Changes of Carbohydrates in Pepper (Capsicum annuum L.) Flowers in Relation to Their Abscission Under Different Shading Regimes

Abscission of pepper flowers is enhanced under conditions oflow light and high temperature. Our study shows that pepperflowers accumulate assimilates, particularly in the ovary, duringthe day time, and accumulate starch, which is then metabolizedin the subsequent dark period. With the exception of the petals,the ovary contains the highest total amounts of sugars and starch,compared with other flower parts and contains the highest totalactivity, as well as activity calculated on fresh mass basis,of sucrose synthase, in accordance with the role of this enzymein starch biosynthesis. Low light intensity or leaf removaldecreased sugar accumulation in the flower and subsequentlycaused flower abscission. The threshold of light intensity fordaily sugar accumulation in the sink leaves was much lower thanin flowers, resulting in higher daytime accumulation of sugarsin the sink leaves than in the adjacent flower buds under anylight intensity, suggesting a competition for assimilates betweenthese organs. Flowers of bell pepper cv. ‘Maor’and ‘899’ (sensitive to abscission) accumulatedless soluble sugars and starch under shade than the flowersof bell pepper cv. ‘Mazurka’ and of paprika cv.‘Lehava’ (less sensitive). The results suggest thatthe flower capacity to accumulate sugars and starch during theday is an important factor in determining flower retention andfruit set. Pepper; Capsicum annuum L.; abscission; shading; pepper flowers; ovary; leaves; sugars; starch; acid invertase; sucrose synthase     

Partitioning of [14C]sucrose and Acid Invertase Activity in Reproductive Organs of Pepper Plants in Relation to Their Abscission Under Heat Stress

The effect of heat stress on processes related to carbohydratepartitioning was investigated in young bell pepper (Capsicumannum L. cv. Maor) plants in relation to abscission of theirreproductive organs at different stages of development. None of the reproductive organs abscised after 5 d in a normalday/night temperature regime (25/18°C). With a temperatureregime of 35°C day, 25°C night, abscission occurredin only a small portion of the flower buds and none of the flowersand fruitlets. However, when temperatures in the day and nightwere reversed (25/35°C, day/night) all the buds and someof the flowers abscised during that time period. The young fruitat the first node did not abscise under any temperature regime.The abscission rate of the flower buds was reduced under heatstress if the developing fruit at the first node had been removed. High temperature during either the light or dark periods reducedthe export of [¹⁴C]sucrose from the source leaf (fed for 48h with [¹⁴C]sucrose). Both heat stress and fruit presence reduced the relative amountof [¹⁴C]sucrose which was exported to the flower buds, flowersand roots. Likewise, these treatments reduced the concentrationof reducing sugars in the reproductive organs. Concomitantly,the heat stress and fruit presence on the first node reducedthe activity of soluble acid invertase in the flower buds andthe roots, but not in young leaves. Overall, the results show that heat stress causes alternationin sucrose distribution in the plant, but may also have specificeffects on metabolic activities related to sucrose import andutilization in flower buds and flowers which in turn may enhancetheir abscission. Bell pepper, (Capsicum annuum L. cv. Maor), abscission, acidinvertase, heat stress, reproductive organs, sink leaves. Bell pepper, (Capsicum annuum L. cv. Maor), abscission, acid invertase, heat stress, reproductive organs, sink leaves.     

Partitioning of [14C]sucrose and Acid Invertase Activity in Reproductive Organs of Pepper Plants in Relation to Their Abscission Under Heat Stress

The effect of heat stress on processes related to carbohydratepartitioning was investigated in young bell pepper (Capsicumannum L. cv. Maor) plants in relation to abscission of theirreproductive organs at different stages of development. None of the reproductive organs abscised after 5 d in a normalday/night temperature regime (25/18 °C). With a temperatureregime of 35 °C day, 25 °C night, abscission occurredin only a small portion of the flower buds and none of the flowersand fruitlets. However, when temperatures in the day and nightwere reversed (25/35 °C, day/night) all the buds and someof the flowers abscised during that time period. The young fruitat the first node did not abscise under any temperature regime.The abscission rate of the flower buds was reduced under heatstress if the developing fruit at the first node had been removed. High temperature during either the light or dark periods reducedthe export of [¹⁴C]sucrose from the source leaf (fed for 48h with [¹⁴C]sucrose). Both heat stress and fruit presence reduced the relative amountof [¹⁴C]sucrose which was exported to the flower buds, flowersand roots. Likewise, these treatments reduced the concentrationof reducing sugars in the reproductive organs. Concomitantly,the heat stress and fruit presence on the first node reducedthe activity of soluble acid invertase in the flower buds andthe roots, but not in young leaves. Overall, the results show that heat stress causes alternationin sucrose distribution in the plant, but may also have specificeffects on metabolic activities related to sucrose import andutilization in flower buds and flowers which in turn may enhancetheir abscission. Bell pepper, (Capsicum annuum L. cv. Maor), abscission, acid invertase, heat stress, reproductive organs, sink leaves     

Carbohydrate Metabolism in Drought-Stressed Leaves of Pigeonpea (Cajanus cajan)

Pigeonpea is a tropical grain-legume, which is highly dehydrationtolerant. The effect of drought stress on the carbohydrate metabolismin mature pigeonpea leaves was investigated by withholding waterfrom plants grown in very large pots (50 kg of soil). The moststriking feature of drought-stressed plants was the pronouncedaccumulation of D-pinitol (1D-3-methyl-chiro-inositol), whichincreased from 14 to 85 mg g^–1 dry weight during a 27d stress period. Concomitantly, the levels of starch, sucroseand the pinitol precursors myo-inositol and ononitol all decreasedrapidly to zero or near-zero in response to drought. The levelsof glucose and fructose increased moderately. Drought stressinduced a pronounced increase of the activities of enzymes hydrolysingsoluble starch (amylases) and sucrose (invertase and sucrosesynthase). The two anabolic enzymes sucrose phosphate synthase(sucrose synthetic pathway) and myo-inositol methyl transferase(pinitol synthetic pathway) also showed an increase of activityduring stress. These results indicate that pinitol accumulatedin pigeonpea leaves, because the carbon flux was diverted fromstarch and sucrose into polyols. Key words: Drought, polyols, pinitol, sucrose, starch, pigeonpea     

Soluble carbohydrates in Delphinium and their influence on sepal abscission in cut flowers

A carbohydrate other than sucrose, glucose, fructose and myo -inositol was detected in sepal extracts of Delphinium . This compound was identified as mannitol by ¹H-NMR. Mannitol was the major carbohydrate in all examined organs: the sepal, the other parts of the flower, the stem and leaves. Mannitol as well as glucose (both at 0.55 M ), fed to cut Delphinium flowers, similarly delayed the abscission of sepals. 3- O -methyl glucose (3-OMG) and polyethylene glycol 200 at the same molar concentrations had no such effect. The treatment with glucose markedly increased the concentrations of glucose and fructose in the sepals without changing the concentrations of sucrose and mannitol. On the other hand, the treatment with mannitol increased the concentrations of glucose and fructose in addition to mannitol in the sepals, suggesting that mannitol is metabolized in Delphinium flowers. The treatment with 3-OMG increased the concentration of 3-OMG but not other carbohydrates. Mannitol and glucose similarly delayed the increase in ethylene production in flowers, but 3-OMG did not. The sensitivity to ethylene was similarly reduced by the treatment with glucose and mannitol, but not by 3-OMG. These results suggest that the treatment with mannitol, a major carbohydrate in Delphinium , delayed the abscission of sepals by reducing the sensitivity to ethylene. Mannitol further acted, not merely as an osmolyte, but as an apparent source for carbohydrate metabolism in the flower.     

The Abscission of Rose Petals

Petal abscission was studied in twelve hybrid tea rose (Rosahybrida L.) cultivars. At about 20 °C the time to petalabscission in uncut stems in greenhouses was the same as incut stems placed in water in the greenhouse or in a climate-controlledroom. The time between petal unfolding and abscission dependedon the cultivar, and varied between 12 and 35 d. The time topetal abscission of the cultivars was inversely correlated withtheir flower diameter at full bloom (linear regression, r² =0·82). In the cultivars with a relatively large flowerdiameter (10-18 cm) the petals fell without visible desiccationsymptoms, whereas in the group with a small diameter the petalswere partially or fully desiccated when shed. Fertilization occurred in some flowers of a few cultivars studied.In cultivars with a relatively large flower diameter (Papa Meilland,Cocktail, Dr. Verhage, Tineke) it had no effect on the timeto abscission in Motrea, Europa, and Carolien roses, which bearsmall flowers, the petals fell after fertilization, whereasin unfertilized flowers of the latter group of cultivars anabscission zone just above the uppermost node became activeand all parts above this node (pedicel and flower) turned brownand desiccated, though remained attached for more than a month. It is concluded that in the cultivars investigated: (a) thetime to petal abscission was inversely related to their flowerdiameter, (b) abscised petals were more desiccated in cultivarsin which the time to abscission was longer, (c) fertilizationhad little effect on the time to abscission in most cultivars,whereas the absence of fertilization prevented petal abscissionin a number of the small-diameter cultivars where it was replacedby flower abscission, and (d) cutting and placement in waterat 20 °C did not affect the time to abscission.Copyright1995, 1999 Academic Press Abscission, fertilization, flowers, petals, Rosa hybrida L., rose, water stress, carbohydrate stress     

Modulation of Sucrose Content by Fructose 2,6-bisphosphate during Photosynthesis in Rice Leaves Growing at Different Light Intensities

Reddy, A. R. and Das, V. S. R. 1987. Modulation of sucrose contentby fructose 2,6-bisphosphate during photosynthesis in rice leavesgrowing at different light intensities.—J. exp. Bot. 38:828–833. The relationship between the rate of CO₂ fixation and sucroseconcentration in the leaves of rice (Oryza sativa L.) grownat different light intensities was investigated. Maximum sucrosecontent coincided with maximum rates of CO₂ fixation, achievedat a photon flux density of 1600 µmol m^–2 s^–1.The levels of sucrose and fructose 2,6-bisphosphate were alsocompared in the leaves under different light intensities. Fructose2,6-Msphosphate accumulated during growth at low light. Theactivity of fructose-6-phosphate 2-kinase was high in the leavesgrown at low light while that of fructose-2,6-bisphosphatasewas low. The activities of phosphoglucose isomerase and phospho-glucomutasewere slightly increased by growth at low light The activitiesof UDP glucose pyrophosphorylase were adversely affected invitro with increased concentrations of fructose 2,6-bisphosphatewhile those of sucrose phosphate synthase were moderately affected.Phosphoglucose isomerase and phosphoglucomutase were activatedby fructose 2,6-bisphosphate (8-0 mmol m^–3) by 12-15%.The results suggested that low light intensities during growthresult in an accumulation of fructose 2,6-bisphosphate whichmodulates the key enzymes of sucrose biosynthesis thus regulatingcarbon flow under conditions of limited photosynthesis. Key words: Oryza sativa, photosynthesis, sucrose synthesis, fructose 2,6-bisphosphate, light     

The Effect of Fruit Load, Defoliation and Night Temperature on the Morphology of Pepper Flowers and on Fruit Shape

The shape and regularity of bell pepper (Capsicum annuumL.)fruit are known to be determined at a very early stage of flowerdevelopment. Small, flattened fruit which are commonly parthenocarpicdevelop under low-temperatures (below 16 °C) from flowerswith enlarged ovaries. In such flowers self-pollination is notefficient because of the large distance between the stigma andstamens. Flower deformation of this kind is common during thewinter season. In the present study it was found that deformationsof flowers, similar to those found under low temperatures, wereinduced in 15 d by complete removal of fruit from plants growingunder night-time temperatures of 18 °C. Only flowers whichwere at the pre-anthesis stage at the time of fruit removalwere deformed by this treatment. Removal of leaves from thelower part of the plant (source leaves) partially reduced theeffect of fruit removal on the shape of the flowers and on subsequentfruit morphology. Fruit removal induced significant increasesin the concentrations of starch and reducing sugars, but notsucrose, in the flower buds. Likewise, flower buds of plantswhich grew under a night-time temperature of 12 °C containedmore carbohydrate than those which grew at 18 °C. Theseresults suggest that flower morphology in pepper is at leastpartly controlled by source-sink relationships. Assimilateswhich are normally transferred to developing fruit may be transported,upon fruit removal, to the flower buds which subsequently swell.A similar increase in assimilate translocation to flower budsmay occur under low temperatures, subsequently causing deformationof fruit.Copyright 1999 Annals of Botany Company Pepper, (Capsicum annuumL), flower shape, low temperatures, source-sink relationship, fruit shape, seeds, reducing sugars, sucrose, starch.     

Density Studies on Lupins: I. Flower Development

In an August-sown experiment the pattern of flower developmentwas followed for cv. Ultra (Lupinus albus L.) and cv. Unicrop(L. angustifolius L.) grown at low (10 plants m^–2) andhigh (93 and 83 plants m^–2, Ultra and Unicrop respectively)densities. Dry weight increase of flowers on the main-stem inflorescenceand first lateral below the main-stem were compared at differentfloral stages. Maximum flower weight was reached just priorto the open flower stage and remained constant or declined untila pod formed or abscission occurred. The time period betweenmaximum flower weight and pod formation or abscission was upto 10 days. Emergence of the inflorescence was earlier and thefirst flower of Ultra opened 10 days before Unicrop. Developmentof each terminal raceme (inflorescence) was acropetal, withpods having formed on lower flower nodes when terminal flowerswere still quite immature. Laterals forming the next generationof inflorescences grew from axillary leaf buds below an inflorescencewhile it was in full flower. Sources of competition from connectedreproductive and vegetative metabolic sinks are discussed. Lupinus spp., lupins, flower development, planting density     

Ethylene-Enhanced Transport of 14C-Labeled Metabolites in Rice Seedlings in Relation to Ethylene-Stimulated Coleoptile Growth

To examine the effects of ethylene on sugar transport from endospermsto coleoptiles in rice (Oryza sativa L. cv. Sasanishiki) seedlings,the contents of free sugars in the coleoptiles of explants fedcold glucose and the distributions of ¹⁴C-activities after feedingof ¹⁴C-glucose to the scutella were determined at various timesafter ethylene application. Changes in sucrose, glucose andfructose in the cold glucose-fed explants exposed to ethylenewere similar to those in the ethylene-treated intact seedlingshaving endosperms. Ethylene enhanced the transport of ¹⁴C-labeledmetabolites from the scutella to the coleoptiles. Most of the¹⁴C accumulated in the ethylene-treated coleoptiles were presentas neutral substances in the ethanol-soluble fraction. Regardlessof the presence or absence of ethylene, the incorporation of¹⁴C into sucrose preceded that into glucose and fructose. Theglucose and fructose moieties of ¹⁴C-sucrose in the coleoptileswere almost equally labeled, and the specific activities of¹⁴C-sucrose were higher than those of ¹⁴C-glucose and ¹⁴C-fructose.These results suggested that sucrose synthesized in the scutellawas exported to the coleoptiles, and cleaved there into glucoseand fructose. Ethylene may accelerate the transport of ¹⁴C-labeledmetabolites by activating sucrose cleavage in the coleoptiles. (Received July 1, 1985; Accepted September 17, 1985)     

Action Mechanism of Ethylene in the Control of Sugar Translocation in Relation to Rice Coleoptile Growth I. Sucrose Metabolism

The fate of ¹⁴C-glucose fed through scutella of rice (Oryzasativa L. cv. Sasanishiki) seedling explants was investigatedin relation to ethylene action on sugar translocation to growingcoleoptiles and leaves. In the scutellum, sucrose, UDPglucoseand F6P were rapidly labeled, and sucrose-phosphate synthaseactivity was higher than sucrose synthase activity. Radioactivesucrose soon appeared in both coleoptiles and leaves, and increasedrapidly. Its specific activity in both tissues became almostequal to that in the scutella. The specific activities of ¹⁴C-glucosein both coleoptiles and leaves changed almost in parallel tothose of ¹⁴C-fructose. These results suggest that sucrose wassynthesized in the scutellum and exported to the coleoptileand leaf, where it was cleaved to glucose and fructose. Ethylene slightly increased the specific activities of ¹⁴C-sucrosein all tissues, but markedly increased those of ^l4C-glucoseand -fructose only in the coleoptile. We assume that the ethyleneenhancement of sucrose transport from scutellum to the coleoptileresults from the activation of sucrose unloading in the growingcoleoptile where imported sucrose is cleaved into glucose andfructose. (Received May 25, 1987; Accepted October 30, 1987)     

Activity of Key Enzymes Involved in Carbohydrate Metabolism in Phaseolus vulgaris Cell Suspension Cultures

Activities of some key enzymes of glycolysis and sucrose metabolismwere investigated in relation to the physiological growth stagein bean cell suspension cultures. Activities of sucrose synthase,pyrophosphate:fructose-6-phosphate phosphotransferase, ATP:fructose-6-phosphatephosphotransferase, UDP glucose pyrophosphorylase, acid andalkaline invertase were detected. Both pyrophosphate:fructose-6-phosphatephosphotransferase and sucrose synthase activities increasedduring the active phase of cell division. Thereafter activitiesbegan to decline when sugar in the medium was depleted. Theincrease in enzyme activities coincided with a sharp decreasein the endogenous sucrose, glucose and fructose levels. Thelargest change occurred in the activity of sucrose synthase,which was more than seven fold higher in logarithmic phase cellsthan in lag hase cells. Transfer of mid-logarithmic phase cellsto fresh medium, containing 93 mmol dm^–3 sucrose, or additionof sucrose to existing medium, resulted in a further increasein PPjifructose- 6-phosphate phosphotransferase and sucrosesynthase activities. ²Present address: Plant Biotechnology Research Centre, PrivateBag X293, Pretoria 0001, Republic of South Africa.     

Influence of Different Carbon Sources on Invertase Activity and Growth of Sour Cherry (Prunus cerasus L.) Shoot Cultures

The influence of different sugars on shoot multiplication invitro and on the activity of invertase was studied with sourcherry (Prunus cerasus L.) cultures. The sugars sucrose, glucoseand fructose, and the sugar alcohol sorbitol, were investigatedat a wide range of concentrations. The optimum concentrationsof all carbon sources were 2% and 3% (w/v). Sucrose and glucosefavoured a similar rate of proliferation. However, in the presenceof fructose, proliferation was lowest but was coupled with thehighest frequency of formation of long shoots. The highest activityof total invertase was for tissues growing on a sucrose-containingmedium while, in a sugar-free medium, invertase activity wasmainly found in the ‘salt extracted’ fraction. Inthe remaining treatments, ‘soluble invertase’ dominated.For each sugar investigated, the activity of both forms of invertasewas significantly higher at 2% than at 3% (w/v) sugar. Key words: Tissue culture, sugars, invertase, shoot multiplication, sour cherry     

The Role of Glucose on Flower Bud Formation in Thin-Layer Tissue 12Nicotiana tabacum L.

The effect of glucose on flower bud formation was studied inthin-layer tissue cultures of epidermal strips from flower stalksof Nicotiana tabacum L. cv. Samsun. A minimum concentration of 30 mol m^–3 glucose in the MS-mediumcontaining 1.0 mmol m^–3 of both NAA and BA was necessaryfor flower bud formation. With 150 mol m^–3 glucose a minimumstay of 10 d was required for optimal flower bud formation. Withholding glucose for a limited period at different time intervalsafter the onset of culture caused a delay in flower bud formationand did not affect previous development on glucose. The resultsindicated that competence for flower bud initiation is not restrictedto the early stage of culture. The process may start at anytime later at the appropriate glucose concentration. However,for both optimal initiation and further development of flowerbuds the presence of a metabolizable sugar is required. Incubationof the tissue on glucose is associated with higher respirationrate. Key words: Flower formation, Glucose, mannitol, Nicotiana tabacum, Respiration, tissue culture     

Regulation of Stem Abscission and Callus Growth in Shoot Explants of Sweet Orange [Citrus sinensis (L.) Osbeck]

Two-node explants from Sweet Orange cv. St Ives Valencia orangeshoots produced prolific callus and formed secondary abscissionzones within internodes when cultured in vitro with abscisicacid (ABA, 5 µM) or -naphthaleneacetic acid (NAA, 5 µM).Benzyladenine (BA, 1 µm) induced callus but had littleeffect on abscission. Secondary abscission zone formation wasassociated with ABA-induced and auxin-induced ethylene formation.Treatment of explants with inhibitors of ethylene synthesis[aminoethoxyvinyl glycine (AVG), Co²⁺, PO₄^3–] preventedformation of secondary abscission zones but had variable effectson callus formation. Newly made explants contained high concentrationsof endogenous ABA (up to 6000 ng g^–1 f.wt), as measuredby GC/MS/SIM. Long-term subculture of explants (two years) inmedia containing BA (1 µm) led to a reduction in endogenousABA level (40 ng g^–1 f. wt) and to loss of capacity toform extensive callus and secondary abscission zones. Citrus sinensis (L.) Osbeck cv. St Ives Valencia, sweet orange, secondary abscission zones, in vitro, ethylene, endogenous ABA, endogenous IAA     

Effect of Temperature on Starch Degradation in Chlorella vulgaris 11h Cells

Analysis of products formed in Chlorella vulgaris 11 h cellsduring photosynthesis in air containing 3,000 ppm ¹⁴CO₂ at varioustemperatures revealed that the level of ¹⁴C-starch was maximumaround 20–24?C and decreased with further rise in temperatureuntil 40?C, while ¹⁴C-sucrose greatly increased at temperaturesabove about 28?C. Elevating the temperature from 20 to 38?Cduring photosynthetic ¹⁴CO₂ fixation resulted in a remarkabledecrease in ¹⁴C in starch and a concomitant increase in ¹⁴Cin sucrose. This conversion of starch to sucrose when shiftingthe temperature from 20 to 38?C proceeded even in the dark.Hydrolysis of sucrose by rß-fructosidase showed that,irrespective of the experimental conditions, the radioactivitiesin sucrose were equally distributed between glucose and fructose.The enhancement of starch degradation with temperature risewas more remarkable than that of the activity of ribulose bisphosphatecarboxylase from the same cells. When Chlorella cells whichhad been preloaded with ¹⁴C-starch after photosynthesis for30 min at 20?C were incubated in the dark for an additional30 min at 20?C, ¹⁴C-starch was degraded by only about 4%. However,the values after 30-min dark incubation at 28, 32, 36 and 40?Cwere increased by about 10, 19, 36 and 50%, respectively. Duringthe temperature-dependent conversion of starch to sucrose, nosignificant amount of radioactivity accumulated in free glucoseand maltose. (Received October 27, 1981; Accepted January 9, 1982)     

Invertase Activity, Soluble Carbohydrates and Inflorescence Development in the Tomato (Lycopersicon esculentum Mill.)

The concentration of reducing sugars in the developing firstinflorescence of the tomato (Lycopersicon esculentum Mill.)increased steadily between the macroscopic appearance of theflower buds and the initial stages of fruit expansion. Overthis period sucrose concentrations remained relatively constant.The rise in reducing sugar concentration was accompanied byan increase in the activity of an acid invertase. In individualflower buds invertase activity rose to a maximum shortly beforeanthesis and declined sharply as the anthers dehisced. Increased planting densities and removal of source leaves reducedthe rate of dry matter accumulation by the first inflorescenceand increased the incidence of flower bud abortion. These changeswere correlated with reductions in reducing sugar concentrations,in reducing sugar/sucrose ratios and in acid invertase levels.Removal of young leaves at the shoot apex significantly increasedthe relative growth rate of the inflorescence and led to a substantialincrease in its invertase content. These treatments had relativelylittle effect on sucrose concentration in the inflorescence. The data are consistent with the operation of an invertase-mediatedunloading mechanism for transported sucrose at sinks in theflower buds. It is suggested that the retarded development ofthe first inflorescence and the high incidence of flower budabortion observed under conditions of reduced photoassimilateavailability are causally related to the decline in invertaseproduction in the flower buds. Possible mechanisms for the regulationof invertase synthesis in the flowers are discussed. Lycopersicon esculentum Mill, tomato, inflorescence development, invertase, sink activity     

Characteristics of Nectar Secretion by the Extrafloral Nectaries of Ricinus communis

The characteristics of nectar secretion by excised extrafloralnectaries of Ricinus have been examined. Secreted nectar wasfound to contain three sugars: sucrose, glucose and fructose,with glucose and fructose occurring in a 1: 1 ratio. All threesugars supported secretion when used in the culture medium andthe yield of nectar sugar was found to be concentration-dependent.Other sugar sources failed to support secretion. Experimentsusing ¹⁴C-sugars and ¹⁴CO₂ fed to intact plants allowed themovement of sugars through the nectary to be examined. Sucrosesynthesis occurs when excised glands are fed glucose and thisoccurs very early in the transport through the nectary. Themain sugar transported was sucrose, with little hydrolysis occurringuntil the final step of secretion. There was no evidence thatsucrose hydrolysis occurs either by invertase in the nectaror by a microbial flora. Inhibitors of respiration were foundto inhibit secretion as did anaerobiosis. Temperature also hada marked effect, with a temperature coefficient of 1.8. However,secretion of sucrose was not affected by anaerobic conditions,low temperatures or inhibitors of respiration as markedly asthat of glucose and fructose. Electron microscopy revealed the presence of a thickened andheavily stained wall at the inner border of the secretory epidermallayer. This wall contained numerous plasmodesmata at a frequencyof 14 per µm² and may represent an apoplastic barrier.Light microscope cytochemistry revealed that acid phosphataseis primarily located in the nectiferous tissue, while ATPaseis concentrated in the epidermis. The possibility that the nectarycontains two pathways for sucrose secretion, both apoplasticand symplastic, is discussed. Key words: Invertase, nectary, plasmodesmata, Ricinus communis, sucrose     

Invertase Activity and its Relation to Hexose Accumulation in Potato Tubers

Hexose accumulation was shown to occur in freshly harvestedmature potato tubers (Solanum tuberosum L.) both after storageat 10 ?C and when subsequently transferred to low temperature(3 ?C) storage. In general, changes in hexoses and sucrose werefound to be related to changes in acid invertase activity. Totalacid invertase activity (i.e. assayed after destroying the endogenousinvertase inhibitor present in the extracts) generally reflectedsugar changes more closely than did basal activity (i.e. assayedwith the inhibitor present). There was no evidence of a specificalkaline invertase. A comparison of the temperature responsesof cultivar Record with that of two SCRI² clones demonstrateddistinct genotypic variation in the extent of hexose accumulation.However, these differences were not always reflected by genotypicdifferences in total invertase activity. Key words: Invertase inhibitor, glucose, fructose, sucrose