Effect of abscisic acid on exudation from deficient and aged sunflower roots

The effect of abscisic acid (ABA) was observed on exudation from roots of sunflower ( Helianthus annuus L. cv. Habad) plants whose mineral nutrition was cut off or which were deprived of K⁺ or NO⁻₃ for 90 h prior to excision. In spite of a marked decrease in exudation rate, the magnitude of the promotive effect of ABA on both volume flow and release of ions to the xylem was similar to that obtained in roots of plants grown in full nutrient solution. Application of ABA to the medium at different times after excision increased the promotive effect of ABA as the time from excision increased. The magnitude of the ratio ABA-treated/control in roots which were treated 74 h after excision was twice that in freshly-excised roots. The effect of ABA lasted up to 50 h and during that period it followed the endogenous rhythm in exudation from the control roots. It is concluded that since a steady promotive effect of ABA persists under a variety of experimental conditions, this may be considered a genera! phenomenon in sunflower roots.     

Effect of Potassium, and Abscisic and Indole-3-Acetic Acids, on Maize Root Xylem Exudation and Potassium Efflux

The influence of potassium sulfate, abscisic acid (ABA) and indole-3-acetic acid (IAA) solutions on xylem exudation rate and potassium efflux from the apical cut end of root tips of intact maize (Zea mays L. cv. Dnepropetrovskaya) seedlings was studied. Foliar application of 5 mM K₂SO₄ considerably stimulated the exudation rate. The application of ABA and IAA (1 mM) also induced a high rate of xylem exudation, K⁺ efflux being simultaneously increased. This revised version was published online in July 2006 with corrections to the Cover Date.     

Der Einfluss verschiedener Salze und verschiedener Inhibitoren auf den Wurzeldruck von Zea mays

The effect of different salts and inhibitors on the root pressure of Zea mays.— The influence of various salt solutions and inhibitors on the exudation rate has been investigated with young excised primary roots of Zea mays. The following results were obtained. — The effect of chlorides on the exudation rate was higher than the effect of sulphates K⁺ and Na⁺ effected higher flux rates than Ca²⁺ and Mg²⁺ The highest exudation rate was obtained with KCl. — In comparison to an isotonic Lutrol-solution (a liquid condensed polyethylenoxid) a 0.5525 molar KCl-solution, applicated on the root stump, increased the exudation rate considerably. — Metabolic inhibitors and anaerobic conditions decreased the exudation rate. — Experiments, concerning the influence of metabolic inhibitors on the exudation and on the Rb-uptake showed a highly significant positive correlation (r =+0.72***) between the exudation rates and the Rb-concentrations in the exudates. The Rb-accumulation in the root tissue was not correlated to the exudation rate. — The experimental data agree with the concept of a transversal water transport in the root tissue, effected by osmotic forces. The root pressure is based on the osmotic gradient between the xylem sap and the outer solution. This gradient is built up by the metabolic secretion of ions into the xylem sap. It is supposed that the transversal water transport in the roots mainly goes through the free space of the cortex.     

The effect of plant hormones on phosphate uptake and translocation in maize roots

In short-term (1 h) uptake experiments GA₃(10^-5M) stimulated P_i uptake into maize root cortex cells by 28.7 %, Ethrel (10^-3M) inhibited it by 18.5 % and BA, IAA, and ABA were inactive. In long-term (5 h) experiments ABA remained inactive, GA₃ lost its stimulatory effect, and BA (5. 10^-6M), IAA (10^-4 -10^-5M), and Ethrel (10^-3 -5. 10^-4M) decreased P_i uptake. When the hormones were present only during 3 h preincubation (“augmentation”) period ABA was inactive, GA₃ slightly raised and BA, IAA, and Ethrel slowed down subsequent P_i uptake. BA(10^-7 –10^-5M) decreased xylem sap volume flow and P_i translocation. ABA in all tested concentrations (10^-8 –10^-5M) reduced exudation rate and P_i translocation, its effect declining with time. IAA effect strongly depended on concentration used and on application time and varied from strong inhibition to moderate stimulation of both volume flow and P_i translocation. GA₃ (10^-7M) slightly stimulated xylem volume flow but inhibited phosphate translocation. Ethrel (10^-4 and 10^-5M) increased both parameters, but P_i transloeation much more than volume flow. IAA, BA, and ABA influenced volume flow and P transloeation to the same extent leaving P_i concentration in the xylem sap unchanged. GA₃ and Ethrel influence P_i concentration in the xylem sap and it is thus probable that these hormones regulate release of phosphate ions into the xylem sap.     

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     

A new concept of root exudation1

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch-activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m^?3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.     

Movement of Abscisic Acid into the Apoplast in Response to Water Stress in Xanthium strumarium L

The effect of water stress on the redistribution of abcisic acid (ABA) in mature leaves of Xanthium strumarium L. was investigated using a pressure dehydration technique. In both turgid and stressed leaves, the ABA in the xylem exudate, the `apoplastic' ABA, increased before `bulk leaf' stress-induced ABA accumulation began. In the initially turgid leaves, the ABA level remained constant in both the apoplast and the leaf as a whole until wilting symptoms appeared. Following turgor loss, sufficient quantities of ABA moved into the apoplast to stimulate stomatal closure. Thus, the initial increase of apoplastic ABA may be relevant to the rapid stomatal closure seen in stressed leaves before their bulk leaf ABA levels rise.

Following recovery from water stress, elevated levels of ABA remained in the apoplast after the bulk leaf contents had returned to their prestress values. This apoplastic ABA may retard stomatal reopening during the initial recovery period.

     

Effects of synthetic plant growth retardants and abscisic acid on root functions of Brassica rapa plants exposed to low root-zone temperature

Possible interactions of two synthetic plant-growth retardants during the short-term response of Brassica rapa L. ssp. oleifera (DC.) Metzger plants to low root-zone temperature were investigated by pretreating with mefluidide or paclobutrazol. Water and solute transfers were studied by measuring xylem sap volume flow (under root pressure exudation) and ion flow from the roots. Relations with nitrate uptake rate were also considered. Root pretreatment with paclobutrazol strongly restricted the cold-inducible processes which normally restore water and solute flow from the root xylem. Paclobutrazol decreased the rates of nitrate uptake and exudation flow from the root xylem (principally by reducing root hydraulic conductivity) with dramatic consequences for ion flow, especially that of nitrate.
The effects of root ABA pretreatment on plant response to root cooling were then studied separately or in association with a pretreatment with paclobutrazol. Despite a slight decrease in nitrate uptake rate, ABA pretreatment of the roots enabled the plant to develop rapid mechanisms for adaptation to cold constraint at the root level. Moreover, this action of exogenous ABA greatly reduced the effect of a simultaneous paclobutrazol pretreatment and partly restored water and solute flows.
Thus, the improvement of plant resistance to cold conditions brought about by treatments with mefluidide and paclobutrazol (previously shown in long-term experiments) cannot simply be explained by their short-term effects.     

Water transport in sunflower root systems: effects of ABA, Ca2+ status and HgCl2

Excised 20-d-old sunflower roots (Helianthus annuus L. cv. Sun-Gro 380) with different Ca²⁺ status were used to study the effects of root Ca²⁺ status and abscisic acid (ABA) on the exudation rate (Jv), the hydraulic conductivity of the root (Lpr), the flux of exuded Ca²⁺ (JCa, and the gradient of osmotic pressure between the xylem and the external medium. Jv and Lpr increased in direct proportion to the Ca²⁺ status of the root. Addition of ABA (4 M) at the onset of exudation in the external medium made Jv and Lpr rise, and this effect also increased with the Ca²⁺ status. The effects of HgCl2 and its interaction with ABA on water transport in the root were also studied. Addition of HgCl2 (1 M) 2 h after the onset of exudation in the external medium quickly inhibited Jv, independently of the presence of ABA in the root medium. The results recorded here point to the involvement of ABA and Ca²⁺ in the regulation of root water flow, as well as the existence of aquaporins in the cell membranes of sunflower roots.     

Studies in Sieve-tube Exudation through Aphid Mouth-parts II. The Effects of Pressure Gradients in the Wood and Metabolic Inhibitors

Exudation was studied from the mouth-parts of the willow aphidTuberolachnus salignus (Gmelin) inserted in the phloem of isolated15 -cm. stem segments of Salix viminalis. By supplying waterunder pressure to the cut surface of the wood at one end ofa segment and allowing it to run freely from the other, a pressuregradient could be set up in the xylem. By closing the free end,the pressure in the xylem could be raised uniformly. This lasttreatment caused the rate of exudation to increase. This effectof pressure could be balanced by an equivalent osmoticum introducedinto the xylem. The increase was therefore due to the DPD inthe xylem rather than any intrinsic effect of pressure. The effect of a pressure gradient (4 atm. applied) dependedon the position of the mouth-parts in relation to the gradient.If they were near the low-pressure end, exudation from themincreased. If they were near the high-pressure end, exudationdecreased. In neither case was the total carbohydrate or totalnitrogen concentration of the exudate changed. When longer segments were used (e.g. 41 cm.) the effect of asimilar pressure (4 atm.) was greater than with 15-cm. segmentsalthough the point of application of the pressure was more remotefrom the mouth-parts. When the wood at the midpoint of a segmentwas severed leaving a narrow bridge of bark joining the twohalves, raising the pressure in the xylem of one half led toan increased exudation from mouth-parts situated in the otherhalf. Thus the effect of pressure was transmitted along thephloem since there was no rise in xylem pressure in the halfsegment bearing the mouth-parts. 10^-4 M. dinitrophenol applied to a bark abrasion a few centimetresaway from mouth-parts, reduced exudation. Applied to the cambialsurface, exudation was reduced by 0.1 per cent, phloridzin andstopped completely by 10^-4 M. dinitrophenol and 10^-6 M. potassiumcyanide. Inhibitors had no effect on the concentration of theexudate. No simple interpretation of the effects of pressure gradientsis possible, but it is suggested they give qualified supportto the Munch hypothesis.     

Re-examining the role of ABA as the primary long-distance signal produced by water-stressed roots

The role of ABA as the primary long-distance signal produced by water-stressed roots and transported to stomata continues to be challenged. We have recently reported that expression of ABA biosynthetic genes in roots only increases in the later stage of water stress. Our results support the hypothesis that in early water stress, increased levels of ABA in xylem sap are due to leaf biosynthesis and translocation to roots and from there to xylem. If so, other xylem-borne chemicals may be the primary stress signal(s) inducing ABA biosynthesis in leaves. We found that apart from ABA, sulfate was the only xylem-borne chemical that consistently showed higher concentrations from early to later water stress. We also found increased expression of a sulfate transporter gene in roots from early water stress onwards. Moreover, using bioassays we found an interactive effect of ABA and sulfate in decreasing maize transpiration rate, as compared to ABA alone. While ABA is undoubtedly the key mediator of water stress responses such as stomatal closure, it may not be the primary signal produced by roots perceiving water stress.Key words: abscisic acid, ABA biosynthesis, corn, drought, maize, malate, pH, stomatal conductance, sulfate, Zea mays     

Does Xylem Sap ABA Control the Stomatal Behaviour of Water-Stressed Sycamore (Acer pseudoplatanus L.) Seedlings?

Sycamore seedlings were grown with their root systems dividedequally between two containers. Water was withheld from onecontainer while the other container was kept well-watered. Effectsof soil drying on stomatal behaviour, shoot water status, andabscisic acid (ABA) concentration in roots, xylem sap and leaveswere evaluated. At 3 d, root ABA in the drying container increased significantly,while the root ABA in the unstressed container of the same plantsdid not differ from that of the control. The increase in rootABA was associated with the increase in xylem sap ABA and withthe decrease in stomatal conductance without any significantperturbation in shoot water status. At 7 d, despite the continuous increase in root ABA concentration,xylem sap ABA showed a marked decline when soil water contentwas depleted below 013 g g^–1. This reduction in xylemsap ABA coincided with a partial recovery of stomatal conductance.The results indicate that xylem sap ABA is a function of rootABA as well as the flow rate of water from roots to shoots,and that this ABA can be a sensitive indicator to the shootof the effect of soil drying. Key words: Acer pseudoplatanus L., soil drying, stomatal behaviour, xylem sap ABA     

Stomatal conductance in relation to xylem sap abscisic acid concentrations in two tropical trees, Acacia confusa and Litsea glutinosa

Two tropical trees, Acacia confusa and Litsea glutinosa, were grown under controlled conditions with their roots subjected to soil drying and soil compaction treatments. In both species, a decline in stomatal conductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resulted in a substantial decrease in stomatal conductance but had little effect on leaf water potential. A rapid and substantial increase in xylem abscisic acid (ABA) concenation ([ABA]), rather than hulk leaf ABA, was closely related to soil drying and soil compaction. A significant relationship between stomatal conductance (g_s) and xylem [ABA] was observed in both species. Artificially feeding ABA solutions to excised leaves of both species showed that the relationship bet ween g_s and [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between g_s and xylem [ABA]. These results suggest that xylem ABA may act as a stress signal in the control of stomatal conductance.     

Diurnal change in the relationship between stomatal conductance and abscisic acid in the xylem sap of field-grown peach trees

To evaluate whether abscisic acid (ABA) in the xylem sap playsan important role in controlling stomatal aperture of field-grownPrunus persica trees under drought conditions, stomatal conductance(g) and xylem ABA concentrations were monitored both in irrigatedand non-irrigated trees, on two consecutive summer days (threetimes a day). Stomata1 conductance of non-irrigated trees hada morning maximum and declined afterwards. The changes in gduring the day, rather than resulting from variations in theconcentrations of ABA in the xylem sap or the delivery rateof this compound to the leaves, were associated with changesin the relationship between g and xylem ABA. The stomata ofwater-stressed trees opened during the first hours of the day,despite the occurrence of a high concentration of ABA in thexylem sap. However, stomatal responsiveness to ABA in the xylemwas enhanced throughout the day. As a result, a tight inverserelationship between g and the logarithm of xylem ABA concentrationwas found both at midday and in the afternoon. A similar relationshipbetween g and ABA was found when exogenous ABA was fed to leavesdetached from well-watered trees. These results indicate thatABA derived from the xylem may account for the differences ing observed between field-grown peach trees growing with differentsoil water availabilities. Several possible explanations forthe apparent low stomatal sensitivity to xylem ABA in the morning,are discussed, such as high leaf water potential, low temperatureand high cytokinin activity. Key words: Prunus persica L., stomata, xylem ABA, water deficits, root-to-shoot communication     

Root hypoxia reduces leaf growth : role of factors in the transpiration stream

This study examined the potential role of restricted phloem export, or import of substances from the roots in the leaf growth response to root hypoxia. In addition, the effects of root hypoxia on abscisic acid (ABA) and zeatin riboside (ZR) levels were measured and their effects on in vitro growth determined. Imposition of root hypoxia in the dark when transpirational water flux was minimal delayed the reduction in leaf growth until the following light period. Restriction of phloem transport by stem girdling did not eliminate the hypoxia-induced reduction in leaf growth. In vitro growth of leaf discs was inhibited in the presence of xylem sap collected from hypoxic roots, and also by millimolar ABA. Disc growth was promoted by sap from aerated roots and by 0.1 micromolar ZR. The flux of both ABA and ZR was reduced in xylem sap from hypoxic roots. Leaf ABA transiently increased twofold after 24 hours of hypoxia exposure but there were no changes in leaf cytokinin levels.     

The effect of drought hardening and chilling on ABA content in xylem sap and ABA - delivery rate from root of tomato plant

This paper is a continuation of our studies related to the response of two tomato cultivars: Robin and New Yorker to chilling: the later is more tolerant to chilling than the former one (Starck et al. 1994). The concentration of ABA in the xylem sap and ABA delivery rate (calculated as the amount of ABA exuded in 2h from the cut stump, following shoot removal) were estimated by ELISA. The relative water content (RWC) of the leaf blades and stomatal resistance (RS) were also measured. Tomato plants were grown in a greenhouse, under noncontrolled conditions. Before chilling some of the plants were drought hardened for 10 days (H). As an consequence of water deficit only New Yorker growth slightly decreased. Plants were chilled to 2–5 °C during three consecutive, 16-h nights, preceded by warm days, which caused a decrease in the RWC of leaf blades. Chilling did not decreased leaf blade hydration significantly, but drastically increased the concentration of ABA in the xylem sap in more chilling tolerant cv. New Yorker only. The delivery rate of ABA was markedly enhanced in both cultivars, but much more in New Yorker. Drought hardening increased ABA delivery rate in cv. Robin only, especially after chilling. The lack of correlation between changes in the RWC of leaf blades after low temperature treatment and the concentration of ABA in the xylem sap as well as its delivery rate suggest, that in both tomato cultivars chilling increased ABA level directly, not as an secondery effect of temperature-induced water deficit.     

Abscisic acid and water transport in sunflowers

The role of abscisic acid (ABA) in the transport of water and ions from the root to the shoot of sunflower plants (Helianthus annuus) was investigated by application of ABA either to the root medium or to the apical bud. The exudation at the hypocotyl stump of decapitated seedlings was measured with and without hydrostatic pressure (0–0.3 MPa) applied to the root. All ABA concentrations tested (10^-10–10^-4 mol·l^-1) promoted exudation. Maximal amounts of exudate (200% of control) were obtained with ABA at 10^-6·mol·l^-1 and an externally applied pressure of 0.1 MPa. The effect was rapid and long-lasting, and involved promotion of ion release to the xylem (during the first hours) as well as an increase in hydraulic conductivity. Abscisic acid applied to the apical bud had effects similar to those of the rootapplied hormone. Increased rates of exudation were also obtained after osmotic stress was applied to the root; this treatment increased the endogenous level of ABA in the root as well as in the shoot. Water potentials of the hypocotyls of intact plants increased when the roots were treated with ABA at 5°C, whereas stomatal resistances were lowered. The results are consistent with the view that ABA controls the water status of the plant not only by regulating stomatal transpiration, but also by regulating the hydraulic conductivity of the root.Abbreviations and symbols ABA abscisic acid - Tv volume flow - Lp hydraulic conductivity - PEG polyethyleneglycol - water potential - osmotic potential - osmotic value - P hydrostatic pressure     

Does ABA in the Xylem Control the Rate of Leaf Growth in Soil-Dried Maize and Sunflower Plants?

Maize (Zea mays L.) and sunflower (Helianthus annuus L.) plantswere grown in large volumes of soil and leaf growth rate wasmonitored on a daily basis. Half the plants were given a soildrying treatment and when they showed a significant restrictionof growth rate (compared to both their daily growth rate beforedrying and the average growth rate of well-watered plants onthe same day), leaf water relations were measured and xylemsap was extracted using several techniques. There was a significant negative log-linear relationship betweenthe rate of leaf growth and the concentration of ABA in thexylem for both species. There was no clear relationship betweenleaf growth rate and leaf water potential or turgor for eitherspecies. Assessment of different methods for sampling xylemsap suggests that exudates collected from stem stumps or samplescollected by pressurizing the whole root system are suitablefor estimating ABA concentration in xylem, at least with largeplants of maize or sunflower, provided the first few hundredcubic millimetres of collected sap are used for the assay. Centrifugationof sections of stems resulted in dilution of ABA in the xylemsap with sap squeezed from parenchyma tissue. This is because,at least in plants subjected to mild soil drying, the concentrationof the ABA in the xylem is far higher than that in the cellsap of stem tissue. Results support the proposal that ABA plays a major role asa chemical signal involved in the root-to-shoot communicationof the effects of soil drying. The non-hydraulic restrictionof leaf growth by a chemical signal can be explained by theextra root-sourced ABA in the xylem and may be an importantcomponent of the modification of growth and development whichresults from prolonged soil drought. Key words: Soil drying, ABA, leaf growth, Zea mays L., Helianthus annuus L.     

Stomatal control by both [ABA] in the xylem sap and leaf water status: a test of a model for draughted or ABA-fed field-grown maize

A model of maize stomatal behaviour has been developed, in which stomatal conductance is linked to the concentration of abscisic acid ([ABA]) in the xylem sap, with a sensitivity dependent upon the leaf water potential (Ψ₁). It was tested against two alternative hypotheses, namely that stomatal sensitivity to xylem [ABA] would be linked to the leaf-to-air vapour pressure difference (VPD), or to the flux of ABA into the leaf. Stomatal conductance (g_s) was studied: (1) in field-grown plants whose xylem [ABA] and Ψ₁ depended on soil water status and evaporative demand; (2) in field-grown plants fed with ABA solutions such that xylem [ABA] was artificially raised, thereby decreasing g_s and increasing Ψ₁ and leaf-to-air VPD; and (3) in ABA-fed detached leaves exposed to varying evaporative demands, but with a constant and high Ψ₁. The same relationships between g_s, xylem [ABA] and Ψ₁, showing lower stomatal sensitivity to [ABA] at high Ψ₁, applied whether variations in xylem [ABA] were due to natural increase or to feeding, and whether variations in Ψ₁, were due to changes in evaporative demand or to the increased Ψ₁ observed in ABA-fed plants. Conversely, neither the leaf-to-air VPD nor the ABA flux into the leaf accounted for the observed changes in stomatal sensitivity to xylem [ABA]. The model, using parameters calculated from previous field data and the detached-leaf data, was tested against the observations of both ABA-fed and droughted plants in the field. It accounted with reasonable accuracy for changes in g_s (r² ranging from 0.77 to 0.81). These results support the view that modelling of stomatal behaviour requires consideration of both chemical and hydraulic aspects of root-to-shoot communication.     

Effects of pod removal on the transport and accumulation of abscisic Acid and indole-3-acetic Acid in soybean leaves

Concentrations of abscisic acid (ABA) and indole-3-acetic acid (IAA) in the second most recently expanded trifoliolate leaf were determined during reproductive development of soybean (Glycine max [L.] Merr cv `Chippewa 64'). The concentration of ABA in leaves was constant during most of the seed filling period until the seeds began to dry. The concentration of IAA in the leaves decreased throughout development. Removal of pods 36 hours prior to sampling resulted in increased concentrations of ABA in leaves during the period of rapid pod filling but had little effect on the concentration of IAA in leaves. ABA appears to accumulate in leaves after fruit removal only when fruits represent the major sink for photosynthate.

ABA and IAA moving acropetally and basipetally in petioles of soybean were estimated using a phloem exudation technique. ABA was found to move mostly in the basipetal direction in petioles (away from laminae). IAA, primarily in the form of ester conjugate(s), was found to be moving acropetally (toward laminae) in petioles. The highest amount of IAA ester(s) was found in petiole exudate during the mid and late stages of seed filling. Removal of fruits 36 hours prior to exudation reduced the amount of IAA ester recovered in exudate, suggesting that fruits were a source of the IAA conjugate in petiole exudate.