Oxidation Rate of Monomeric Radish Cytochrome f with Plastocyanin

The reaction rate of reduced monomeric cytochrome f with oxidizedplastocyanin, both purified from Japanese radish, was determinedby a stopped-flow method. The oxidation rate constant was 6.0x 10⁷ M¹sec¹ at pH 7.0 and 25°C, which is slightly higherthan the value reported by Wood [(1974) Biochim. Biophys. Acta357 : 370] for oligomeric parsley cytochrome f Thermodynamicparameters also were determined to be 56 KJ M^–1 for activationenthalpy and 90 J M^–1 K^–1 for activation entropy.Neither a pH from 6 to 9 nor the addition of NaCl, polylysine,histone or polyaspartate affected the rate constant. ¹Present address: The National Institute for Environmental Studies,Yatabe, Ibaraki 305, Japan. (Received October 9, 1980; Accepted November 17, 1980)     

Effect of growth irradiance on the maximum photosynthetic capacity of water hyacinth [Eichhornia crassipes (Mart.) Solms]

We grew water hyacinth [Eichhornia crassipes (Mart.) Solms]for 60 days in a greenhouse under natural light and in a controlledenvironment room at 31/25?C day/night temperatures and 90, 320and 750/µEm^–2sec^–1. We then determined maximumphotosynthetic rates in 21% and 1% oxygen, stomatal diffusionresistances, contents of chlorophyll and soluble protein, andthe size and density of the photosynthetic units (PSU) in representativeleaves from the four treatments. In air containing 21% oxygen,maximum photosynthetic rates were 14, 27 and 29 mg CO₂ dm^–2hr^–1for plants grown in artificial light at 90, 320 and 750µEm^–2sec^–1,respectively. Plants grown in natural light (maximum of 2000µEm^–2sec^–1) had maximum photosynthetic ratesof 34 mg CO₂ dm^–2hr^–1. In all treatments, photosyntheticrates in 1% oxygen were about 50% greater than rates in normalair, indicating the presence of photorespiration in water hyacinth.There was no apparent relationship between maximum photosyntheticrate per unit leaf area and stomatal conductance, chlorophyllcontent per unit area, or PSU density per unit area. However,the higher maximum photosynthetic rates were associated withgreater mesophyll conductances, specific leaf weights and proteincontents per unit area. When plants grown at 90µEm^–2sec^–1for 120 days were transferred to 750µEm^–2sec^–1for 5 days, only young leaves that were just beginning to expandat the time of transfer exhibited adaptation to the higher irradiance.The 40% increase in light-saturated photosynthetic rate in theseyoung leaves was associated with increases in mesophyll conductance,soluble protein content per unit area, and specific leaf weight. ¹ Mississippi Agricultural and Forestry Experiment Station cooperating. (Received July 19, 1978; )     

Longitudinal Water Movement in the Primary Root of Zea mays

The rates of transfer of tritiated water (THO) along lengthsof excised primary roots of Zea mays have been measured undera variety of conditions. The following values of ‘apparentdiffusion coefficients’ for THO in the root tissue havebeen evaluated: 1.5±0.1x10^-5 cm² sec^-1 in roots boiledfor 3 min before use,0.5±0.03x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaF,0.9±0.07x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaN₃,and 2.1±0.2x10^-5 cm² sec^-1in normal roots. The bathing medium in all cases was 1.0 mMKCl/0.1 mM CaCl₂ with the addition of the inhibitors where appropriate.Thefourfold increase in the rate of THO transfer in normal rootscompared with poisoned ones is attributed to the existence ofa long-distance convective flow in the first case, which isterminated by the addition of inhibitors. Since experimentsshow that this convective flow must occur both acropetally andbasipetally with equal velocity, it is thought to occur in thephloem.By assuming the ‘streaming transcellular strands’model for phloem transport, the rate of movement required togive the observed transfer has been computed as approximately4.5x10^-2 cm sec^-1 (160 cm h^-1).The earlier report of the existenceof a highly impermeable barrier surrounding the xylem vesselshas been further substantiated by the experiments reported here.     

A Study of the Exudation of Excised Maize Roots after Removal of the Epidermis and Outer Cortex

The fluid exudation rates and the ionic composition of the exudatesof isolated maize roots were compared with those of similarroots from which the epidermal and outer cortical cells hadbeen removed. The results indicate that in low ionic strengthmedia the salt fluxes into the exudation stream are severaltimes larger in the control than in the treated roots. As theionic strength of the bathing medium increases to 10 mM KClor greater, the salt fluxes become equal. A mechanism to accountfor this behaviour is discussed. The values of L_p, the hydraulic conductivity of the roots toradial water flow, are 0.26 x 10^–6 cm sec^–1 atm^–1for treated and 0.14 x 10^–6 cm sec^–1 atm^–1for control roots, but a statistical technicality precludeda significance test on this apparent difference. The kinetics of tritiated water exchange from external mediumto the exudation fluid in both control and treated roots werenot significantly different. The entry of labelled water seemsto be achieved by the fluid exudation rate in both cases; thusthe barrier to water movement to the exudate compartment remainsintact after removal of the epidermis.     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

Water Potential, Growth, and Polyribosome Content of the Stressed Wheat Apex

The effect of drought on the growth, ribosomal content, andwater potential of the immature floral apex of wheat plantswas studied under controlled environment conditions. Duringdrought the water potential of the apex (measured with a thermocouplepsychrometer) decreased at approximately the same rate as thatof expanded leaves. Elongation and differen tiation of the floralapex ceased at approximately –12 x 10⁵ Pa and the polyribosomalcontent decreased from 50% of the total ribosomal populationto less than 10%. At this water potential also, elongation ofexpanding leaves was severely inhibited. With continued drought the water potential of the apex continueddecreasing. The exposed leaves died at a water potential ofabout –35 x 10⁵ Pa but the apex was still alive at a waterpotential of –60 x 10⁵ Pa and after rewatering it eventuallyresumed growth.     

Translocation in Pteridium

The distributions of radioactivity with distance have been mappedin the rachis of Pteridium aquilinum at different times afterapplication of ¹⁴CO₂ to a pinna. Profiles change in shape withtime, from wave to transition and then to diffusion. The apparentdiffusion constant of sucrose may be obtained from transitionand diffusion profiles. Values varied from 2.2 x 10^–3to 3.6 x 10^–2 cm₂ sec^–1. These results are comparedwith those for various angiosperm species.     

Carbon Dioxide Exchange of Spring-wheat in Relation to Age and Photon-flux Density at Different Growth Temperatures

CO₂-exchange rates (CER) of the sixth and the flag leaves oftwo spring-wheat varieties, Kolibri and Famos, were comparedusing an open-circuit infrared gas analysing system. Measurementswere repeated every two weeks starting when leaf blades werefully expanded. Single plants were grown in a controlled environmenthaving a photopuiod of 15 h and a day/night temperature of 24/19°C(H), 18/13 °C (M), and 12/7 °C (L) respectively untilapprox. 2 weeks after anthesis and at 18/13 °C until maturity.The photosynthetic photon-flux density (PPFD) at the top ofthe plants was 500 µE m^–2 sec^–1. During themeasurements PPFD was gradually reduced from 2000 to 0 µEm^–2 sec^–1 whereas the temperature was maintainedat the respctive growth-temperatures during the light period.The CER of the sixth leaf declined fairly similarly for bothvarieties, except for Kolibri where a faster decline was observedduring the first two weeks after full leaf expansion. The CERof the flag leaf declined more slowly than that of the sixthleaf. With the flag leaf of Famos, the decline was nearly linear,whereas with Kolibri it was very slow during the first few weeksbut rapid as the leaves further senesced. This pattern becamemore pronounced as the growth temperature decreased. The declinein relation to leaf age was much smaller at low PPFD than athigh PPFD during the same period. At full leaf expansion Kolibrireached higher maximum CER than Famos except at H. As the PPFDwas reduced the difference became smaller and at very low PPFDsuch as 50 µE m^–2 sec^–1 was reversed for thesixth leaf. Under optimum growth conditions maximum values ofCER were greater than 50mg CO₂ dm^–2h^–1 and PPFDfor light saturation was close to 2000 µE m^–2 sec^–1.A comparison between the actual CER and a fitted curve widelyused, PN=(a+b/l)^–1–DR, showed that the goodnessof fit strongly depends on cultivar, treatment and leaf ageas well as on the number and the level of PPFD from which datafor calculations are taken. Triticum aestivum, L., wheat, photosynthesis, photon-flux density, light response, carbon, dioxide exchange     

Effects of Colchicine, Vinblastine, Cytochalasin B and Phalloidin on the Seismonastic Movement of Mimosa pudica Leaf and on Motor Cell Ultrastructure

Fleurat-Lessard, P., Roblin, G., Bonmort, J. and Besse, C. 1988.Effects of colchicine, vinblastine, cytochalasin B and phalloidinon the seismonastic movement of Mimosa pudica leaf and on motorcell ultrastructure.—J. exp. Bot. 39: 209–221. Colchicine at 1 x 10^–3 mol dm^–3 does not affectthe seismonastic movement of Mimosa pudica leaves but disruptsmicrotubules in motor cells. Vinblastine at 5 x 10^–3 moldm^–3 does not affect this movement and partly disruptsmicrotubules. Vinblastine at 1 x 10^–4 mol dm^–3 alwaysdisrupts microtubules, even after a 12 h reversibility whenthe movement is restored. These drugs, applied at the same respectiveconcentrations, do not alter cytoplasmic and vacuolar fibrils.Cytochalasin B and phalloidin alter the seismonastic movementof Mimosa leaves when applied at concentrations of 1.25 x 10^–3and 2.4 x 10^–4 mol dm^–3 respectively. These drugs,used at the same respective concentrations, also affect themotor cell structure and, in particular, modify the arrangementand the structure of the fibrils but they do not destroy themicrotubules. These data suggest that microtubules are not directly involvedin the seismonastic reaction whereas fibrils, formed by thin(3.0 nm wide) filaments, may be implicated in this reaction. Key words: Colchicine, cytochalasin B, phalloidin, Mimosa pudica, motor cells, vinblastine     

DNA Sequencing Using Ultra Small Amounts of Reagents and Template

One of the key points in the genome project is finding waysto reduce the running cost in DNA sequencing. One way is touse a highly-sensitive fluorescent DNA sequencer, where onlytrace amounts of template DNA and reagents are needed. An experimentalprotocol optimized for the trace amounts of DNA analysis wasestablished by using the hybridization reaction rate coefficientof primers on template DNA, which was estimated to be 7.5x10⁵M^–1 sec^–1 at 37°C. One femtomole of templateDNA with 0.001 unit of modified T7 DNA polymerase (SequenaseVer. 2.0) and also 0.45 fmol of M13 template DNA with 0.01 unitof Taq DNA polymerase were enough to sequence DNA of up to 400bases.     

Chloroplast DNA in Expanding Spinach Leaves

The proportion of chloroplast DNA in total DNA from spinachleaves has been measured using the second order reassociationkinetics of a ³H-labelled chloroplast DNA probe in total DNAextracts. There was no significant difference between the proportionof chloroplast DNA in the basal and distal halves of 2 cm leavesand in the distal halves of 5, 8, and 10 cm leaves. The meanof all the observations was 21.1 ± 0.7%. There was littlechange in the average total DNA content of cells from any ofthe leaves but cells from larger leaves contained 130–170chloroplasts while cells from the basal half of 2 cm leavescontained about 20 chloroplasts which were smaller than thosefrom the larger leaves. Consequently the average number of copiesof the plastome per chloroplast in large leaves was about 30(5 x 10^–15 g DNA) and in the smaller chloroplasts in thebase of 2 cm leaves was 200 (32 x 10^–15 g DNA). Stainingwith the DNA fluorochrome 4, 6-diamidino-2 phenyl indole (DAPI)showed 10–15 plastid nucleoid areas in chloroplasts oflarger leaves, suggesting there are 2–3 copies of theplastome per plastid nucleoid.     

Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. II. The Effects of Abscisic Acid and Cytokinins

Guttation was used as a non-destructive way to study the flowof water and mineral ions from the roots and compared with parallelmeasurements of root exudation. Guttation of the leaves of barley seedlings depends on age andon the culture solution. Best rates of guttation were obtainedwith the primary leaves of 6- to 7-day-old seedlings grown onfull mineral nutrient solution. The growing leaf tissue becomessaturated with K⁺ below 1.5 mM K⁺ in the medium, whereas K⁺concentration in the guttated fluid still increases furtheras K⁺ concentration in the medium is raised. At 3 mM K⁺ averagevalues of guttation were 1.4–2.4 mm³ h^–1 per plantwith a K⁺ concentration of 10–20 mM; for exuding plantsthe flow was 4.2–7.6 mm³ h^–1 per plant and K⁺ concentration35–55 mM. Abscisic acid (ABA) at 10^–6 to 10^–4 M 0–2h after addition to the root medium increased volume flow ofguttation and exudation and the amount of K⁺ exported. Threeh after addition of ABA both volume and amount of K⁺ were reduced.There was an ABA-dependent increase in water permeability (Lp)of exuding roots shortly after ABA addition. Later Lp was decreasedby 35 per cent and salt export by 60 per cent suggesting aneffect of ABA on salt transport to the xylem apart from itseffect on Lp. Benzyladenine (5 x 10^–8 to 10^–5 M)and kinetin (5 x 10^–6 M) progressively reduced volumeflow and K⁺ export in guttation and exudation and reduced Lp. Guttation showed a qualitatively similar response to phytohormonesas found here and elsewhere using exuding roots. Hordeum vulgare L., barley, guttation, abscisic acid, cytokinins, benzyl adenine, kinetin     

Purification of an Auxin-Binding Protein from Etiolated Mung Bean Seedlings by Affinity Chromatography

An auxin-binding protein with high affinity for 2,4-D and IAAwas purified from the extract of etiolated mung bean seedlingsby affinity chromatography on 2,4-D-linked Sepharose 4B andby gel nitration on Sepharose 4B. Its molecular weight was estimatedto be about 390,000 by gel nitration on Sepharose 4B and itconsisted of two different subunits with molecular weights ofabout 47,000 and 15,000. This protein had no ribulose-l,5-bisphosphatecarboxylase activity. Its dissociation constants for 2,4-D andIAA were 9.3 x 10^–6 M and 3.2 x 10^–6 M, respectively,as determined by Scatchard's method. (Received December 21, 1982; Accepted March 23, 1983)     

The Effect of Morphactin (Methyl 2-Chloro-9-hydroxyfluorene-9-carboxylate) on Basipetal Transport of Indol-3-ylacetic Acid in Hypocotyl Sections of Phaseolus vulgaris L.

The effect of morphactin (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate)on basipetal transport of auxin (Indol-3-ylacetic acid-2-¹⁴C)was studied in bean (Phaseolus vulgaris) hypocotyl with thedonor-receiver block method. Morphactin (5 x 10^–6m) reduced IAA (5 x 10^–6m) transportintensity by an average of 83 per cent and auxin transport capacityby 90 per cent, but transport velocity was not affected. Morphactin did not inhibit uptake of IAA into hypocotyl tissue,but it did prevent transfer of IAA from the tissue into receiverblocks. Chromatographic analysis of the tissue after 4 h IAA-2-¹⁴Ctransport showed that 54 per cent of the total activity wasin the form of IAA in the control and 42 per cent in the morphactintreated tissue. No difference was found in the rate of decarboxylationof IAA-1-¹⁴C between control and morphactin treated tissue sections.Nor could any difference between control and morphactin be shownin the radioactivity associated with a TCA ppt fraction. Ina study of the transportable auxin pool, morphactin decreasedthe size of the pool and increased the half-life of decay ofauxin transport from 1•22 h to 3•85 h. In a kineticanalysis of the reversal of morphactin (5 x 10^–6m) inhibitionby increasing concentration of IAA-2-14C (5 x 10^–6m to2 x 10^–5m), it was shown that IAA transport resemblesMichaelis-Menten enzyme reaction kinetics, and that inhibitionby morphactin fitted a ‘mixed type’ model. IAA hada dissociation constant of 8•5 x 10^–6m and morphactinthat of 4•3 x 10^–7m with a K_m for the transport processof 8•5 x 10^–6m.     

Matric Water Potential of Leaf Tissue--Measurement and Significance

A thermocouple psychrometer method, previously described foruse in determining tissue and sap water potentials, has beenadapted for determining matric potentials. Matric values ranging from approximately zero to –10 x10⁵ Pa were observed with wilting wheat leaves. Mean valuesof total tissue water potential and of its solute (osmotic),matric, and cell wall pressure components were –16.6,–18.3, –30 and +4.7 Pa x 10⁵, respectively. Matric potentials are often ignored in investigations of plantwater relations. This practice is shown to result in underestimationof wall pressures (in this case by a mean of 64 per cent) andsometimes in spuriously negative values.     

Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing and ingestion rates of laboratory-born Thalia democraticaaggregates and Dolioletta gegenbauri gonozooids, phorozooidsand oozooids were determined while fed Isochrysis galbana (4–5µm diameter) alone or in combination with Peridinium trochoideum(16–18 µm diameter) at concentrations of 0.15–0.70mm³ x 1^–1. Grazing rates (ml x zooid^–1 x 24 h ^–1)ranged from 10 to 355, and at zooid weights greater than 5 µgcarbon were in order oozooid > gonozooid > aggregate.Grazing rates increased exponentially with increasing zooidweight. Weight-specific grazing rates (ml x µgC^–1x 24 h^–1) were independent of the four-fold initial foodconcentration. Mean weight-specific grazing rates increasedlinearly with increasing zooid weight for the aggregates andoozooids, but gonozooid mean rates were independent of zooidweight. Aggregate and gonozooid ingestion rates (10⁶ µm³x zooid^–1 x 24 h^–1) ranged from 4 to 134 while oozooidrates ranged from 3 to 67. All ingestion rates were independentof the initial food concentration but increased linearly withincreasing zooid weight at similar rates. All mean weight-specificingestion rates (ml x µgC^–1 x 24 h^–1) wereindependent of zooid weight. The mean aggregate daily ration(µgC ingested x µg body C^–1) was 59% and themean doliolid ration was 132%. Field studies indicate that normalconcentrations of D. gegenbauri in the Georgia Bight clear theirresident water volume (1 m³) in about 4 months, but that highlyconcentrated, swarm populations which occur along thermohalinefronts clear their resident water volume in less than 1 day. ¹Current address: MacLaren Plansearch Ltd., P.O.Box 13250, sta.A.,St.John's, Nfld. A1B 4A5     

Microbial dynamics during the summer ice-loss phase in maritime Antarctic lakes

The dynamics of bacterioplankton and protozooplankton in twomaritime Antarctic lakes (Heywood Lake and Sombre Lake, SignyIsland, South Orkneys) were studied during the phase of icebreak-out (December and early January 1994/95). The lakes aresuffering animal-induced (fur seal) eutrophication, though HeywoodLake is most severely affected. Both lakes had morphologicallydiverse bacterial communities which increased during the studyperiod, reaching maxima of 80 x 10⁸ l^–1 in Heywood Lakeand 31.8 x 10⁸ l^–1 in Sombre Lake. Heterotrophic nanoflagellates(HNAN) reached a peak in late December with maxima of 40.6 x10⁸ l^–1 in Sombre Lake and 174 x 10⁵ l^–1 in HeywoodLake. Phototrophic nanoflagellates (PNAN) peaked in late Decemberafter ice loss in Heywood Lake (63 x 10⁵ l^–1), which coincidedwith a peak in a bloom of Chroomonas acuta which reached abundancesof 1.0 x 10⁸ l^–1. In Sombre Lake, ice persisted for alonger period and here PNAN reached their highest density atthe end of the study period (around 70.0 x 10⁵ l^–1). Ciliateabundance reached high levels in Heywood Lake (>60001^–1),while in Sombre Lake maximum abundance was 568l^–1. Protozooplanktondiversity was greater in the less-enriched Sombre Lake. Grazingrates of ciliates averaged 70.6 bacteria indiv.^–1 h^–1in Heywood Lake and 119.3 bacteria indiv.^–1 h^–1in Sombre Lake. The difference was a reflection of the differenttaxonomic make-up of the community in the lakes. HNAN grazingrates varied between 0.51 and 0.83 bacteria indiv.^–1 h^–1in Sombre and Heywood Lakes, respectively. Specific growth rates(r) h^–1 in Sombre Lake were 0.028 for ciliates and 0.013for HNAN, and in Heywood Lake 0.010 for ciliates and HNAN 0.012.These growth rates result in doubling times ranging between38 and 69 h for ciliates and around 55 h for HNAN.HNAN grazingon bacteria was curtailed in Heywood Lake in early January asa result of predation by microcrustacean larvae feeding on theplankton. Thus, for a short phase top-down control was apparentin the dynamics of Heywood Lake, a feature uncommon in Antarcticlake ecosystems. The impact of natural eutrophication on thesesystems is discussed in relation to other unaffected Antarcticlakes.     

The NADP$-specific isocitrate dehydrogenase of Rhodospirillum rubrum

The NADP^$-specific isocitrate dehydrogenase was partially purifiedfrom photosynthetically-grown Rhodospirillum rubrum. The pHoptimum is between 7.5 and 9.0 in phosphate buffer. The apparentKm is 3.1x10^–5 M for isocitrate, 5.1x10^–5 M forNADP^$, 1.7x10^–5 M for manganese, 1.5x10^–4 M formagnesium, and 3.5x10^–3 M for inorganic orthophosphate.Arsenate exerts a slight inhibition. The Q₁₀ between 17.5°Cand 40°C is 1.62, and the energy of activation at 25°Cis 9.74 Kcal/mole. Glyoxylate and oxalacetate cause concertedinhibition of the enzyme activity. Various nucleotides inhibitthe activity. The kinetics of inhibition by ATP was found tobe mixed type with respect to NADP^$ and isocitrate, the K_i valuesbeing 1.17x10^–3 M and 1.10x10^–3 M respectively.The inhibition between ATP and orthophosphate is competitivewith a K_i of 10^–4M. Thiol binding reagents are inhibitory;this inhibition is reversed by cysteine or reduced glutathione. (Received October 1, 1971; )     

Varietal Differences in the Kinetics of Iron Uptake by Excised Rice Roots

Iron uptake by excised rice roots can be described in kineticterms of active absorption and the formation of ion-carriercomplexes. Conventional interpretation indicates two carriers,one of which is responsible for iron absorption at low concentrations,with the second primarily functioning at high iron levels. Themaximum absorption of both carriers vanes greatly between varieties,the variety Pebifun having much greater absorptive capacitythan either Siam-29 or Paldal. Varietal differences in uptakecapacity primarily depend on total carrier concentration, 0.93,0.40,and 0.31 µ moles per g roots of Pebifun, Siam-29, andPaldal respectively. Although the kinetic treatment suggeststwo carriers, the chromatographic evidence indicates the presenceof the iron primarily in one major initial complex, with suggestionsof accumulation rather than turnover. An alternative to thecarrier hypothesis is therefore put forward which regards theaccumulation of iron as binding initially on to one or severalclosely related substances, followed by incorporation into secondaryproducts when a critical level of the initial product is exceeded.Manganese was found to competitively inhibit iron uptake byrice roots. Inhibition was more severe in Siam-29 than in Paldaland can be explained on the basis of carrier concentration andcompetition for the same carrier site. Copper was found notto be competitive for the iron absorption sites. The effectof copper was not significant with the low levels of iron butonly with the 5 x 10^–4 M Fe level. The effect varied betweenvarieties, in Pebifun both 5 x 10^–5 and 5 x 10^–6M Cu inhibited iron uptake compared with the control. In thecase of Siam-29 both copper levels accelerated iron uptake,5 x 10^–5 M Cu giving the greatest uptake. With 5 x 10^–5M Cu Pebifun took up much less iron than Siam-29. The mecharosmof copper inhibition/stimulation is highly speculative.