Nitrate and Other Anions in the Rice Phloem Sap

The anionic composition was determined for pure rice phloemsap which had been collected from brown planthopper styletsby cutting them with a YAG laser beam. The most abundant inorganicanion was Cl^– at 52.1 mM. PO₄^3– and SO₄^2–were found at 8.1 mM and 1.8 mM, respectively. NO₃^– waspresent at 1.9 mM when the rice had been grown with nutrientsolution containing 0.35 m of NO₃^– The phloem sap alsocontained malic, succinic and citric acid at 7.3, 3.0 and 3.2mM respectively. Small amounts of other organic acids were alsofound. (Received October 4, 1984; Accepted December 14, 1984)     

The Composition of Phloem Exudate and Xylem Sap from Tree Tobacco (Nicotiana glauca Grah.)

The composition of xylem sap and exudate from stem incisionsof Nicotiana glauca Grah. was compared in detail. Exudationfrom stem incisions occurred over a 5 min period in certainplants, enabling collection of 5–30 µl of sap. Therate of exudation showed an exponential decline. Exudate hada high dry matter content (170–196 mg ml^–1) andhigh sugar (sucrose) levels. Xylem sap had a low pH (5.8) andexudate a pH of 7.9. Glutamine dominated the amino compoundsin xylem sap and exudate, and K⁺ was the major cation. Totalamino compounds in stem exudate reached 10.8 mg ml^–1 whereasxylem sap contained much lower levels (0.28 mg ml^–1).All mineral elements and amino compounds with the exceptionof calcium were more concentrated in stem exudate than in xylemsap. Sucrose was labelled heavily in stem exudate following pulsingof an adjacent leaf with ¹⁴CO₂. A concentration gradient ofsugar (2.1 bar m^–1) was recorded for stems. Levels ofsucrose, amino compounds and K⁺ ions in stem exudate showeda diurnal periodicity. Each commodity reached maximum concentrationat or near noon and minimum concentration about dawn. The evidencesuggests that exudate from stem incisions of N. glauca is arepresentative sample of solutes translocated in the phloem. Nicotiana glauca Grah., phloem sap, xylem sap, sucrose, amino compounds, mineral ions     

Apoplastic Solute Concentrations of Organic Acids and Mineral Nutrients in the Leaves of Several Fagaceae

Ion chromatographic methods determined organic acids and mainnutrient minerals in the apoplastic solution from leaves ofseveral Fagaceae (Quercus ilex L., Quercus cerris L., Quercusvirgiliana (Ten.) Ten, and Fagus sylvatica L.). The anions oforganic acids found in high amounts (250 to 650 µM) werequinate, malate, and oxalate. Lactate, pyruvate, formate andacetate were detected in relatively low amounts with concentrationsbetween 20 and 200 µM. The total concentration of organicacids in the apoplastic sap ranged between 1.5 and 2 mM. Thetotal concentration of inorganic cations (K⁺, Mg²⁺, NH₄⁺, Ca²⁺,Na⁺) and anions (C1^–, NO₃^–, SO^2–₄ and PO^3–₄)in the apoplastic sap varied between 5 and 10 mM, and 0.35 and1.8 mM, respectively. We conclude that the concentration oforganic acid ions in the leaf apoplast depends mainly on theexchange with the leaf cells and is influenced by the electrochemicalgradient between the symplast and the apoplast in relation tothe water potential of the leaf. The determination of formateand acetate in the apoplastic compartment of leaves lend weightto the argument that the production of these acids by treesis a important emission source to the atmosphere. (Received June 9, 1998; Accepted April 8, 1999)     

Chemical Composition of Bleeding Xylem Sap from Kiwifruit Vines

A study of the chemical composition and charge balance was madeof bleeding xylem sap collected from excised one-year-old extensionshoots of healthy, Mn-deficient, Mn-toxic and Zn-deficient kiwifruitvines (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson)immediately prior to leafburst. The exudates were analysed formacronutrient cations and anions, trace elements, amino acids,organic acids and sugars. Major charged species measured wereCa (13.3 mM), K (8.9 mM), Mg (5.6 mM), malate (12.5 mM) andphosphate (5.8 mM). Glutamine (12 mM) was the predominant Ncarrier identified, accounting for 58 per cent of the totalN followed by NO₂^–-N (4.5 per cent), NH₄⁺-N (3.5 per cent)and arginine-N (2.9 per cent). Approximately 22 per cent ofthe N was in a hydrolysable proteinaceous fraction comprisingmainly glutamine and glutamate. Eighteen free proteinaceousamino acids were idetified in sap, the most abundant being glutamine,glutamic acid, valine, isoleucine and phenylalanine. Computersimulation of the chemical composition predicted that in additionto hydrated cations, ion pairs formed between inorganic components(SO₄^2–, HPO₄^2–, H₂PO₄^–) and cations (Ca²⁺,Mg²⁺, Mn²⁺), plus metal-organic ligand complexes (Ca Malate,Zn Malate, FeCit, CuHis, CuGln) are important species involvedin translocation. The solubility product of hydroxyapatite wasexceeded in all exudates although in vitro precipitation wasnot observed. To achieve electroneutrality with the componentsmeasured, however, formation of precipitate precursors (hydroxyapatitenuclei) had to be assumed. Irregularities in Mn nutrition (butnot Zn) were clearly indicated by the elemental compositionof exudate suggesting the use of sap analysis as a possiblepre-season indicator of nutritional status for this species. Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson, kiwifruit, xylem sap composition, trace metals, amino acids, organic acids     

IONIC COMPOSITION AND ELECTRIC RESPONSE OF LAMPROTHAMNIUM SUCCINCTUM

The concentrations of K⁺, Na⁺ and Cl^– in the cytoplasmof Lamprothamnium succinctum, a brackish water Characeae, areabout 137, 47 and 86 mM respectively. The concentration of K⁺in the cytoplasm is of the same order as that in the cell sap,while the concentrations of Na⁺ and Cl^– are much lowerthan those in the cell sap (i.e., 140 mM Na⁺, 370 mM Cl^–).In the brackish water, in which the plant grows, the internodesis never excitable electrically. However, it acquires excitabilitywhen it is kept in a mannitol-pond water. The action potentialthus elicited is accompanied by a temporary cessation or slowdown of the cytoplasmic streaming. ¹This work was supported by Research Grants from the Ministryof Education of Japan     

Nicotianamine and histidine/proline are, respectively, the most important copper chelators in xylem sap of Brassica carinata under conditions of copper deficiency and excess

The effect of two different copper conditions (deficiency andexcess) on the amino acid composition in B. carinata xylem sapwas analysed. When the Cu in the nutrient solution was increasedfrom 0.12 to 2.5 or 5 µM, the concentrations of histidine,threonine, glutamine, proline, methionine, and glycine weremuch increased in the xylem sap. When Cu was made deficientin the nutrient solution by decreasing its concentration from0.12 µM to 0 µM, nicotianamine, glutamine, and threoninewere significantly increased in the xylem sap. Aqueous solutionscontaining different Cu–amino acid complexes (simulatedsaps) responded in a specific way to the changes in pH, providinga signature that was used to evaluate, by comparison with thereal xylem sap, the importance of each amino acid in the xylemtransport of Cu. For a single amino acid, the free solutionCu²⁺ concentration versus pH titration curves for histidineand proline were the most similar to that for xylem under Cuexcess. Under Cu deficiency, this Cu concentration versus pHtitration curve appeared to be very similar to that for nicotianamine.It is concluded that increased Cu concentrations induced theselective synthesis of certain amino acids in the sap, of whichhistidine and proline are the most important. Under Cu deficiency,the concentration of nicotianamine was induced the most. Thefact that nicotianamine is induced under Cu starvation and notunder Cu excess, is in contrast to similar studies indicatingspecies-specific reactions. However, the induction of nicotianamineunder Cu starvation is in line with recent molecular data ofthe role of nicotianamine in intracellular Cu delivery. Key words: Brassica carinata, copper, histidine, nicotianamine, proline, xylem sap Received 30 September 2008; Revised 16 October 2008 Accepted 20 October 2008     

Osmotic and ionic regulation in Nitella

When the osmotic value of an internodal cell of Nitella flexiliswas modified by the method of transcellular osmosis, the normalosmotic value was chiefly restored by the release or absorptionof K⁺. The release or uptake of Na⁺ was observed only when themodification of osmotic value was significant. Both the uptakeand release of K⁺ were linearly dependent on the degree of modificationof the osmotic value. The effectiveness of alkali metal cationsin restoring the osmotic value in cells of lower osmotic valueswas in the order K⁺>Rb⁺>Na⁺, Cs⁺>Li⁺. The absorptionof K⁺ by cells of lower osmotic values depended strongly ontemperature, while the release of K⁺ from cells of higher osmoticvalues did not. To clarify whether the Nitella cell regulates the osmotic valueor regulates the concentration of K⁺ in the vacuole, the cellsap was exchanged for artificial cell saps whose osmotic valuesand ionic concentrations were varied independent of each other.It was shown that in Nitella two regulating mechanisms are operating,one which regulates the osmotic value of the cell sap irrespectiveof the level of vacuolar K⁺ (0.1–140 mM) and another whichregulates the vacuolar K⁺-level when it is abnormaly high (>160mM). Both mechanisms are assumed to operate in order to keepthe concentration of K⁺ in the cytoplasm at a constant level.The presence of Na⁺ (0–100 mM) and Ca²⁺ (5–40 mM)did not affect the movement of K⁺ during osmoregulation. ¹Present address: Sanki Engineering Limited, Nagaokakyo, Kyoto,Japan. (Received December 19, 1973; )     

Inhibition of Flowering in the Long-Day Plant Lemna gibba G3 by Hutner's Medium and its Reversal by Medium Modification

The long-day plant Lemna gibba G3 flowers normally in E medium(Hoagland-type medium plus 30 µM EDTA) but in 0.5 H mediumthere is no flowering. Ammonium is present in 0.5 H medium andis known to inhibit flowering in L. gibba G3, but even in NH₄⁺-free0.5 H medium there is virtually no flowering under continuouslight. Increasing the phosphate concentration of the NH₄⁺-free0.5 H medium from 1.15 ITIM to 12 or 16 mM results in substantialflowering. Decreasing the EDTA concentration from 850 µIMto 250 µM, or raising the nitrate concentration from 4mM to 12 mM, results in only a small increase in flowering.If the decrease in EDTA and increase in nitrate are combinedwith the increase in phosphate, however, the flowering responseis nearly as good as that obtained using E medium. Thus, withthese three changes the inhibitory effect of NH₄⁺free 0.5 Hmedium for flowering in L. gibba G3 is almost completely reversed In the above studies flowering was not limited by daylength.When plants were grown on E medium under an 11 hour daylengthwhere flowering is limited by daylength, decreasing the phosphateconcentration in the medium reduced flowering, but increasingthe phosphate concentration in the medium did not stimulateflowering. Thus, when flowering is limited by daylength, highphosphate will not cause flowering, but a certain level of phosphateappears to be necessary for the expression of photoinductionunder long days. (Received January 14, 1986; Accepted June 24, 1986)     

Chemical Composition of Phloem Sap from the Uppermost Internode of the Rice Plant

The chemical composition of phloem sap from the uppermost internodeof rice plants (Oryza sativa L., var. Kantou), one week afteranthesis, was compared with that of phloem sap from the leafsheath of a young seedling. The pure phloem sap from rice plantswas collected by an insect laser technique. The phloem sap from the uppermost internode contained a highlevel of sucrose (573.8 mM) which was the only sugar detected.The concentrations of total amino acids, potassium and ATP were124.8 mM, 40.4 mM and 1.76 mM, respectively. The concentrationof sucrose was three times higher and the potassium level wasone third as high in the internode sap as in the phloem sapfrom the leaf sheath. The total concentration of amino acidswas almost the same, but the relative amount of each amino acidwas quite different. The ratios of levels of Glu to Gln andof levels of Asp to Asn in the phloem sap from the uppermostinternode were smaller than those in the phloem sap from theleaf sheath. The adenylate energy charge was 0.92–0.93in both types of phloem sap. The amino acid composition of the phloem sap from the uppermostinternode was compared with that of the phloem sap of the flagleaf and the endosperm sap of the same plant, one week afteranthesis. The differences in composition along the phloem pathwaysuggest the selective translocation of amino acid. (Received July 21, 1989; Accepted December 11, 1989)     

Accumulation and Conversion of Sugars by Developing Wheat Grains: V. THE ENDOSPERM APOPLAST AND APOPLASTIC TRANSPORT

The volume and composition of the endosperm apoplast of thedeveloping wheat grain, comprising endosperm cavity and intercellularfree-space, was examined in relation to kernel growth rate andsize. Samples of the cavity sap were collected by centrifugationof kernels during the linear phase of grain growth. The cavitysap contained 10–50 mM sucrose, a small amount of hexosesbut a high concentration of oligosaccharides (up to 9 timesthat of sucrose). In comparing cvs Yandilla King and Cleveland,high growth rate was associated with high cavity sap sucroseconcentration but with low K⁺ concentration. K⁺ concentrationin the endosperm cells (124 mM) was about 5 times higher thanin the cavity sap (10–40 mM). Cavity sap pH was 6.3–6.6.The uptake of sucrose by endosperm cells was partly inhibitedby PCMBS, an inhibitor of membrane-bound carriers. Several necessaryconditions for proton cotransport during sucrose uptake by endospermcells were met. The volume of the intercellular free-space, estimated by membranepermeating (¹⁴C-mannitol, ¹⁴C-sucrose) or non-permeating (³H-PEG900)markers averaged 2.2 µl or 5–7% of the water ingrains of cvs Yandilla King, Cleveland and SUN 9E. The cavityvolume was highly variable but tended to be larger in largergrains. Pulse labelling of ¹⁴CO₂ to flag leaves showed that ¹⁴C-sucrosewas the principal ¹⁴C-assimilate in the cavity sap and was convertedto insoluble compounds in the endosperm while the cavity sapoligosaccharides acquired negligible label in 6 h. Key words: Wheat, Endosperm apoplast, Sugars     

Stimulatory Effect of Chloride Ions on NADP Malic Enzyme from Leaves of two C4 Species of Cyperus

NADP malic enzyme (EC 1.1.1.40 [EC] ) from leaves of two C₄ speciesof Cyperus (C. rotundus and C. brevifolius var leiolepis) exihibiteda low level of activity in an assay mixture that contained lowconcentrations of Cl^–. This low level of activity wasmarkedly enhanced by increases in the concentration of NaClup to 200 mM. Since the activity of NADP malic enzyme was inhibitedby Na₂SO₄ and stimulated by relatively high concentration ofTris-HCl (50–100 mM, pH 7–8), the activation ofthe enzyme by NaCl appears to be due to Cl^–. Variationsin the concentration of Mg²⁺ affected the K_A (the concentrationof activator giving half-maximal activation) for Cl^–,which decreased from 500 mM to 80 mM with increasing concentrationsof Mg²⁺ from 0.5 mM to 7 mM. The K_m for Mg²⁺ was decreased from7.7 mM to 1.3 mM with increases in the concentration of NaClfrom zero to 200 mM, although the increase of V_max was not remarkable.NADP malic enzyme from Cyperus, being similar to that from otherC₄ species, was able to utilize Mn²⁺. The K_m for Mn²⁺ was 5mM, a value similar to that for Mg²⁺. The addition of 91 mMNaCl markedly decreased the K_m for Mn²⁺ to 20 +M. NADP malicenzyme from Setaria glauca, which contains rather less Cl^–than other C₄ species, was inactivated by concentrations ofNaCl above 20 mM, although slight activation of the enzyme wasobserved at low concentrations of NaCl at pH7.6. (Received February 20, 1989; Accepted June 12, 1989)     

Osmotic Adjustment and Osmotic Constituents in Roots of Mung Bean Seedlings

Osmotic adjustment in roots of mung bean seedlings (Vigna mungo(L.) Hepper) and the effect of cotyledon excision on the osmoticadjustment were investigated. The major osmotic constituentsin roots of intact seedlings were K⁺, Cl^–, free aminoacids and sugars (glucose, fructose and sucrose). All theseintracellular concentrations distinctly increased under osmoticstress and contributed to about 80% of the intracellular osmoticpressure of the root cell sap. Cotyledon excision remarkablysuppressed both the osmotic adjustment and the elongation inroots. However, the effect of cotyledon excision on intracellularK⁺ and Cl^– concentrations in roots was quite small. Twodifferent mechanisms are likely for the osmotic adjustment inroots. One is the K⁺ and Cl^–-dependent osmotic adjustmentwhich is cotyledon-independent, and the other is the osmoticadjustment dependent on the supply of free amino acids and sugarsfrom cotyledons. (Received September 20, 1986; Accepted January 14, 1987)     

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     

Apoplastic Sugar Concentration and pH in Barley Leaves Infected with Brown Rust

Soluble sugars were extracted by low speed centrifugation fromthe apoplast of leaves of barley (Hordeum distichum L.) infiltratedwith water. Infection of the leaf with the brown rust fungus(Puccinia hordeii) resulted in a reduction in the concentrationof sucrose, glucose and fructose in the apoplast. Sugars werepresent in an apoplastic space occupying 12 and 17 cm³ m^–2of leaf area in healthy and infected tissue, respectively. Uptakeof hexoses by intercellular hyphae is suggested as a cause ofthis reduction. The pH of apoplastic sap extracted from rust-infectedleaves was increased to pH 7·3 from pH 6·6 incontrols. The effect of a reduced apoplastic sugar pool andincreased pH on export from infected leaves is discussed. Key words: Apoplast, barley (Hordeum distichum L.), brown rust (Puccinia hordeii Otth.), pH, sucrose, hexose     

Growth and Osmoregulation of Chaetoceros muelleri in Relation to Salinity

The growth and osmoregulation of Chaetoceros muelleri Lemmermann(Bacillariophyceae) were investigated as a function of salinity.This centric diatom grew well over a wide range of salinityand required concentrations of NaCl above 10 mM for growth.Using gas chromatography- mass spectroscopy (GC-MS) analysisof cell extracts, we demonstrated that the alga contains anisomer of cyclohexanetetrol. The level of this isomer increasedwith increasing salinity. Levels of free amino acids also increasedwith increasing salinity, and quantitative determination withan amino acid analyzer revealed that the level of glutamic acidincreased the most with increases in salinity. Levels of intracellularK⁺ and Cl^– also increased significantly with increasesin salinity. Thus, in C. muelleri, not only organic solutessuch as the cyclohexanetetrol isomer and glutamic acid, butalso inorganic solutes such as K⁺ and Cl^– contribute toosmoregulation. (Received November 7, 1994; Accepted April 10, 1995)     

Plasma Membrane H+-ATPase in Guard-Cell Protoplasts from Vicia faba L.

The activity of plasma membrane H⁺-ATPase was measured withmembrane fragments of guard-cell protoplasts isolated from Viciafaba L. ATP hydrolytic activity was slightly inhibited by oligomycinand ammonium molybdate, and markedly inhibited by NO₃^–and vanadate. In the presence of oligomycin, ammonium molybdateand NO₃^–, the ATP-hydrolyzing activity was strongly inhibitedby vanadate. It was also inhibited by diethylstilbestrol (DES),p-chloromercuribenzoic acid (PCMB) and Ca²⁺, but slightly stimulatedby carbonyl cyanide m-chlorophenylhydrazone (CCCP). The acitivityhad higher specificity for ATP as a substrate than other phosphoricesters such as ADP, AMP, GTP and p-nitrophenylphosphate; theK_m was 0.5 mM for ATP. The activity required Mg²⁺ but was notaffected by K⁺, and it was maximal around pH 6.8. When guard-cellprotoplasts were used instead of membrane fragments, the ATPaseactivity reached up to 800µmol Pi.(mg Chl)^–1.h^–1in the presence of lysolecithin. These results indicate thatthe guard cell has a high plasma membrane H⁺-ATPase activity. (Received December 23, 1986; Accepted April 28, 1987)     

Effect of Potassium on Sucrose and Amino Acid Release from the Seed Coat of Developing Seeds of Pisum sativum

After removal of the embryo from developing seeds of Pisum sativum,the ‘empty’ ovules (seed coats without enclosedembryo) were filled with a solution (pH 5.5) containing mannitol(usually 400 mM) to which various salts were added. A solutioncontaining two isotopes ((a) [²H]-sucrose/[–¹⁴C]aminoisobutyricacid (AIB) or (b) [³H]valine/[₁₄C]asparagine mixture) was administeredto the plant via the petiole subtending the fruiting node, and[²H]solute and [¹⁴C]solute unloading from the seed coat wasmeasured, in pulse-labelling experiments of about 5 h. The presenceof 25 or 50 mM K⁺ in the ‘empty’ ovule enhancedthe release of sucrose from the seed coat particularly duringthe first hours of the experiment, but the stimulating effectof K⁺ on the release of labelled solutes derived from aminoacids was much smaller. The presence of 25 mM CaCl₂ did notaffect the release of sucrose or amino acids from the seed coat.The effect of K⁺ on sucrose and amino acid release is explainedas an inhibition of sucrose and amino acid resorption from theseed coat apoplast into seed coat cells, after unloading fromthe seed coat unloading sites. It is suggested that amino acidrelease is much less affected by K⁺ than sucrose release, becausefar less resorption of amino acids by seed coat parenchyma cellstakes place during amino acid transport into the seed coat cavity. Pisum sativum, pea, assimilate transport, assimilate unloading, seed-coat exudate, seed development, sucrose resorption, surgical treatment     

Thermodynamics of Malate Transport across the Tonoplast of Leaf Cells of CAM Plants

The electrochemical potential difference for each dissociationstate of malic acid across the tonoplast of leaf cells was examinedin two CAM plants, Graptopetalum paraguayense and Kalanchoëdaigremontiana. The concentration of malic acid in each dissociationstate was estimated from an analysis of pH and concentrationsof ionic species that included calcium, malate and isocitrate.The vacuoles contained 30–40 mM isocitrate and 50–70mM calcium in G. paraguayense, and 20–30 mM isocitrateand 70–100 mM calcium in K. daigremontiana. For the calculationof the pattern of dissociation of malic acid, the formationof chelates of calcium with malate and isocitrate, which havedifferent stability constants depending on the dissociationof the acids, were also taken into consideration. The vacuolarconcentrations of the divalently dissociated form of malic acid(mal^2– were 4–7 mM and 1-3 mM in G. paraguayenseand in K. daigremontiana, respectively. To obtain informationabout the cytoplasmic concentration of malate, the apparentinhibition constant for malate of phosphoenolpyruvate carboxylasewas measured. It was about 330 µM in the dark period and60 µM in the light period. Considering an inside-positivemembrane potential, we conclude that mal^2– can be takenup passively into the vacuole during the dark period and canbe released passively from the vacuole during the light period.Two types of channel (the "SV-type" channel and a novel "MU-type"channel) which we found recently in G. paraguayense [Iwasakiet al. (1992) Plant Physiol. 98: 1494] are probably involvedin the uptake and the release of malate in the diurnal CAM rhythm.The existence of a large pH-buffering capacity due to isocitricacid in the vacuole allows the accumulation of a large amountof malic acid during the diurnal CAM rhythm. (Received February 12, 1992; Accepted July 10, 1992)     

Effect of soil compaction on barley (Hordeum vulgare L.) growth: II Are increased xylem sap ABA concentrations involved in maintaining leaf expansion in compacted soils?

The abscisic acid (ABA)-deficient mutant of barley, Az34, exhibiteda much reduced rate of leaf expansion at a bulk density of 1.6g cm^–3 as compared to the isogenic wild-type variety,Steptoe. Az34 had a consistently lower xylem sap ABA concentrationat 7 d and 14 d after emergence (DAE). The xylem sap data suggestthat ABA present at Steptoe concentrations may have a directrole in maintaining leaf expansion at the sub-critical bulkdensity (1.6 g cm^–3 To test this hypothesis, additionof synthetic ABA either to the rooting environment (100 nM)or directly to the xylem sap (5 pg µl^–1 to reproducethe xylem sap ABA concentrations found in Steptoe, increasedleaf expansion in Az34 to the wild-type level. Furthermore,feeding Steptoe xylem sap to Az34 produced similar effects.These experiments provide direct evidence of a positive rolefor ABA as a root-to-shoot signal which assists in maintainingleaf growth in plants experiencing subcritical levels of compactionstress. Key words: ABA-deficient mutant, leaf expansion, xylem sap, ABA     

Pyruvate Uptake by Mesophyll and Bundle Sheath Chloroplasts of a C4 Plant, Panicum miliaceum L.

Intact chloroplasts were isolated from mesophyll and bundlesheath protoplasts of a C₄ plant, Panicum miliaceum L., to measurethe uptake of [1-¹⁴C]pyruvate into their sorbitol-impermeablespaces at 4?C by the silicone oil filtering centrifugation method.When incubated in the dark, both chloroplasts showed similarslow kinetics of pyruvate uptake, and the equilibrium internalconcentrations were almost equal to the external levels. Whenincubated in the light, only mesophyll chloroplasts showed remarkableenhancement of the uptake, the internal concentration reaching10–30 times of the external level after 5 min incubation.The initial uptake rate of the mesophyll chloroplasts was enhancedabout ten fold by light and was saturated with increasing pyruvateconcentration; K_m and V_max were 0.2–0.4 mM and 20–40µmol(mg Chl)^–1 h^–1, respectively. The lightenhancement was abolished by DCMU and uncoupling reagents suchas carbonylcyanide-m-chlorophenylhydrazone and nigericin. Theseresults indicate the existence of a light-dependent pyruvatetransport system in the envelope of mesophyll chloroplasts ofP. miliaceum. The uptake activity of mesophyll chloroplastsboth in the light and the dark was inhibited by sulfhydryl reagentssuch as mersalyl and p-chloromercuriphenylsulfonate, but thebundle sheath activity was insensitive to the reagents. Thesefindings are further evidence for the differentiation of mesophylland bundle sheath chloroplasts of a C₄ plant with respect tometabolite transport. (Received July 3, 1986; Accepted October 8, 1986)