Metabolism of Exogenous Malonate by Coffee Leaf Tissue

When [l-¹⁴C]-malonate was supplied to discs cut from matureleaves of Coffea arabica, ¹⁴CO₂ was released (approximately12% of the total CO₂ respired) and organic acids of the Krebscycle, uronic acids, sugars and amino acids became radioactive.There was no incorporation of MC into either lipids or phenoliccompounds. The formation of glucose from malonate has not beenobserved in other studies with plant tissues. The synthesisof labelled glucose together with an active pentose phosphatepathway that is stimulated by malonate explains the accumulationof radioactive phosphogluconate in the leaf discs. Tentativeproposals are made for pathways to account for the results obtained. Key words: Coffee leaves, Malonate metabolism, Pentose phosphate pathway     

Nectar Production and Floral Characteristics of Tropaeolum majus L. Grown in Ambient and Elevated Carbon Dioxide

Tropaeolum majus (nasturtiums) were grown from seed in growthcabinets, under 380 and 750 ppmv CO₂. Elevated CO₂significantlyincreased nectar secretion rate, both in flowers milked of nectardaily and in once sampled, 3-d-old flowers. Elevated CO₂didnot affect time to flowering, total number of flowers produced,pollen to ovule ratio, or the total or individual concentrationsof nectar amino acids. The dry weight and longevity of individualflowers was also unchanged. Nectar sugar content was unchangedby elevated CO₂in a subset of flowers used to assess the 3-d-oldnectar volume. This subset did not show the same increase innectar volume under elevated CO₂as the full set, resulting inthe concentration of sugars remaining unchanged. Overall, thequantity rather than the quality of the nectar changed underelevated CO₂while flower characteristics remained constant,implying that the identity of pollinators may remain the samewhile foraging behaviour (e.g. number of visits per plant, distancetravelled) may change in the future. Copyright 1999 Annals ofBotany Company Tropaeolum majus, nasturtiums, elevated CO₂, nectar, phenology, floral characteristics, amino acids, pollinator-plant interactions.     

Estimation of the Freezing Injury in Flower Buds of Evergreen Azaleas by Water Proton Nuclear Magnetic Resonance Relaxation Times

Temperature dependence of longitudinal relaxation times (T₁)of water protons in flower buds of six azalea species differingin cold hardiness and ecological distribution was investigatedby pulse nuclear magnetic resonance spectroscopy. Thermal hysteresiswas observed for T₁ following a slow freeze-thaw cycle. TheT₁ ratio (the ratio obtained from the difference between theoriginal T₁ value in an unfrozen sample and the final T₁ aftera freeze-thaw treatment, both at 20C, divided by the originalT₁) was closely correlated with the viability of florets innon-acclimated buds of R. kiusianum. If the buds were frozento a lethal temperature and then thawed to 20C, the T₁ ratioincreased. The T₁ ratios of acclimated winter buds for the sixspecies used were correlated with the level of cold hardiness(supercooling ability of florets determined by differentialthermal analysis). The T₁ ratio of deacclimated spring buds,especially those from hardier species, markedly increased uponcooling to a lethal temperature. Species differences observedin acclimated winter buds disappeared upon deacclimation. TheT₁ ratio appears to be related to the viability of florets andthe degree of freezing damage (membrane disruption) in florets. (Received December 28, 1984; Accepted May 24, 1985)     

14C Photoassimilate Partitioning in Developing Sunflower Seeds

Changes with age of protein and oil content in field-grown Helianthusannuus seeds were followed during the grain filling period.Seeds were sampled from different zones on the flower head:peripheral, intermediate, and central. Regardless of seed position,at maturity protein and oil content accounted for approximately18% and 50% of the dry weight respectively. In an attempt todetermine the importance of the role of photosynthesis in grain-filling,¹⁴CO₂ was incorporated into sunflower leaves and translocatedradioactive photoassimilates in the seed were studied. ¹⁴C-labelledproducts including carbohydrates, nitrogenous compounds andlipids were determined as a function of seed position, lengthof chase period, and seed age. Within 8 h, ¹⁴C-labelled photosynthateis detected in the seed, up to 80% of seed-incorporated radioactivitybeing in the form of free sugars and organic acids. The conversionrate from free sugars to storage compounds (lipids, proteinsand starch) varies according to seed position and age. Lipidsconstitute the major photosynthetic sink, reaching levels ofgreater than 80% of the total seed-incorporated radioactivity.The effects of abscisic acid on uptake and partitioning of ¹⁴Csucrose into immature excised cotyledons were also studied. Key words: Sunflower, photosynthate, abscisic acid     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

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)     

Anthocyanin Influence on Water Proton NMR Relaxation Times and Water Contents in Leaves of Evergreen Woody Plants During the Winter

An investigation was made to determine the effect of anthocyaninin red-colored evergreen leaves during the mid-winter on waterproton NMR relaxation times (T₁). Water contents, anthocyanincontentsand histologic localization of red-coloration in mesophyllswere determined by using both red-colored and green leaves fromthe same branches of Rhododendron, Viburnum and Mahonia, respectively.Although the decrease of water contents in the red-colored leavesin Mahonia was insignificant, decreases in the former two specieswere clearly observable. T₁ differences between red-coloredand green leaves for the three species were insignificant. Increasesof anthocyanin contents and histologic localization of red colorationin mesophylls for the red-colored leaves were more pronouncedin Rhododendron and Viburnum than in Mahonia. These observationssuggest that the pronounced increases of histologic localizationof red-colored mesophyll cells and anthocyanin contents in red-coloredleaves for the former two species contribute to maintenanceof T₁ relaxation times in spite of the marked decrease of watercontents in leaves. It is assumed that the increase of localizationareas of red-colored parenchymatous cells in mesophylls is moreeffective than the total contents of anthocyanins in leavestowards the maintenance of the T₁ level in red-colored leaves,and this appears to be dependent on the vacuolar compartmentationof anthocyanin in mesophyll cells. (Received August 3, 1991; Accepted December 12, 1991)     

Relationship of Nuclear Magnetic Resonance Relaxation Time to Water Content and Cold Hardiness in Flower Buds of Evergreen Azalea

The longitudinal relaxation time (T₁) of water protons in floretsof R. ? akebono flower buds was measured with a pulse NMR spectrometerto determine the relationship of T₁ to water content and coldhardiness (supercooling ability). Seasonal changes of T₁ inflorets were closely correlated with water content and supercoolingability of florets. T₁ of florets was short for acclimated budshaving a low water content and long for non-acclimated budshaving a high water content. Flower buds collected in Novemberand stored at 0 and 5?C for 4 weeks had shorter T₁ values thanbuds stored at 10?C even though the floret water content andsupercooling ability were similar. Thus, the short T₁ of coldacclimated buds hardened naturally or by storage at low temperaturesis due to a combination of both reduced water content and temperature. (Received August 27, 1983; Accepted May 26, 1984)     

Changes in Anthocyanin, Carotenoid, Chlorophyll, and Protein in Developing Florets of the Chrysanthemum

Florets of a purple cultivar (Fandango) of the horticulturalchrysanthemum (Chrysanthemum morifolium Ramat) were removedfrom flower heads at seven stages of opening (from unopenedbud to dying flower) and segregated into different lengths,each of which was analysed. Wet weight per floret increased from 0.25–1 mg in thebud to about 3 mg (tubular florets) or to 20–40 mg (rayflorets) in the fully open flower. Protein decreased from 6per cent of the wet weight in the bud to about 2 per cent inthe open flower. In the ray florets anthocyanin concentrationreached a maximum in the half-open flower and then decreasedsharply, whereas carotenoid and chlorophyll declined continuouslyfrom the bud stage. Almost no anthocyanin was formed by thetubular florets and chlorophyll declined as in the ray florets,but carotenoid concentration increased to a maximum in the half-openflower and then decreased. In another cultivar (Light Bronze Fandango) the content of anthocyaninwas lower and that of carotenoid higher but similar changesin pigment levels were observed except that carotenoid roseto a maximum in the ray florets. In two other cultivars, Redand Cerise Fandango, the anthocyanin content was the same asin Fandango but the carotenoid concentration was the same orabout half that in Light Bronze Fandango respectively.     

Seasonal Changes in Activities of Enzymes Responsible for the Formation of C6-aldehydes and C6-alcohols in Tea Leaves, and the Effects of Environmental Temperatures on the Enzyme Activities

When tea leaves were homogenized and incubated, the volatileC₆-compounds hexanal, cis-3-hexenal, cis-3-hexenol and trans-2-hexenalwere formed much more by summer leaves than by winter leavesof tea plants (Camellia sinensis). The enzymes lipolytic acylhydrolase (LAH), lipoxygenase, fatty acid hydroperoxide lyase(HPO lyase) and alcohol dehydrogenase (ADH) and an isomerizationfactor were responsible for the sequential reactions of C₆-compoundformation from linoleic and linolenic acids in tea leaf lipids,and there were seasonal changes in their activities. The tealeaf enzymes were of 3 types: LAH and lipoxygenase, which hadhigh activities in summer leaves and low activities in winterleaves; ADH, which had low activity in summer leaves and highactivity in winter ones; and HPO lyase and the isomerizationfactor, which did not seem to have any effect on the rate ofC₆-compound formation throughout the year. Changes in enzymeactivities were induced by shifts in the environmental air temperaturerather than by the age of the leaves. The combined activitiesof these enzymes determined the amounts and compositions ofthe volatile C₆-compounds formed, which are the factors thatcontrol the quality of the raw leaves processed for green tea. (Received October 6, 1983; Accepted December 20, 1983)     

Free Sugars and Organic Acids in the Leaves of Various Plant Species and their Compartmentation Between the Tissues4

The distribution of free sugars and organic acids between theepidermis and mesophyll of Tulipa gesneriana L., Vicia fabaL., and Commelina communis L. leaves was studied using mainlygas-liquid chromatography. Fructose, glucose, sucrose, and myo-inositol were found in theepidermis and mesophyll of all three species. In T. geenerianaleaf tissues arabinose (trace levels), stachyose, tuliposidesA and B (mainly in the mesophyll), and xylose (trace levelsalso in V. faba tissues) were also detected. The acids were more difficult to detect and identify, beingat considerably lower concentrations than the sugars in bothtissues. Fumaric, citric, malic, ascorbic (trace levels), andan unidentified acid were common to the epidermis and mesophyllof all three species. Of special interest was the detectionof large amounts of glyceric acid in the epidermis and mesophyllof V. faba; this acid was not detected in the tissues from theother species. Fumaric acid was also very abundant in the epidermisof V.faba. A special study was made of the compartmentation of acids andsugars between the epidermis and mesophyll of T. geenerianaleaves after light and dark treatments. No changes in free acidor sugar levels were detected in the epidermis or mesophyllafter these treatments. Except for suceinic acid (P < 0·05),there were no statistically significant differences in acidlevels between the epidermis and mesophyll but for most of thesugars (myo-inositol, arabinose, and xylose being exceptions)differences were highly significant (P < 0·001), highestlevels occurring in the mesophyll. The differences in sugarlevels and the similarity in acid levels between epidermis andmesophyll of tulip leaves were considered to be essentiallydue to the different CO₂ fixing mechanisms and capacities ofthe two tissues. The energy source for the essentially non-greenepidermal tissue was discussed.     

The Effects of Light, Osmotic Potential and Atmospheric Gases on Germination of the Mistletoe Amyema preissii

The high osmotic concentration of the viscid layer and non-volatilegermination inhibitors cannot account for the prolonged dormancyof the seeds of Amyema preissii while they remain within theintact fruit. Once excised, however, germination could not beprevented by depriving the seed of an external supply of water,oxygen or light. Elongation of the hypocotyl, bearing the primaryhaustorium of the young seedling, was enhanced by conditionsthat favoured photosynthesis as well as respiration. The pericarpis considered to prevent germination by acting as a barrierto influx of exogenous O₂ (critical under dark conditions),and efflux of endogenous CO₂ (critical under light conditions).     

Water Stress Induced Changes in the Amounts of Some Photosynthetic Assimilation Products and Respiratory Metabolites of Sunflower Leaves

Sunflower leaves, water-stressed under controlled conditions,contained greater amounts of amino acids as their water potentialdecreased, with glycine, serine, and glutamate increasing morethan alanine and aspartate. Proline accumulated only at severestress. Low O₂ concentration altered the amounts of amino acids,principally decreasing the amount of glycine and increasingserine. The changes in total pool size are related to previousresults on the accumulation of 14C and the specific activityof products. Photorespiration was large under water stress,where leaves accumulated carbon in glycine of low specific activity,and in 21% O₂, where both total amount and specific activityof glycine was greater than in 1.5% O₂. This suggests that thereare two pools of glycine, one controlled by O₂ and closely relatedto photosynthesis, the other non-photosynthetic and affectedby water stress. The organic acids suocinate, citrate, and fumarate increasedat small leaf-water potentials. Sucrose decreased in amountwith stress and was absent at the most severe stress; therewas less glucose and fructose. The amount of carbon lost fromsugars was similar to the amount accumulated in amino acidstogether with the carbon lost in respiration. It is concluded that stress decreased the flux of carbon fromphotosynthesis for the synthesis of amino acids and sugars butmore carbon from stored materials, principally sucrose, wasused in the production of organic acids and amino acids.     

The Role of Nitrate in the Osmoregulation of Lettuce (Lactuca sativa L.) Grown at Different Light Intensities

Blom-Zandstra, M. and Lampe, J. E. M., 1985. The role of nitratein the osmoregulation of lettuce (Lactuca sativa L.) grown atdifferent light intensities.—J. exp. Bot. 36: 1043–1052. The effect of different light intensities on the nitrate accumulationvis-à-vis the concentration of other solutes in plantsap expressed from lettuce leaves was studied. After growinglettuce plants under constant environmental conditions for 52d, they were transferred to different light intensities andharvested periodically. A quantitative analysis of componentsin solution in the expressed plant sap showed a decrease innitrate concentration and an increase in the organic acids (mainlymalate) and sugars (mainly glucose) with increasing light intensity.The light intensity only slightly increased the osmolarity ofthe expressed plant sap. The measured osmolarity correspondedvery well with the value estimated from the quantitative analysesimplying that all osmotically active compounds had been accountedfor. The decrease in nitrate concentration in the expressedplant sap was fully compensated for by an increase in the dissociatedorganic acids that partly dissociate twofold to sustain electroneutralityand by an increase in both organic acids and sugars to maintainthe osmolarity. The suggestion is supported that nitrate mayserve as osmoticum at low light conditions to compensate forthe shortage of carbohydrates resulting from suboptimal photosynthesis. Key words: Nitrate accumulation, osmoregulation, Lactuca saliva L.     

Root Respiration and Carbohydrate Status of Two Wheat Genotypes in Response to Hypoxia

To investigate root respiration and carbohydrate status in relationto waterlogging or hypoxia tolerance, root respiration rateand concentrations of soluble sugars in leaves and roots weredetermined for two wheat (Triticum aestivum L.) genotypes differingin waterlogging-tolerance under hypoxia (5% O₂) and subsequentresumption of full aeration. Root and shoot growth were reducedby hypoxia to a larger extent for waterlogging-sensitive Coker9835. Root respiration or oxygen consumption rate declined withhypoxia, but recovered after 7 d of resumption of aeration.Respiration rate was greater for sensitive Coker 9835 than fortolerant Jackson within 8 d after hypoxia. The concentrationsof sucrose, glucose and fructose decreased in leaves for bothgenotypes under hypoxia. The concentration of these sugars inroots, however, increased under hypoxia, to a greater degreefor Jackson. An increase in the ratio of root sugar concentrationto shoot sugar concentration was found for Jackson under hypoxicconditions, suggesting that a large amount of carbohydrate waspartitioned to roots under hypoxia. The results indicated thatroot carbohydrate supply was not a limiting factor for rootgrowth and respiration under hypoxia. Plant tolerance to waterloggingof hypoxia appeared to be associated with low root respirationor oxygen consumption rate and high sugar accumulation underhypoxic conditions.Copyright 1995, 1999 Academic Press Oxygen consumption rate, sugar accumulation, Triticum aestivum L., waterlogging tolerance     

Assimilation of gaseous ammonia and the transport of its products in barley and spinach leaves

The interactions between the assimilation and transport of nitrogenand carbon were investigated in barley and spinach leaves. Bothplants were fumigated with NH₃ (1 mg m^–3 and the contentof amino acids, sucrose and carbon intermediates of amino acidmetabolism were analysed in the leaves, apoplast and phloemsap. The following changes took place in the C- and N-metabolismof barley leaves during 5 h of fumigation with NH₃ (a) The contentsof amino acids, especially glutamine, largely increased andthe contents of sucrose, 2-oxoglutarate, phosphoenolpyruvate,and glycerate-3-phosphate declined. (b) A decrease in the phophoenolpyruvatecontent was accompanied by an increased activity of phosphoenolpyruvatecarboxylase. (c) The altered cytosolic concentrations of aminoacids and sucrose during NH₃ fumigation correlated with similarchanges in the apoplast and phloem sap. The altered percentageof each amino acid relative to the total amino acid concentrationin the cytosol, caused by NH₃ fumigation, is reflected in theapoplast and the phloem sap. The results indicate that the concentrations of amino acids in the cytosol determine their concentrationsin the phloem. Key words: Amino acids, ammonia fumigation, barley leaves, C: N partitioning, phosphoenolpyruvate carboxylase, phloem sap, spinach leaves     

Relationship between photosynthetic carbon metabolism and essential oil biogenesis in peppermint under Mn-stress

Changes in growth and yield parameters, and ¹⁴CO₂ and (U-¹⁴C)sucrose incorporation into the primary metabolic pool, and essentialoil have been investigated under Mn-deficiency and subsequentrecovery in Mentha piperita, grown in solution culture. UnderMn-deficiency, CO₂ exchange rate, total chlorophyll, total assimilatoryarea, plant dry weight, and essential oil yield were significantlyreduced, whereas chlorophyll a/b ratio, leaf area ratio andleaf stem ratio significantly increased. In leaves of Mn-deficientplants, ¹⁴CO₂ incorporation into the primary metabolic pool(ethanol-soluble and -insoluble) and essential oil were significantlylower, whereas (U-¹⁴C) sucrose incorporation into these componentswas significantly higher as compared to the control. Among theprimary metabolites, the label was maximum in sugars, followedby organic acids and amino acids. A higher label in these metaboliteswas, in general, observed in stems of Mn-deficient plants ascompared to the control. Mn-deficient plants supplied with completenutrient medium for 3 weeks exhibited partial recovery in growthand yield parameters, and essential oil biogenesis. Thus, underMn-deficiency and subsequent recovery, the levels of primaryphotosynthetic metabolites and their partitioning between leafand stem significantly influence essential oil biogenesis. Key words: Mentha piperita, Mn-stress, ¹⁴CO₂ and [U-¹⁴C] sucrose incorporation, oil accumulation, primary photosynthetic metabolites     

The Photosynthetic Activity of Cox's Orange Pippin Apple Flowers in Relation to Fruit Setting

The ability of green parts of Cox's apple flowers (sepals, receptacleand pedicel) to photosynthesise was demonstrated using ¹⁴CO,feed experiments and an infra-red gas analysis (IRGA) system.The sepals had the greatest ability, comparable with that ofleaves, followed by the receptacle. Sepals and leaves fixedthe same amount of CO₂ in the dark. Photosynthesis and respirationrates in orchard flowers were measured in comparison to thoseof leaves. Flower gross photosynthesis (GP) was about a thirdof that of leaves at the balloon stage and 15 d after full bloomin 1987, but this ratio was much less at full bloom (FB) inthe same year. GP in flowers decreased rapidly with fruitletenlargement. It was estimated that flower photosynthesis significantly contributedto their carbohydrate balance (15-33%) during the floweringand fruit setting periods. The highest contributions were made at the green cluster stage(33%) and between 6 and 11 d after FB (27%), the period whenthe fruit set had been initiated. Removal of sepals, the mainphotosynthetic parts of the flower, at the pink bud stage decreasedinitial set. The results suggest that flower photosynthesismight play an important role in flower growth and fruit setting.Copyright1994, 1999 Academic Press Apple, photosynthesis, respiration, fruit setting, flower photosynthesis, sepal removal     

Inhibition of taste nerve responses to sugars and amino acids by cupric and zinc ions in mice

Inhibition of chorda tympani nerve responses to sugars and aminoacids by CuCl₂ and ZnCl₂ was studied in ddy mice. Responsesto sugars (sucrose, fructose, glucose and maltose) and Na saccharinwere markedly depressed after adaptation of the tongue to 0.1mM CuCl₂ or ZnCl₂. No significant difference in the amount ofinhibition by either salt was found among sugars. The concentration-responsecurve for sucrose was shifted towards the right along the abscissaby treating the tongue with CuCl₂ or ZnCl₂ at 0.1 mM. Reciprocalplots of the concentration-response relationship yielded straightlines, which intersected at a point along the ordinate. Theresults indicate that binding of surcrose as well as other sugarsto sweet receptor molecules is competitively inhibited by Cu²⁺or Zn²⁺. Responses to amino acids (Gly, L-Ala, L-Ser, L-Pro,L-Val and D-Trp) were either slightly or scarcely inhibitedby 0.1 mM CuCl₂ or ZnCl₂. Reciprocal plots of the concentration-responserelationship for Gly before and after adaptation of the tongueto either metal salt failed to yield straight lines. It is proposedthat amino acids would bind to a small proportion of sweet receptormolecules and, in addition, stimulate other receptor mechanismsresponsible for initiating impulses in fibers responsive totaste stimuli other than sucrose.     

Exposure of Leaves of Cucumis sativus L. to Low Temperatures in the Light Causes Uncoupling of Thylakoids I. Studies with Isolated Thylakoids

Chilling of leaves of cucumber (Cucumis sativus L.) at 5?C inmoderate light for 5 h caused almost complete suppression ofphotosynthetic oxygen evolution in the leaves. Comparison ofelectron transport activities determined in the presence andabsence of an uncoupler, methylamine, indicated that thylakoidsprepared from such treated leaves were uncoupled without anysignificant changes in the capacity for electron transport.Immunoblotting revealed that the amount of coupling factor 1(CF₁) associated with membranes was reduced by the chillingtreatment in the light. Thylakoids prepared from leaves thathad been chilled in moderate light for 5 h and then re-warmedfor 1 h were coupled and capable of synthesising ATP. However,the capacity of leaf photosynthesis was not restored by therewarming. These results indicate that the thylakoids are uncoupledby the dissociation of some CF₁ complexes from the thylakoidmembranes during the chilling treatment of leaves in the lightand that thylakoids are recoupled by reassociation of CF₁ duringthe subsequent rewarming of the treated leaves at 25?C. It alsoappears that chilling in the light causes irreversible damageto reaction(s) other than those involved in electron transportand photophosphorylation. ¹ Present address: Department of Biological Science, Facultyof Science, Himeji Institute of Technology, Kamigori, Ako-gun,Hyogo, 678-12 Japan (Received July 1, 1991; Accepted October 4, 1991)