Antitranspirant Activity in Xylem Sap of Maize Plants

Xylem sap from unwatered maize plants was collected and testedfor antitranspirant activity. Two assays were used. These werea transpiration assay with detached wheat leaves and a stomatalbio-assay involving the direct microscopic observation of epidermisof Commelina communis. The reduction in transpiration of detached wheat leaves promotedby xylem sap could be duplicated almost exactly by the applicationof solutions of ABA of equivalent concentration to that foundin the xylem sap. Removal of virtually all the ABA from thexylem sap, using an immunoaffinity column, removed virtuallyall the antitranspirant activity in both assays. These results are discussed in the context of other resultswhich suggest the presence of as-yet unidentified inhibitorsin the xylem sap of unwatered plants. We suggest that with maize plants at least, stomatal responsesto soil drying can be entirely explained by enhanced concentrationof ABA in the xylem stream. Key words: Antitranspirant activity, ABA, ABA bio-assay, xylem sap     

Stored xylem sap from wheat and barley in drying soil contains a transpiration inhibitor with a large molecular size

Xylem sap was collected from wheat and barley growing in a drying soil, and the effect of the sap on transpiration was detected by a bioassay with detached wheat leaves. The inhibitory activity of fresh sap was small, and could be largely accounted for by the abscisic acid content (about 2×10^-5mol m^-3). When fresh sap was stored at -20°C for several days, the activity increased. Maximum activity developed after a week. This increase in activity was due to a compound that increased in size with storage at -20°C. When fresh sap was fractionated with filters of different molecular size exclusion characteristics, and the separated fractions stored at -20°C for a week, activity developed only in the fraction containing compounds smaller than 0·3 kDa. However, when sap already stored at -20°C was fractionated, activity was only in fractions containing compounds larger than 0·3 kDa. The increase in activity and in size did not occur with storage in liquid nitrogen (-196°C) or at -80°C. These results suggest that storage at -20°C causes the aggregation or polymerization of a small compound with low activity to form a large compound with high activity. This change is not catalysed by an enzyme because it can occur in a fraction from which molecules larger than 0·3 kDa are removed. It is probably promoted by high solute concentrations when ice crystals form. Sap collected from plants in soils of high water potential had little or no activity after storage at -20°C.     

Abscisic Acid is not the only stomatal inhibitor in the transpiration stream of wheat plants

Xylem sap was collected from the transpiration stream of wheat (Triticum aestivum L.) plants and assayed for the presence of an inhibitor of transpiration using leaves detached from well-watered plants. Transpiration of detached leaves was reduced by nearly 60% by sap collected from plants in drying soil, and to a lesser extent (about 25%) by sap from plants in well-watered soil. As the soil dried the abscisic acid (ABA) concentration in the sap increased by about 50 times to 5 × 10⁻⁸ molar. However, the ABA in the sap did not cause its inhibitory activity. Synthetic ABA of one hundred times this concentration was needed to reduce transpiration rates of detached leaves to the same extent. Furthermore, inhibitory activity of the sap was retained after its passage through an immunoaffinity column to remove ABA. Xylem sap was also collected by applying pressure to the roots of plants whose leaf water status was kept high as the soil dried. Sap collected from these plants reduced transpiration to a lesser extent than sap from nonpressurised plants. This suggests that the inhibitory activity was triggered partly by leaf water deficit and partly by root water deficit.     

Calcium in Xylem Sap and the Regulation of its Delivery to the Shoot

Amounts of total and free calcium in root and shoot xylem sapwere quantified for a number of species grown in comparableenvironments and in a rooting medium not deficient in calcium.The potential for the shoot to sequester calcium was also examined,along with the ability for ABA to regulate calcium flux to theleaf. Xylem sap calcium showed considerable interspecific and diurnalvariation, even though the plants were grown with similar rhizosphericcalcium concentrations. The potential for the shoot to sequesterxylem sap calcium was also highly variable between species andimplied an ability, at least in some species, to regulate thecalcium reaching the shoot in the transpiration stream. Long distance transport of calcium in the xylem was not primarilyby mass flow, because neither calcium uptake nor distributionwere closely related to water uptake or transpiration. The diurnalchanges in xylem sap total ion concentration appeared to benegatively correlated with transpiration while, in contrast,the calcium ion concentration showed two peaks, one occurringin the dark and the other in the light period. The application of ABA to roots caused an increase in the rateof exudation from the xylem of detopped well-watered plants.These experiments suggest that changes in root water relationsdriven by ionic fluxes were the likely cause for enhanced sapexudation from ABA-treated roots. The steady-state concentrationof calcium in the xylem sap was unaffected by ABA when exudationrate increased and, consequently, the flux of calcium must alsohave increased. Key words: Abscisic acid, calcium, xylem sap, ionic fluxes     

Control of Leaf Expansion Rate of Droughted Maize Plants under Fluctuating Evaporative Demand (A Superposition of Hydraulic and Chemical Messages?)

We have analyzed the possibility that chemical signaling does not entirely account for the effect of water deficit on the maize (Zea mays L.) leaf elongation rate (LER) under high evaporative demand. We followed time courses of LER (0.2-h interval) and spatial distribution of elongation rate in leaves of either water-deficient or abscisic acid (ABA)-fed plants subjected to varying transpiration rates in the field, in the greenhouse, and in the growth chamber. At low transpiration rates the effect of the soil water status on LER was related to the concentration of ABA in the xylem sap and could be mimicked by feeding artificial ABA. Transpiring plants experienced a further reduction in LER, directly linked to the transpiration rate or leaf water status. Leaf zones located at more than 20 mm from the ligule stopped expanding during the day and renewed expansion during the night. Neither ABA concentration in the xylem sap, which did not appreciably vary during the day, nor ABA flux into shoots could account for the effect of evaporative demand. In particular, maximum LER was observed simultaneously with a minimum ABA flux in the droughted plants, but with a maximum ABA flux in ABA-fed plants. All data were interpreted as the superposition of two additive effects: the first involved ABA signaling and was observed during the night and in ABA-fed plants, and the second involved the transpiration rate and was observed even in well-watered plants. We suggest that a hydraulic signal is the most likely candidate for this second effect.     

Anti-transpirant activity in xylem sap from flooded tomato (Lycopersicon esculentum Mill.) plants is not due to pH-mediated redistributions of root- or shoot-sourced ABA

In flooded soils, the rapid effects of decreasing oxygen availability on root metabolic activity are likely to generate many potential chemical signals that may impact on stomatal apertures. Detached leaf transpiration tests showed that filtered xylem sap, collected at realistic flow rates from plants flooded for 2 h and 4 h, contained one or more factors that reduced stomatal apertures. The closure could not be attributed to increased root output of the glucose ester of abscisic acid (ABA-GE), since concentrations and deliveries of ABA conjugates were unaffected by soil flooding. Although xylem sap collected from the shoot base of detopped flooded plants became more alkaline within 2 h of flooding, this rapid pH change of 0.5 units did not alter partitioning of root-sourced ABA sufficiently to prompt a transient increase in xylem ABA delivery. More shoot-sourced ABA was detected in the xylem when excised petiole sections were perfused with pH 7 buffer, compared with pH 6 buffer. Sap collected from the fifth oldest leaf of "intact" well-drained plants and plants flooded for 3 h was more alkaline, by approximately 0.4 pH units, than sap collected from the shoot base. Accordingly, xylem [ABA] was increased 2-fold in sap collected from the fifth oldest petiole compared with the shoot base of flooded plants. However, water loss from transpiring, detached leaves was not reduced when the pH of the feeding solution containing 3-h-flooded [ABA] was increased from 6.7 to 7.1 Thus, the extent of the pH-mediated, shoot-sourced ABA redistribution was not sufficient to raise xylem [ABA] to physiologically active levels. Using a detached epidermis bioassay, significant non-ABA anti-transpirant activity was also detected in xylem sap collected at intervals during the first 24 h of soil flooding.     

Assimilate Transport in the Xylem Sap of Pedunculate Oak (Quercus robur) Saplings

Abstract: The rates of photosynthesis and transpiration, as well as the concentrations of organic compounds (total soluble non-protein N compounds [TSNN], soluble carbohydrates), in the xylem sap were determined during two growth seasons in one-year-old Quercus robur saplings. From the data, the total C gain of the leaves, by both photosynthesis and the transpiration stream, was calculated. Large amounts of C were allocated to the leaves by the transpiration stream; depending on the time of day and the environmental conditions the portion of C originating from xylem transport amounted to 8 to 91% of total C delivery to the leaves. Particularly under conditions of reduced photosynthesis, e.g., during midday depression of photosynthesis, a high percentage of the total C delivery was provided to the leaves by the transpiration stream (83 to 91 %). Apparently, attack by phloem-feeding aphids lowered the assimilate transport from roots to shoots; as a consequence the portion of C available to the leaves from xylem transport amounted to only 12 to 16 %. The most abundant organic compounds transported in the xylem sap were sugars (sucrose, glucose, fructose) with concentrations of ca. 50 to 500 μmol C ml^-1, whereas C from N compounds was of minor significance (3 to 20 μmol ml^-1 C). The results indicate a significant cycling of C in the plants because the daily transport of C with the transpiration stream exceeded the daily photosynthetic CO₂ fixation in several cases. This cycling pool of C may sustain delivery of photosynthate to heterotrophic tissues, independent of short time fluctuations in photosynthetic CO₂ fixation.     

Is the ABA concentration in the sap collected by pressurizing leaves relevant for analysing drought effects on stomata? Evidence from ABA-fed leaves of transgenic plants with modified capacities to synthesize ABA.

Most studies on the role of ABA in the stomatal response of the whole plant to drought rely on a good estimate of ABA concentration in xylem sap. In this report, varying volumes of sap (V(sap)) were collected by pressurizing leaves cut from several lines of N. plumbaginifolia with modified capacities to synthesize ABA. Leaves were fed with solutions of known ABA concentration ([ABA](solution) from 0-500 micromol m(-3)) for 2-3 h before sap collection. ABA concentration in extruded sap ([ABA](sap)) was compared with [ABA](solution). In low-volume extracts (less than 0.35 mm(3) cm(-2) leaf area) collected from leaves of well-watered plants, [ABA](sap) was close to [ABA](solution). For all lines, [ABA](sap) decreased with increasing V(sap). The same dilution effect was observed for leaves pressurized just after sampling on droughted plants, suggesting, as for detached leaves fed with ABA, that [ABA](sap) in low-volume extracts approximated well with the concentration of ABA entering leaves still attached on droughted plants. However, ABA-fed leaves sampled from droughted plants yielded higher [ABA](sap) than ABA-fed leaves sampled from well-watered plants. [ABA](sap) was also increased, although very slightly, when leaves were preincubated in highly enriched ABA solution. This indicates that some leaf ABA contributed to the ABA concentration returned in the extruded sap. Consistently, [ABA](sap) in medium-volume extracts (0.35-0.65 mm(3) cm(-2) leaf area) was lower for leaves sampled on under-producing lines than on the wild type. Despite these distortions between [ABA](solution) and [ABA](sap) in medium-volume extracts, stomatal conductance of ABA-fed leaves closely correlated with [ABA](sap) with a similar relationship in all cases, whilst relationships with [ABA](solution) were more scattered.     

Effect of partial rootzone drying on the concentration of zeatin-type cytokinins in tomato (Solanum lycopersicum L.) xylem sap and leaves

Decreased cytokinin (CK) export from roots in drying soil might provide a root-to-shoot signal impacting on shoot physiology. Although several studies show that soil drying decreases the CK concentration of xylem sap collected from the roots, it is not known whether this alters xylem CK concentration ([CK(xyl)]) in the leaves and bulk leaf CK concentration. Tomato (Solanum lycopersicum L.) plants were grown with roots split between two soil columns. During experiments, water was applied to both columns (well-watered; WW) or one (partial rootzone drying; PRD) column. Irrigation of WW plants aimed to replace transpirational losses every day, while PRD plants received half this amount. Xylem sap was collected by pressurizing detached leaves using a Scholander pressure chamber, and zeatin-type CKs were immunoassayed using specific antibodies raised against zeatin riboside after separating their different forms (free zeatin, its riboside, and nucleotide) by thin-layer chromatography. PRD decreased the whole plant transpiration rate by 22% and leaf water potential by 0.08 MPa, and increased xylem abscisic acid (ABA) concentration 2.5-fold. Although PRD caused no detectable change in [CK(xyl)], it decreased the CK concentration of fully expanded leaves by 46%. That [CK(xyl)] was maintained and not increased while transpiration decreased suggests that loading of CK into the xylem was also decreased as the soil dried. That leaf CK concentration did not decline proportionally with CK delivery suggests that other mechanisms such as CK metabolism influence leaf CK status of PRD plants. The causes and consequences of decreased shoot CK status are discussed.     

Do increases in xylem sap pH and/or ABA concentration mediate stomatal closure following nitrate deprivation?

Stomatal conductance (g(s)) of pepper (Capsicum annuum L.) plants decreased during the second photoperiod (day 2) after withholding nitrate (N). Stomatal closure of N-deprived plants was not associated with a decreased shoot water potential (Psi(shoot)); conversely Psi(shoot) was lower in N-supplied plants. N deprivation transiently (days 2 and 3) alkalized (0.2-0.3 pH units) xylem sap exuded from de-topped root systems under root pressure, and xylem sap expressed from excised shoots by pressurization. The ABA concentration of expressed sap increased 3-4-fold when measured on days 2 and 4. On day 2, leaves detached from N-deprived and N-supplied plants showed decreased transpiration rates when fed an alkaline (pH 7) artificial xylem (AX) solution, independent of the ABA concentration (10-100 nM) supplied. Thus changes in xylem sap composition following N deprivation can potentially close stomata. However, the lower transpiration rate of detached N-deprived leaves relative to N-supplied leaves shows that factors residing within N-deprived leaves also mediate stomatal closure, and that these factors assume greater importance as the duration of N deprivation increases.     

Xylem sap calcium concentrations do not explain liming-induced inhibition of legume gas exchange

Background and aims

Liming is considered normal agricultural practise for remediating soil acidity and improving crop productivity; however recommended lime applications can reduce yield. We tested the hypothesis that elevated xylem sap Ca²⁺ limited gas exchange of Phaseolus vulgaris L. and Pisum sativum L. plants that exhibited reduced shoot biomass and leaf area when limed.

Methods

We used Scholander and whole-plant pressure chamber techniques to collect root and leaf xylem sap, a calcium-specific ion-selective electrode to measure xylem sap Ca²⁺, infra-red gas analysis to measure gas exchange of limed and unlimed (control) plants, and a detached leaf transpiration bioassay to determine stomatal sensitivity to Ca²⁺.

Results

Liming reduced shoot biomass, leaf area and leaf gas exchange in both species. Root xylem sap Ca²⁺ concentration was only increased in P. vulgaris and not in P. sativum. Detached leaves of both species required 5 mM Ca²⁺ supplied to via the transpiration stream to induce stomatal closure, however, maximum in vivo xylem sap Ca²⁺ concentrations of limed plants was only 1.7 mM and thus not high enough to influence stomata.

Conclusion

We conclude that an alternative xylem-borne antitranspirant other than Ca²⁺ decreases gas exchange of limed plants.     

Role of Pressure in Xylem Transport of Coconut and Other Palms

The significance of root pressure in the transport of xylem sap has been investigated in Cocos nucifera L. and a few other palms. Despite the fact that excised palm roots can generate considerable pressures in situ, the quantity of water transported is only a small fraction of the demand resulting from transpiration. Most water transport is induced by negative pressure gradients, as in other higher plants. The development of considerable negative pressures has been demonstrated both directly and indirectly. Acoustic detection was used for the first time to monitor cavitation in water-stressed Cocos leaves. Its detection implies the ready disruption of xylem sap under these tensions. We suggest that root pressure might serve to refill cavitated xylem conduits when water is abundantly available and transpiration practically zero. However, little or no positive pressure could be demonstrated in intact palms subjected to low water stress: experimentally.     

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.     

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.     

Metabolic origin of acetaldehyde emitted by poplar (Populus tremula x (P. alba) trees

The metabolic origin and emission by the leaves of the tropospheric trace gas acetaldehyde were examined in 4-month-old poplar trees (Populus tremula x P. alba) cultivated under controlled environmental conditions in a greenhouse. Treatments which resulted in increased ethanol concentration of the xylem sap caused significantly enhanced rates of acetaldehyde and ethanol emission by the leaves. Leaves fed [¹⁴C]-ethanol via the transpiration stream emitted [^<14C]-acetaldehyde. These findings suggest that acetaldehyde in the leaves is synthesized by a metabolic pathway that operates in the opposite direction of alcoholic fermentation and results in oxidation of ethanol. Enzymatic studies showed that this pathway is mediated either by alcohol dehydrogenase (ADH; EC 1.1.1.1) or catalase (CAT; EC 1.11.1.6), both constitutively present in the leaves of poplar trees. Labelling experiments with [¹⁴C]-glucose indicated that the ethanol delivered to the leaves by the transpiration stream is produced in anaerobic zones of submersed roots by alcoholic fermentation. Anoxic conditions in the rhizosphere caused by flooding of the root system resulted in an activation of alcoholic fermentation and led to significantly increased ethanol concentrations in the xylem sap. These results support the hypothesis that acetaldehyde emitted by the leaves of trees is derived from xylem transported ethanol which is synthesized during alcoholic fermentation in the roots.Keywords: Acetaldehyde, emission, ethanol, anaerobiosis, Populus tremula x P. alba      

Collection of Xylem Sap at Flow Rate Similar to in vivo Transpiration Flux

We have explored a method to collect xylem sap using a Scholanderpressure chamber for potted plants. Intact root system in potswhich fitted the pressure chamber was pressurised at a pneumaticpressure numerically equal to the absolute value of shoot waterpotential. The rate of xylem flow obtained from the stem stumpunder such pressure was found similar to the rate of transpirationbefore detopping. The rate of pressurised flow from detop-pedroots was linearly related to the pressure applied in both well-wateredand soil-dried plants. The osmotic concentration of the xylemsap was negatively related to the rate of volume flow, suggestingthe necessity to collect xylem sap at in vivo flow rate if originalsolute concentration is to be evaluated. The concentration ofABA in the xylem sap, however, did not show such a relationshipwith water flux. Both well-watered and soil-dried plants showedthe concentration of ABA in xylem sap largely stable with arange of volume flow rate, indicating a linear relationshipbetween the rate of ABA delivery through xylem and that of volumeflow. We also compared the concentrations of ABA in xylem sapsequentially collected from pressurised roots with that fromdetached shoots of the same plants. The concentration of ABAin the initial saps from shoots showed to be similar to thatfrom roots. However, a decrease in the concentration of ABAin the xylem sap collected from detached leaf or twig was observedwhen more volume of sap was collected, which might also be dependenton the plant species and the volume of xylem vessels concerned. (Received February 3, 1997; Accepted October 7, 1997)     

Induction of xylem sap methylglycine by a drought and rewatering treatment and its inhibitory effects on the growth and development of plant organs

In higher plants, the xylem vessels functionally connect the roots with the above-ground organs. The xylem sap transports various organic compounds, such as proteins and amino acids. We examined drought and rewatering-inducible changes in the amino acid composition of root xylem sap collected from Cucurbita maxima roots. The major free amino acids in C . maxima root xylem sap were methylglycine (MeGly; sarcosine) and glutamine (Gln), but MeGly was not detected in the xylem sap of cucumber. MeGly is an intermediate compound in the metabolism of trimethylglycine (TMG; betaine), but its physiological effects in plants are unknown. Drought and rewatering treatment resulted in an increase in the concentration of MeGly in root xylem sap to 2.5 m M . After flowering, the MeGly concentration in the xylem sap dropped significantly, whereas the concentration of Gln decreased only after fruit ripening. One milli molar MeGly inhibited the formation of adventitious roots and their elongation in C . maxima , but glycine, dimethylglycine, or TMG had no effect. Similar effects and the inhibition of stem elongation were observed in shoot cuttings of cucumber and Phaseolus angularis . These observations seem to imply a possible involvement of xylem sap MeGly in the physiological responses of C . maxima plants to drought stress.     

Carbon balance in leaves of young poplar trees

In the present study, important components of carbon metabolism of mature leaves of young poplar trees (Populus x canescens) were determined. Carbohydrate concentrations in leaves and xylem sap were quantified at five different times during the day and compared with photosynthetic gas exchange measurements (net assimilation, transpiration and rates of isoprene emission). Continuously measured xylem sap flow rates, with a time resolution of 15 min, were used to calculate diurnal balances of carbon metabolism of whole mature poplar leaves on different days. Loss of photosynthetically fixed carbon by isoprene emission and dark respiration amounted to 1% and 20%. The most abundant soluble carbohydrates in leaves and xylem sap were glucose, fructose and sucrose, with amounts of approx. 2 to 12 mmol m(-2) leaf area in leaves and about 0.2 to 15 mM in xylem sap. Clear diurnal patterns of carbohydrate concentration in xylem sap and leaves, however, were not observed. Calculations of the carbon transport rates in the xylem to the leaves were based on carbohydrate concentrations in xylem sap and xylem sap flow rates. This carbon delivery amounted to about 3 micromol C m(-2) s(-1) during the day and approx. 1 micromol C m(-2) s(-1) at night. The data demonstrated that between 9 and 28 % of total carbon delivered to poplar leaves during 24 h resulted from xylem transport and, hence, provide a strong indication for a significant rate of carbon cycling within young trees.     

The chemical form of trivalent chromium in xylem sap of maize (Zea mays L.)

Abstract

The toxicity, mobility and bioavailability of Cr, a versatile industrial metal and a contaminant, depends on its chemical form, viz: Cr(lll) and Cr(VI). It may enter humans through plants grown on contaminated soil or irrigated by contaminated water. The phytoavailability and transfer through agricultural food chains requires an understanding of mechanisms of Cr uptake and translocation by plants. Xylem sap transports both nutrient and non-nutrient ions after absorption by roots to aerial parts of the plant. lt transports cations by complexation with organic ligands. Trivalent chromium, though prone to hydrolysis, also complexes O donor ligands. The chemical form in which Cr(lll) is transported by xylem sap was investigated. ln vitro studies were performed by mixing the xylem sap of maize plants at three stages of plant growth with radiotagged Cr(III). The speciation change was investigated after 10 days and 30 days by anion and cation exchange elution chromatography. The elution curves were compared with those of pure Cr(III) and Cr(III) complexes of different synthetic acids. Complexation of Cr(III) with ligands of xylem sap especially with carboxylates was evident. Cationic Cr(III) was vitally being transported as anionic organic complex species. The major species seemed to be that of Cr(III)-citrate. Citric acid was the major complexing acid of xylem sap as determined by HPLC. These mobile and soluble complexes may get immobilized and stored in leaves and other edible plant parts. This may also be a mechanism used by plants for detoxification of toxic Cr(VI) which may become reduced and then complexed.