tubes were 'spliced' into existing ones in the unmassaged zone to re-establish vascular continuity. Plants with a greater capacity to exude phloem sap from a given stem location had a greater cross-sectional area of sieve tubes in the vicinity.
The significance of observations with respect to other sap exudation phenomenon is discussed. The importance of the present work in understanding the technique of palm tapping, on which the palm sugar industry depends, is also considered.