Uptake and distribution of 65Zn and 54Mn in wheat grownat sufficient and deficient levels of Zn and Mn: I. During vegetative growth

The uptake and distribution of ⁶⁵Zn and ⁵⁴Mn by wheat (Triticumaestivum cv. Aroona) was investigated. Plantswere grown in achelate-buffered nutrient solution with either sufficient Znand Mn, low Zn or low Mn. A single representative seminal rootfrom 14-d-old and 42-d-old plants was dual-labelled with ⁶⁵Znand ⁵⁴Mn. The 14-d-old plants were harvested every 10 min from10–140 min of labelling, whilst the 42-d-old plants wereharvested after 2 h of labelling. At harvest, each plant wasseparated into leaves, main stem, unexposedroots, and tillers.In addition, the crown was separatedfrom the stem in the 14-d-oldplants In the control plants labelled at 14 d, ⁶⁵Zn was firstdetectedand accumulated in the crown of the roots after 40–60min. Labelled Zn was then detected in the stem, followed bythe leaves. The oldest and youngest leaves received less ⁶⁵Znthan the second and third oldest leaves. The plants grown underlow Zn conditions accumulated more ⁶⁵Zn in their older leavesand transferred ⁶³Zn to the unexposed roots. Distribution of⁵⁴Mn was similar in the controls to that of ⁶⁵Zn, except theolder leaves received no HMn, At the second harvest, a similardistribution pattern of ⁶⁵Zn and ⁵⁴Mn was observed with regardto leaf age. Large amounts of ⁶⁵Zn and ⁵⁴Mn were detected withinthe unexposed roots of all treatments. It is suggested thatthe distribution of root-supplied Zn and Mn may be determinedby micronutrient status and its relationship with leaf transpirationrates. Key words: Distribution, manganese, vegetative growth, wheat, zinc     

THE DISTRIBUTION PATTERN OF CARBON-14 ASSIMILATED BY A SINGLE LEAF IN TOBACCO PLANT

When ¹⁴CO₂ was administered to a fully expanded leaf (12th leaf)of tobacco plant at the stage just before flower budding, about30% of ¹⁴C assimilated was translocated to other organs after3 hours. After 21 hours, 20{small tilde}30% of the radioactivitywas translocated to the roots, about 20% to upper stem, 10%to lower stem, and 10% to the 17th leaf located directly abovethe 12th leaf. The amount of ¹⁴C translocated to other leaveswas small after 31 hours. When ¹⁴CO₂ was applied to the 17th leaf, radioactivity in otherorgans was negligible. Judging from the time course of ¹⁴C-incorporation into organicsubstances, it was inferred that sucrose imported into the rootsfrom the 12th leaf was converted into compounds of cationicfraction and sugar esters. ¹⁴C imported into the 17th leaf was mostly incorporated into80% ethanol-soluble fraction, especially into sucrose. On theother hand, ¹⁴C fixed photosynthetically by the 17th leaf wasmostly recovered in starch and protein fraction after 8 hoursof ¹⁴CO₂ assimilation. ¹A part of this paper was presented at the Japanese Societyof Plant Physiologists, in April, 1965. ²Present address: Central Research Institute, Japan MonopolyCorporation, Shinagawa-ku, Tokyo.     

Carbon and Nitrogen Sources for Protein Synthesis and Growth of Sugar-beet Leaves

Protein synthesis in very young leaves utilizes carbon fromphotosynthesis and from translocated sucrose, and nitrogen translocatedin both xylem and phloem. The carbon of young leaf protein isderived mainly from assimilated CO₂, while translocated sucrosecontributes proportionately more of its carbon to insolublecarbohydrate. Most protein amino-acids become labelled from¹⁴CO₂, glutamate being the notable exception. Glutamine or glutamateis synthesized from sucrose in roots, and is translocated toyoung leaves. It is suggested that a small but significant proportionof the nitrogen requirement of the young leaf is translocatedfrom roots as glutamine, in the phloem. Inorganic nitrogen istranslocated in xylem.     

青葙对土壤锰的耐性和富集特征

通过盆栽试验,研究了青葙(Celosia argentea Linn.)对不同浓度(0、50、100、200、300、500 mg/kg)锰(Mn)污染土壤的吸收和积累特性。结果表明,青葙的锰含量、生物富集系数和生物量均随着土壤锰浓度的增加而增加。当土壤锰含量为300 mg/kg时,青葙生长良好。在锰浓度500 mg/kg时,青葙叶片边缘出现轻微褪绿现象,但是植株的生长未受到抑制,并且叶片生物量显著增加(P0.05)。此时,叶片中锰含量达到最大值42927 mg/kg,生物富集系数为69.20。青葙吸收的锰有95%—97%被转移到地上部分,表明该植物对锰具有很强转运能力。本研究的结果为利用青葙修复锰污染土壤提供了有力证据。     

Distribution and mobility of manganese in the hyperaccumulator plant Phytolacca acinosa Roxb. (Phytolaccaceae)

The distribution and mobility of manganese (Mn) in the hyperaccumulator plant species Phytolacca acinosa Roxb. (Phytolaccaceae) were investigated in a hydroponic system. The plants were exposed to 2 or 5 mM Mn for up to 28 days. For any given plant, the Mn content in the mature leaves (nos. 5–9) was always higher than that in the old (nos. 1–4) and young leaves (nos. 10–14). Within the different parts of a leaf, Mn was preferentially accumulated in the leaf marginal area, where the observed level was threefold higher than that in the midrib. Cross-sectional analysis of the leaf revealed that the concentration of Mn was higher in the leaf epidermis than in the mesophyll. Cell fractionation analysis with P. acinosa leaves showed that most of the Mn (78.4%) was present in the final supernatant fraction (following centrifugation at 20,000 g for 45 min). The distribution of Mn in the leaves of P. acinosa was controlled mainly by the transpiration rate. Our investigation demonstrated that Mn was readily transported from the roots to shoots of P. acinosa but that it could not be remobilized readily after it reached leaves.     

The Strategy of Carbon Utilization in Uniculm Barley I. THE CHEMICAL FATE OF PHOTOSYNTHETICALLY ASSIMILATED 14C

Following exposure of the youngest mature leaf of uniculm barleyto ¹⁴CO₂, groups of plants were harvested over a 72 h periodto determine the fate of 14C in the photosynthesizing leaf andin growing leaves and roots. Initially, ¹⁴C was mainly presentin sucrose with a little in starch and charged compounds; transportout of the fed leaf was rapid and, by 7 and 24 h, 56 and 93%respectively of the ¹⁴C had been translocated about equallyto growing leaves and roots. Sucrose entering meristems wasquickly metabolized to protein and structural carbohydrate (40and 60% of the ¹⁴C in these organs at 7 and 24 h respectively),while the remainder was converted to short-term storage productsor intermediary metabolites. By the end of the first day c.35% of the 14C originally assimilated had been lost in respiration. The metabolism of the leaf appeared to be organized on a diurnalbasis, for it exported nearly all its carbon within 24 h ofassimilation. In contrast, some of the assimilate imported intogrowing leaves and, to a lesser extent, roots was not immediatelyused for growth but persisted as temporary metabolites and wassubsequently used for growth in the following days.     

Functional Manganese Requirement and its use as a Critical Value for Diagnosis of Manganese Deficiency in Subterranean Clover (Trifolium subterraneum L. cv. Seaton Park)

The effects of manganese supply on plant growth and on photosynthesisand manganese concentrations in young leaves were examined inSeaton Park subterranean clover in three glasshouse water cultureexperiments. Plants werc grown initially with a copious supply of manganese,and transferred to solutions either with or without manganese.Sequential harvests were taken to determine the effects of developingmanganese deficiency on dry matter (DM) yield of whole plantsand selected characteristics [manganese, chlorophyll and photosyntheticoxygen evolution (POE)] of youngest open leaf blades (YOL).In addition, the deffect of leaf age and iron supply on POEwerc examined. Manganese concentrations and POE in YOL declined markedly andrapidly in plants transferred to solutions without manganese,while chlorophyll concentrations of YOL and plant DM yield respondedmore weakly and more slowly. As a result, a level of manganesedeficiency which depressed POE in young leaves by more than50 per cent had no effed on DM production. In youngleaves (YOL, YOL + 1, YOL–1), POE declined whentheir manganese concentrations were < 20 µg g^–1DM. Iron supply did not affect this rdationship. When learnwith < 20 µg Mn g^–1 DM were detached and incubatedfor 24 h in solutions containing high concentrations of manganese,their POE increased to normal rates; leaves with higher manganeseconcentrations did not respond. It is suggested that the valueof 20 µg Mn g^–1 DM is the functional manganese requirementfor POE in young subterranean clover leaves It is also suggestedthat this value may be used as a critical value for indicatingmanganese deficiency in subterranean clover. Functional nutrient requirements determined in this way by correlationof nutrient concentrations in young leaves with their biochemicalor physiological activities appear to offer more accurate andconsistent standards for use an critical values for diagnosisof plant nutrient status than do the critical values determinedin the usual way by correlation with plant dry weight. Trifolium subterraneum L. subterranean clover, manganese, functional requirements, deficiency diagnosis, nutrient requirements, critical values, photosynthetic oxygen evolution     

The Translocation of 14C-Labelled Assimilates in Tomato Plants Infected with Alternaria Solani (Ell. and Mart.) Jones and Grout

A quantitative study was made of the effect of infection bya facultative parasite, Alternaria solani, on the translocationof ¹⁴C-labelled assimilates in the tomato plant. In a plantwith a single diseased leaf, this leaf retained a greater proportionof its assimilated radiocarbon during the early stages of infection.The export of assimilates from a diseased leaf when diseasedevelopment was purposely retarded and was found to be similarto that from a control leaf. At a late stage in the disease,where there was extensive chlorosis and necrosis, export fromdiseased leaves was generally increased and the distributionof the translocated assimilates was altered. Non-infected leaveswere at first unaffected by the disease, but later there wasincreased contribution to the actively growing regions of theplant. In some cases there was a slight increase in total exportfrom healthy leaves but, more generally, a change in the distributionof assimilates occurred. In particular, there was a consistentincrease in activity in the roots. This response to infectionwas not related to the amount of disease present. No evidencewas obtained for an increased import into infected leaves fromnon-infected leaves at any stage of the disease.     

Uptake and distribution of 65Zn and 54Mn in wheat grown at sufficient and deficient levels of Zn and Mn II. During grain development

We investigated the uptake and distribution of Zn and Mn inwheat during grain development. Plants were grown in a chelate-bufferednutrient solution with one of the following treatments: control,low Zn or low Mn. Plants were dual-labelled with ⁶⁵Zn and ⁵⁴Mnat 2 and 8 wk post-anthesis for 5 and 24 h, respectively. Afterlabelling, the plants were separated into individual componentsfor analysis. In the plants harvested at 8 wk after anthesis,spikelets were separated into individual structures and analysedfor radioactivity. Little or no root-supplied ⁵⁴Mn was distributed to the leavesof both the controls and low-Mn plants during the grain developmentstages studied here. More ⁵⁴Mn was distributed to the head at8 wk than at 2 wk post-anthesis. In contrast, root-supplied⁶⁵Zn was transported to the leaves at 2 and 8 wk post-anthesis.More⁶⁵Zn was distributed to the leaves of the low-Zn plants thanthe controls during grain development.More esZn was detectedin the head towards grain maturity. Relatively larger amountsof ⁵⁴Mn than ⁶⁵Zn were found in different parts of the florets.Labelled Mn was found in relatively large quantities in thepalea, lemma and in the glumes, even though most ⁵⁴Mn was foundin the grain. A large percentage of the grain MMn was in theouter pericarp. During grain development leaves may still require Zn but notMn, which may be due to the requirement of Zn in maintainingmembrane structure and function. Distribution of Zn and Mn withinthe spikelets suggests that Zn may enter the grain via the phloemwhile Mnmay enter the grain via the xylem. Key words: Zinc, manganese, nutrient transport, grain development, wheat, Triticum aestivum     

The Distribution Pattern of 14Carbon Assimilated by the Third Leaf of Wheat

The third leaf of wheat, variety Jufy I, was allowed to assimilate¹⁴CO₂ for 2 hrs.; after a further hour the distribution patternof the assimilates was determined. Uptake of ¹⁴CO₂ and assimilatesleaving the leaf increased until the leaf was fully expanded,then slowly decreased. High proportions of labelled translocates were recorded in boththe stem and the the root system, that in the roots increasinggreatly as movement of translocates to the leaves decreased.The two fully grown leaves, L₁ and L₂, imported only slightamounts of labelled translocates. Movement of labelled translocateto each of the younger leaves in turn occurred in a strikingpattern, such that import into a given leaf reached a maximumwhich coincided with its maximum rate of growth, subsequentlyfalling rapidly and reaching a very low level by the time theleaf is fully grown. The results are discussed in relation to what is known of thegeneral pattern of growth and translocation in the wheat plant.     

向日葵(Helianthus annuus L.)对133Cs、88Sr的吸收和分布

通过盆栽试验研究了向日葵(Helianthus annuus L.)对土壤中不同处理浓度¹³³Cs和⁸⁸Sr的吸收,以及¹³³Cs和⁸⁸Sr在向日葵不同部位的分布。结果表明：随着处理浓度的增加,植物中¹³³Cs或⁸⁸Sr的含量增加。同一处理浓度下,⁸⁸Sr含量约比¹³³Cs含量高一个数量级。¹³³Cs和⁸⁸Sr在植物不同部位分布不同。根部中¹³³Cs含量高于植物的其他部位（茎、叶、花）。不同于¹³³Cs在植物中的分布,⁸⁸Sr除在根中的分布外,主要转运到了叶片。¹³³Cs和⁸⁸Sr在向日葵体内的分布与目前对放射性¹³⁷Cs和⁹⁰Sr的研究结果相似,所以¹³³Cs和⁸⁸Sr可分别预测¹³⁷Cs和⁹⁰Sr的运转。向日葵是治理大面积低放核素污染土壤的较佳植物种类。     

Manganese and iron interactions on their uptake and distribution in soybean (Glycine max (L.) Merr.)

Summary The uptake and distribution of iron and manganese were studied in a manganese-sensitive soybean cultivar (‘Bragg’) grown over a range of supply levels of these nutrients in solution culture. At high (90 and 275 μM) manganese levels, increasing the iron concentration in solution from 2 to 100 μM partially overcame the effects of manganese toxicity. Interactions between manganese and iron occurred for dry matter yields, rate of Mn absorption by the roots, and the proportions of manganese and iron transported to the tops. No interaction was observed for the rate of root absorption of iron. The percentage distribution of manganese in the plant top increased with increasing iron, despite a reduced rate of Mn uptake. On the other hand, iron uptake was independent of solution Mn concentration and increased with increasing solution Fe. Also more iron was retained in the roots at high Mn and/or Fe levels in solution. Concentrations of manganese and iron in roots, stems and individual leaves were affected independently by the manganese and iron supplyi.e. without any interaction occurring between the two elements. In general, the concentration in a plant part was related directly to the solution concentration. Symptoms resembling iron deficiency correlated poorly with leaf Fe concentrations whereas high levels of manganese were found in leaves displaying Mn toxicity symptoms.     

Some Effects of General Yellow Rust (Puccinia striiformis) Infection on 14Carbon Assimilation, Translocation and Growth in a Spring Wheat

A general, heavy infection of Yellow Rust(Puccinia StriiformisWestend.) on the leaf laminas of the spring wheat (Triticumvulgare Host) Jufy I, unlike an infection on one leaf only,modified the distribution pattern of ¹⁴C-labelled assimilatetranslocated from the second leaf: the proportion moving tothe roots (in older plants also to the tillers) was decreased,and that moving to the leaves was increased. The proportionof the assimilate translocated out of an infected leaf of asuch plant was, however more than that observed when that leafwas the only one infected, though still less than that froma corresponding leaf in a healthy plant. Age of leaf did notgreatly affect the distribution pattern. The effect of infection on the distribution pattern of assimilatefrom other leaves 15 days after inoculation was comparable tothe effect on that from the second leaf at the same intervalafter inoculation. In the case of the upper leaves the proportionmoving to the tillers was appreciably reduced by infection.These results are considered in relation to data obtained froma parellel growth analysis experiment, with which they are ingood agreement.     

Root Nodules of Phaseolus: Efficient Scavengers of Molybdenum for N2-fixation

Molybdenum is thought to be of intermediate mobility in thephloem and this may limit N₂-fixation by restricting the supplyof molybdenum to the nodules of legumes. When no molybdenumwas supplied to Phaseolus vulgaris nodule Mo content increasedat the expense of shoots and roots even when seed molybdenumcontent was large. Nodules sampled from plants receiving molybdenumin the feeding solution had a concentration of 21–78 µgMo g^-1. In the absence of molybdenum and with deficient seedcontent (<0.5 µg Mo seed^-1) nodule concentrations rangedfrom 1.9 to 3.5 fig Mo g^-1 in a small seeded genotype and 8.7±0.48µg Mo g^-1 in a large seeded genotype. N₂-fixation in theseplants was not impaired except in one instance where noduleconcentration was 1.9 µg Mo g^-1. Evidence that molybdenumis effectively translocated from leaves to roots and noduleswas obtained using foliar treatments. All of the 3.3 µgMo applied to a leaf was recovered in the plant after 10 d.Mo content of the nodules increased by 81%, whilst Mo contentof shoots increased by 56%. Root Mo content was eight timesgreater than that in plants not receiving a foliar treatmentof molybdenum. We conclude that when molybdenum was scarce inthe plant it was mobile and was translocated from roots andshoots to the nodules. As a result, nodule concentrations andcontents of molybdenum were frequently maintained at amountssufficient for N₂-fixation even when the plant was entirelydependent on a small seed reserve of molybdenum.     

Manganese Uptake by Excised Oat Roots

Uptake of ⁵⁴Mn by excised oat roots from dilute manganese chloridesolutions has been investigated. The time-course of uptake hasbeen analysed into the customary but somewhat arbitrary fastand slow phases. Uptake is not metabolic in either of these.The fast phase (‘exchangeable’ manganese) is essentiallycomplete in about 30 minutes and represents the attainment ofequilibrium in a process of ion-exchange. It is shown that analysesappropriate for enzyme kinetics cannot be applied in this situation,and an alternative formulation is based on Donnan equilibration,taking account of the selectivity of the ion exchanger towardsdifferent counter-ions; the predictions of this latter theoryare compared with the experimentally determined uptake. Theslow phase (‘absorbed’ manganese) may also involveexchange sites, either chemically different from, or more difficultof access than, those involved in the fast phase, or both. Equilibriumwas certainly not reached in three hours in this slow-phaseprocess. Release of manganese, taken up by the roots from manganese chloridesolutions, into calcium chloride solutions does not seem tobe simply the reversal of uptake, particularly with very dilutesolutions. This is particularly shown by the kinetics of uptakeand release, uptake being a much faster process than release.Manganese may transfer from the first phase to the second phase,but there is no evidence that uptake by the roots proceeds inseries from first to second phase. It is considered more likelythat the two phases function independently, linked by the surroundingsolution.     

Translocation in Sugar-beet: I. ASSIMILATION OF 14CO2 AND DISTRIBUTION OF MATERIALS FROM LEAVES

¹⁴CO₂ was supplied to leaves, and movement of labelled carbonto other parts of the plant was assessed. Young growing leavesutilized assimilated carbon for their own growth and did notexport carbon to the rest of the plant, while fully expandedleaves exported much of their photosynthate, both to root andto young leaves. Translocation from a particular leaf was tothe two or three younger leaves on the same side of the plant,and to a sector of root below the source leaf. Specific distributionto growing leaves could be modified by partial defoliation.There was no movement of material to leaves which had emergedbefore the source leaf. Part of the carbon entering a leaf by assimilation (and, foryoung leaves, by translocation) was incorporated into insolublematerial, especially in young leaves. Some of the carbon enteringa developing root was permanently stored as sucrose, althoughmuch also entered insoluble material. Loss from the leaf ofcarbon fixed during a short period of photosynthesis was rapidat first but continued at a decreasing rate for several days.Some carbon fixed into the insoluble fraction was translocatedfrom the leaf later, during senescence. Sucrose was the mainmaterial translocated immediately after photosynthesis.     

52Fe Translocation in Barley as Monitored by a Positron-Emitting Tracer Imaging System (PETIS): Evidence for the Direct Translocation of Fe from Roots to Young Leaves via Phloem

The real-time translocation of iron (Fe) in barley (Hordeumvulgare L. cv. Ehimehadaka no. 1) was visualized using the positron-emittingtracer ⁵²Fe and a positron-emitting tracer imaging system (PETIS).PETIS allowed us to monitor Fe translocation in barley non-destructivelyunder various conditions. In all cases, ⁵²Fe first accumulatedat the basal part of the shoot, suggesting that this regionmay play an important role in Fe distribution in graminaceousplants. Fe-deficient barley showed greater translocation of⁵²Fe from roots to shoots than did Fe-sufficient barley, demonstratingthat Fe deficiency causes enhanced ⁵²Fe uptake and translocationto shoots. In the dark, translocation of ⁵²Fe to the youngestleaf was equivalent to or higher than that under the light condition,while the translocation of ⁵²Fe to the older leaves was decreased,in both Fe-deficient and Fe-sufficient barley. This suggeststhe possibility that the mechanism and/or pathway of Fe translocationto the youngest leaf may be different from that to the olderleaves. When phloem transport in the leaf was blocked by steamtreatment, ⁵²Fe translocation from the roots to older leaveswas not affected, while ⁵²Fe translocation to the youngest leafwas reduced, indicating that Fe is translocated to the youngestleaf via phloem in addition to xylem. We propose a novel modelin which root-absorbed Fe is translocated from the basal partof the shoots and/or roots to the youngest leaf via phloem ingraminaceous plants.     

Effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L.)

Summary The effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L. var. ‘Red Kidney’) grown in water culture was studied at different levels of manganese supply. Without silicon, growth depression and toxicity symptoms occurred already at 5 × 10⁻⁴ mM Mn in the nutrient solution. After addition of Aerosil (0.75 ppm Si), the plants tolerated 5 × 10⁻³ mM Mn and, at a higher silicon supply of 40 ppm, as much as 10⁻² mM Mn in the nutrient solution without any growth depression. This increase in manganese tolerance was not caused by a depressing effect of silicon on uptake or translocation of manganese but rather by an increase in the manganese tolerance of the leaf tissue. In absence of silicon, 100 ppm Mn was already toxic for the leaf tissue, whereas with a supply of 40 ppm Si, this ‘critical level’ in the leaves was increased to more than 1000 ppm Mn. At lower manganese levels in the leaf tissue, a molar ratio Si/Mn of 6 within the tissue was sufficient to prevent manganese toxicity. Above 1000 ppm Mn, however, even a much wider Si/Mn ratio (> 20) could not prevent growth depression by manganese toxicity. With⁵⁴Mn and autoradiographic studies, it could be demonstrated that, in absence of silicon, even at optimal manganese supply (10⁻⁴ mM), the distribution of manganese within the leaf blades was inhomogeneous and characterized by spot-like accumulations. In presence of silicon, however, the manganese distribution was homogeneous in the lower concentration range of manganese and still fairly homogeneous in the high concentration range. This effect of silicon on manganese distribution on the tissue level was also reflected on the cellular level. In the presence of silicon, a higher proportion of the leaf manganese could be found in the press sap,i.e., had been transported into the vacuoles, than in the absence of silicon. The increase in manganese tolerance of bean leaves by silicon therefore seems to be primarily caused by the prevention of local manganese accumulation within the leaf tissue which leads to local disorders of the metabolism and, correspondingly, growth depression.     

An Unusual Pattern of 65Zn Distribution in the Leaves of Some Grasses

Autoradiographs of leaves of ryegrass plants (Lolium sp.) takingup radioactive zinc (⁶⁵Zn) from soil showed an unusual patternof label distribution. Tracer accumulated in a regular seriesof pulses along leaf veins. Pulses in adjacent veins occurredin juxtaposition forming the pattern of a band across the leaf. The possibilities of artifacts resulting from plant-culturetechnique or methods of preparing leaves for autoradiographywere eliminated. The band effect in leaves of Loliumand other grass species occurredwith ⁶⁵Zn but not ¹⁹Fe or ⁵⁴Mn. Leaves of Hordeum vulgare, Zeamays, Bromus rigidus, and two non-gramineous species did notshow the band phenomenon with ⁶⁵Zn. There was a close relationshipbetween the frequency of bands and the rate of growth of a leaf.The band pattern was shown to be responsive to changes in daylengthand temperature during growth. Possible origins of the band phenomenon are discussed.     

Partitioning and redistribution of sulphur during S-stress in Macroptilium atropurpureum cv. Siratro

During the first 7 d of sulphate-deprivation stored SO₄^2- wasredistributed and assimilated into organic forms in the tropicallegume Macroptilium atropurpu-reum cv. Siratro. However, whilstthe sulphate content of all tissues declined after removingthe external SO₄^2- supply this was slowest in mature leaves.By contrast, the total S content of mature leaves declined markedlyin the absence of external sulphate whilst that of both youngleaves and roots increased. Furthermore, when radiolabelledSO₄^2- was applied to abraded surfaces of mature leaves, mostof the translocated label was recovered in the root following2 d SO₄^2- deprivation. By contrast, radiolabelled SO₄^2-appliedto young leaves was mostly retained in these tissues and nottranslocated. Within 3 d of removing the SO₄^2- supply there was a large increasein extractable APS-sulphotransferase activity in roots accompaniedby a decline in nitrate reductase activity, but these effectswere not seen in leaves. Five days after the removal of SO₄^2-there was a large increase in the content of asparagine in roots. The results are discussed in relation to the co-ordination ofNO₃^- and SO₄^2- uptake and assimilation and the partitioningof sulphur during S-stress. Key words: Sulphate supply, stomatal conductance, ATP-sulphurylase, APS-sulphotransferase, nitrate reductase