The Differentiation of Contact Cells and Isolation Cells in the Xylem Ray Parenchyma of Populus maximowiczii

A histochemical analysis was made of the differentiation ofcontact cells and isolation cells in the xylem ray parenchymaof Populus maximowiczii. The contact cells formed secondarywalls at approximately the same time as adjoining vessel elements.The lignification of the cell walls of contact cells and vesselelements began earlier than that of wood fibres and isolationcells. Thus, the formation of the secondary wall, includinglignification, of the contact cells might occur at the sametime as that of the vessel elements to which they are directlyconnected. By contrast, the isolation cells began to form secondarywalls later than the vessel elements and wood fibres in thevicinity of the isolation cells. After the deposition of thesecondary wall, a protective layer was formed in contact cellsbut no isotropic layer was observed in isolation cells. Theresults suggest the importance of vessel elements in the determinationof the differentiation of adjoining ray parenchyma cells.Copyright1999 Annals of Botany Company Contact cell, isolation cell, vessel element, xylem differentiation, Populus maximowiczii Henry.     

Studies on Mature Seeds of Cuscuta pedicellata and C. campestris by Electron Microscopy

The seeds of Cuscuta pedicellata have been investigated by transmissionand scanning electron microscopy. Additional observations havebeen made on seeds of C. campestris by SEM only. The seed coatconsists of an outer single epidermis, two different palisadelayers, and an inner multiparenchyma layer. The outer epidermalwall in C. pedicellata has a thick cuticle and zones rich inpectic substances. The thicker ‘U-shaped’ cell wallsin the outer palisade layer are strengthened by a wall layerof hemicellulose. The inner palisade layer has thick walledcells with a ‘light line’. The inner cell wall ofthe compressed multiparenchyma layer has a thin cuticle. A fairlythick cuticle is positioned directly on the endosperm surface.The aleurone cell walls are different from the remaining endospermwalls. The latter are thick and believed to be of galactomannans.There is a ‘clear’ zone between the plasmalemmaand the cell wall in the aleurone cells. The embryo cells arepacked with lipids and proteins. In Cuscuta campestris mostendosperm has been absorbed during the seed development. Theembryo apex has two minute leaf primordia. The features of theCuscuta seeds are discussed in relation to functional and environmentalconditions. Cuscuta pedicellata, Cuscuta campestris, seed, seed coat, cuticle, cell walls, endosperm, aleurone cells, galactomannan, embryo, TEM, SEM     

Differentiation of the Tapetum During Microsporogenesis in Tomato (Lycopersicon esculentum Mill.), with Special Reference to the Tapetal Cell Wall

During microsporogenesis and pollen maturation, the tapetumin anthers of tomato (Lycopersicon esculentum) underwent severalultrastructural changes and ultimately degenerated. The changesobserved related to the secretory function of the tapetum andto the transfer of materials from the cytoplasm to the surfaceof tapetal cells. Electron dense deposits, initially in thevacuoles, disappeared coincident with the appearance of orbiculeson the cell wall. The fibrillar wall of the tapetal cells loosened,presumably to facilitate transfer of materials through the wall.In Addition, membranous fragments were a consistent featurein the tapetum wall and may play a role in transport of materials.The cells of the inner tapetum (towards the connective) andouter tapetum (towards the epidermis) had different ultrastructuralfeatures. The cytoplasm of the outer tapetum was more electrondense and had a higher proportion of dictyosomes and mitochondriathan the inner tapetum, indicating the greater secretory natureof the outer tapetum. The plastids and mitochondria also differedin morphology between the two regions. Degenerations of thetapetal cytoplasm began by the vacuolate microspore stage. Atanthesis, cytoplasm was absent but the orbicular wall of thetapetum remained appressed to the wall of the middle layer ofthe anther.Copyright 1993, 1999 Academic Press Lycopersicon esculentum, microsporogenesis, pollen development, tapetum development, tomato, ultrastructure     

Role of extensin peroxidase in tomato (Lycopersicon esculentum Mill.) seedling growth

 It is proposed that inhibition of extensin peroxidase activity leads to a less rigid cell wall and thus promotes cell expansion and plant growth. A low-molecular-weight inhibitor derived from the cell walls of suspension-cultured tomato cells was found to completely inhibit extensin peroxidase-mediated extensin cross-linking in vitro at a concentration of 260 μg/ml. The inhibitor had no effect upon guaiacol oxidation catalyzed by extensin peroxidase or horseradish peroxidase. We have demonstrated that the light-irradiated inhibition of plant growth may be partially offset by inhibition of endogenous extensin peroxidase activity. Overall plant growth was enhanced by up to 15% in the presence of inhibitor relative to control plants. Inhibitor-treated and illuminated tomato hypocotyls grew up to 15% taller than untreated controls. The inhibitor had no effect upon etiolated plants over a 15-d period, suggesting that only low levels of peroxidase-mediated cross-linking can be found in the cell walls of etiolated plants. SDS-PAGE/Western blots of ionically bound protein from both etiolated and illuminated hypocotyls identified a doublet at 57/58.5 kDa which is immuno-reactive with antibodies raised to tomato extensin peroxidase. Levels of the 58.5-kDa protein, determined by SDS-PAGE, were at least threefold higher in illuminated tomato hypocotyls than in etiolated hypocotyls. Three fold higher levels of extensin peroxidase, elevated in-vitro extensin cross-linking activity and 15% higher levels of cross-linked, non-extractable extensin were observed in illuminated tomato hypocotyls compared with etiolated tomato hypocotyls. This suggests that white-light inhibition of tomato hypocotyl growth appears to be mediated, at least partially, by deposition of cell wall extensin, a process regulated by M_r-58,500 extensin peroxidase. Our results indicate that the contribution of peroxidase-mediated extensin deposition to plant cell wall architecture may have an important role in plant growth. Received: 22 July 1999 / Accepted: 11 October 1999     

Mechanical Defences to Herbivory

The two major mechanical defences of plants are toughness andhardness. These have different material causes and ecologicalfunctions. In any non-metal, high toughness is achieved by compositeconstruction (i.e. by an organized mixture of components). Theprimary source of toughening in plants is the composite cellwall (cellulosic microfibrils set in a hemicellulose and, sometimes,lignin matrix), with a toughness of 3.45 kJ m^-2, which is ten-timesthe probable toughness of its individual components if theycould be isolated. The toughness of most plant tissues is roughlyproportional to the volume fraction of tissue occupied by cellwall (V_c) and, compared to animal tissues and non-biologicalcomposites, is very low. High toughness in plant cells is notproduced by the walls themselves, but by their plastic intracellularcollapse. This is a truly cellular toughening mechanism, oneof the most potent ever discovered by materials scientists,depending on an elongate cell shape with microfibrils directeduniformly at a small angle to the cellular axis. Only ‘woody’cells, tracheids and fibres, have this framework and only inthe S2 layer of their secondary wall. Despite this non-optimumconfiguration, toughness is elevated by this mechanism ten-timesabove that due to cell wall resistance alone. The effectivenessof toughness in preventing herbivory is indisputable, but largelyindirect due to confusion over a false equivalence between nutritional‘fibre content’ and toughness. In contrast, generalizedhardness requires high density. If hardness is due to high V_c, this conflicts with ‘woody’ toughness becausethere is then no lumen for cell walls to collapse into. Thus,dense seed shells may be brittle (i.e. low toughness) even ifbuilt from fibres. However, solid cell wall is not very hard.Instead, high hardness in plants is associated with amorphoussilica and is always localized. The efficacy of hardness ismore difficult to evaluate than toughness because some animalsspecialize in coping with it. Copyright 2000 Annals of BotanyCompany Review, hardness, toughness, mechanical defence, herbivory, cell wall, plastic buckling, silica     

Benzothiadiazole-Mediated Induced Resistance to Fusarium oxysporum f. sp. radicis-lycopersici in Tomato

Benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), a synthetic chemical, was applied as a foliar spray to tomato (Lycopersicon esculentum) plants and evaluated for its potential to confer increased resistance against the soil-borne pathogen Fusarium oxysporum f. sp. radicis-lycopersici (FORL). In nontreated tomato plants all root tissues were massively colonized by FORL hyphae. Pathogen ingress toward the vascular stele was accompanied by severe host cell alterations, including cell wall breakdown. In BTH-treated plants striking differences in the rate and extent of fungal colonization were observed. Pathogen growth was restricted to the epidermis and the outer cortex, and fungal ingress was apparently halted by the formation of callose-enriched wall appositions at sites of fungal penetration. In addition, aggregated deposits, which frequently established close contact with the invading hyphae, accumulated in densely colonized epidermal cells and filled most intercellular spaces. Upon incubation of sections with gold-complexed laccase for localization of phenolic-like compounds, a slight deposition of gold particles was observed over both the host cell walls and the wall appositions. Labeling was also detected over the walls of fungal cells showing signs of obvious alteration ranging from cytoplasm disorganization to protoplasm retraction. We provide evidence that foliar applications of BTH sensitize susceptible tomato plants to react more rapidly and more efficiently to FORL attack through the formation of protective layers at sites of potential fungal entry.     

Cell Locomotion and Contact Guidance in Amphibian Gastrulation

Presumptive mesodermal cells in amphibian gastrulae migratefrom the blastopore toward the animal pole by using the innersurface of the ectodermal layer as their substratum. Duringmigration, the mesodermal cells form lamellipodia and filopodiapredominantly in a direction toward the animal pole. There isa network of the extracellular fibrils on the inner surfaceof the ectodermal layer. The fibrils seem to serve as an adequatesubstratum for attachment of the filopodia and locomotion ofthe mesodermal cells. A significant alignment of the fibrilnetwork along the blastopore—animal pole axis suggestsa hypothesis that it directs morphogenetic cell movements bycontact guidance in combination with contact inhibition of movement.New culture conditions allow the gastrula mesodermal cells tomove actively in vitro with a similar cell shape and at a similarrate as in vivo. Such culture conditions enabled an in vitroexperiment to test the hypothesis of contact guidance. Explantedectodermal layers deposit the fibril network on the surfaceof a cover slip. Dissociated gastrula mesodermal cells seededon such a conditioned surface attach to the surface and moveabout actively. A computer analysis of the time—lapsefilms shows that the cell trails are significantly aligned alongthe blastopore—animal pole axis of the ectodermal layerthat conditioned the surface. The deposited fibril network showsthe alignment along the same axis. There is also a tendencyof the mesodermal cells to move in a polarized fashion preferentiallytoward the animal pole. These results support the hypothesisof contact guidance of mesodermal cell migration in vivo byoriented extracellular fibrils     

Abnormal Stomatal Behaviour and Hormonal Imbalance in flacca, a Wilty Mutant of Tomato: EFFECT OF ABSCISIC ACID AND AUXIN ON STOMATAL BEHAVIOUR AND PEROXIDASE ACTIVITY

The wilty tomato mutant flacca and the normal variety RheinlandsRuhm were compared in terms of: (1) potassium transport intoand out of the guard cells, (2) cell wall properties which includeprotein, hydroxyproline and peroxidase activity, and (3) activityof indol-3yl-acetic acid oxidase. Also studied were the effectsof auxin on stomatal behaviour and peroxidase activity whenapplied to normal plants during development, and the short-termeffect of abscisic acid on the resistance of flacca stomatato closure under plasmolysis. Potassium transport, wall protein and hydroxyproline all seemedto be equal in mutant and normal plants. Peroxidase activitywas higher in the soluble and wall fractions of the mutant,and decreased toward normal in the mutant treated with abscisicacid. More stomata were open and peroxidase activity was higherin normal plants treated with auxin during development. Thepercentage of open stomata under plasmolysis was lower and theiraperture size was smaller in the epidermal strips taken fromabscisic-acid-treated mutant plants than from control mutantplants.     

Cation Exchange Properties of the Cell Walls of Enteromorpha intestinalis (L.) Link. (Ulvales, Chlorophyta

The cell wall of Enteromorpha intestinalis (a marine alga) hasbeen found to behave as a weakly cross-linked cation exchangerin NaCl solutions from 0.1–1020 mMolal (0.1–1000mMolar). Anion adsorption could be described by Freundlich isothermsover this concentration range. The large anion, inulin carboxylate,was found to be a tracer of the anion free space of plant tissuesonly in salt solutions above 10 mMolal. The cell wall of Enteromorphahas a cation exchange capacity of about 2500 µ mol g^–1dry wt. (Na+ form). The cell wallvolume is a complex functionof p^H and the NaCl concentration. As a result, the cation exchangecapacity is only predictable on a dry weight basis. The fixednegative charges of the cell wall have a pK_a of2 in situ and1.75 in vitro, and seem to be a mixture of sulphate and carboxylsugar esters. The applicability of the Donnan equation to plant cell wallsis discussed. Interpretation of the cell wall as a single thermodynamicphase is shown to be inappropriate. A large proportion of thecell wall solution is unaffected by the fixed anions.     

HowStriga Parasitizes its Host: a TEM and SEM Study

The cellular contact betweenStriga hermonthica andStriga asiaticaand their hosts,Zea mays andSorghum bicolor , was investigatedby light, transmission electron and scanning electron microscopy.The xylem connections between parasites and hosts involve veryspecific, clustered intrusions into the host's water conductingelements, predominantly into the large vessel elements. A singlehaustorial cell can penetrate a host vessel element with morethan one intrusion. All intrusions become covered by an additionalelectron-opaque wall layer. During subsequent differentiation,a dissolution of specific wall parts of the cell intrusionsoccurs so that open, cup- or trunk-like structures result. Thevessel-like host contact can comprise up to five openings withina single intrusion. Concomitantly, the intrusions and the haustorialcells to which they belong lose their protoplasts and transforminto elements which take up water. The walls of the haustorialcells and both wall parts of their appendages become stronglylignified. The water and nutrient absorbing structures insertedinto the host vessel are named ‘oscula’. Withinthe whole haustorial complex of bothStriga species no phloemelements were detected. Translocation of substances from hostto parasite are briefly discussed. Striga hermonthica ; Striga asiatica ; haustorial anatomy; xylem contact; osculum     

Response of cultured tomato cells subjected to excess zinc: role of cell wall in zinc compartmentation

The aim of this preliminary study was to evaluate the role of the cell wall in Zn accumulation and tolerance by tomato suspension-cultured cells. Growth parameters, Zn distribution and accumulation by tomato cells were determined in function of zinc concentration. A particular attention was paid to the variations of the total cell wall material (cell wall carbohydrates, proteins, and exopolymers) in relation to extracellular levels of Zn. Cells treated with 0.5–5 mM Zn showed typical symptoms of heavy metal toxicity as testified by various growth parameters. Fresh and dry weights as well as total cell volume per vial decreased with increasing Zn concentration in the culture medium. Concurrently, the cell wall biomass increased, as well as the Zn amount retained by cell wall polymers. Cell wall appeared to assume important roles in Zn fixation and could therefore limit Zn influx into the cell. Our results also suggested that zinc fixation by cell wall was not only due to an increase in cell wall biomass but also to an improvement of its binding capacity.     

Histology of the Morphogenic Response in Thin Cell Layer Explants from Vegetative Tobacco Plants

The morphogenic response of thin cell layers (TCLs) from vegetativetobacco (Nicotiana tabacum L.) plants can be directed very preciselyby varying the concentrations of benzyladenine (BA) and -naphthaleneacetic acid (NAA) in the culture medium. Medium containing 1·6µM BA and 0·5 µM NAA was optimal for shootformation, concentrations of 0·5 µM BA and 1·6µM NAA were optimal for the induction of shoots and rootson the same explant, whereas concentrations of NAA higher than16 µM resulted in callus proliferation only. Polarityin the distribution of the shoot buds was observed, i.e. a switchfrom basal to apical shoot formation occurred with increasingNAA concentrations, suggesting basipetal transport of NAA. Histologicalexamination of TCLs on shoot induction medium revealed thatfirst cell divisions occurred within 2 d in cortical cells whichwere directly in contact with the medium along the longitudinalcut surface, and after 2 d in subepidermal cells along the lateraledges of the explants. Individual lateral buds originated fromone subepidermal and one or more epidermal cells, while apicalbuds originated from single subepidermal or cortical cells locateddirectly at the apical end of the explant. After culture ofTCLs for 2-3 d on root/shoot induction medium cells in the regeneration-competentsubepidermis elongated, while on callus induction medium subepidermalcells elongated and dedifferentiated. The regeneration systemas described in this study will be used to identify cells competentfor regeneration as well as for transformation.Copyright 1994,1999 Academic Press Nicotiana tabacum L., tobacco, thin cell layer explants, cell competence, shoot development, polarity     

Low Temperature Affects Pattern of Leaf Growth and Structure of Cell Walls in Winter Oilseed Rape (Brassica napus L., var. oleifera L.)

Three-week acclimation of winter oilseed rape (Brassica napusL. var. oleifera L.) plants in the cold (2 °C) resultedin a modified pattern of leaf cell enlargement, indicated bythe increased thickness of young leaf blades and modified dimensionsof mesophyll cells, as compared with non-acclimated tissuesgrown at 20/15 °C (day/night). The thickness of leaf cellwalls also increased markedly during cold acclimation but itdecreased in response to a transient freezing event (5 °Cfor 18 h followed by 6 or 24 h at 2 °C, in the dark). Cellwalls of the upper (adaxial) epidermis were most affected. Theirultrastructure was modified by cold and freezing treatmentsin different ways, as revealed by electron microscopy. Possiblereasons for the cold- and freezing-induced modifications inthe leaf and cell wall morphology and their role in plant acclimationto low temperature conditions are discussed. Copyright 1999Annals of Botany Company Acclimation, Brassica napus var. oleifera, cell wall ultrastructure, cold, freezing, leaf structure, winter oilseed rape.     

Development of the Fifth Leaf is Indicative for Whole Plant Performance at Low Temperature in Tomato

In the search for early-detectable selection criteria for growthat low temperature conditions in tomato, first the initiationand growth of individual leaves was analysed. Scanning electronmicroscopy revealed that the first four primordia had alreadydeveloped during the germination period at 25°C. The primordiumof the fifth leaf, however, was initiated after the transferof seedlings to the experimental conditions. The increase inlength of the first three leaves, and to a lesser extent ofthe fourth leaf, was considerably smaller in comparison withthat of later formed leaves. Moreover, the morphology of thefirst three to four leaves was deviant, whereas the others showedthe normal compound leaf architecture. All these results indicatedthat the fifth leaf was the earliest formed leaf with growthcharacteristics that might reflect the growth potential of thewhole plant. Development of the fifth leaf was tested as a marker for wholeplant growth. At three temperature, 18, 15 and 12°C, growthresponses of the fifth leaf were similar to that of whole plantsin four tomato genotypes: Line A, Line B, Premier and MXXIV-13.Significant differences in relative growth rate of dry weightof whole plants and fifth leaves (RGR_W)and of leaf area of thefifth leaves (RGR_LA between two fast growing and two slow growinggenotypes were found. No genotype by temperature interactionfor RGR_W and RGR_LA was found, indicating that the effect oftemperature decrease was similar for the four genotypes. The structure of the mature fifth leaf of one fast and one slowgrowing genotype, Line A and MXXIV-13, was analysed. For bothgenotypes, leaves were small and thick at low temperature, 12°C.The total number of epidermis and palisade parenchyma cellsper leaf was smaller but the size of the cells developed at12°C was larger than at 18°C. Consequently, the slowgrowth at 12°C was due to a low rate of cell division. Atboth temperatures, the fifth leaf to MXXIV-13 was smaller comparedto that of line A. Since the size of the cells were similar,the smaller leaf size was due to lower number of leaf cells. The results confirm the suitability of the growth, especiallyexpressed as RGR_LA , of the fifth leaf as a nondestructive marketfor vegetative development of tomato at low temperature. Growthdifferences between genotypes were mainly reflected by differencesin cell number of leaves, which might be correlated with geneticallydetermined differences in cell number of leaf primordia.Copyright1993, 1999 Academic Press Lycopersicon esculentum Mill. genotypes, plant growth, selection criteria, low temperature, leaf initiation, leaf development, RGR, leaf structure, cell expansion     

The Structure and Orientation of Guard Cells in Plants Showing Stomatal Responses to Changing Vapour Pressure Difference

Transmission electron micrographs revealed that a substantialpart of the guard cell wall of both Quercus robur L. and Populusnigra ‘italica’ L. was either free of cuticle orcovered with a greatly reduced cuticular layer. In Quercus thestructure of the guard cell was such that the area of limitedcuticular development would be exposed to the evaporating powerof the atmosphere even when the stomata were closed. Lanthanumstaining confirmed that this area might be an important siteof evaporation. A similar evaporation site was identified inthe guard cell wall of Pinus sylvestris L. Light micrographsrevealed that this area could also be exposed on the outsideof the leaf when the stomata were closed. It appears that guardcell orientation with respect to the epidermal plane dependsupon epidermal turgor. Changes in orientation of the guard cellcoupled with the exact location of the cuticle-free area inthe guard cell wall may explain the nature of the stomatal responseof individual species to changing VPD and the effect of othervariables, e.g. water deficit, on this response. Quercus robur L, oak, Populus nigra L, poplar, stomata, guard cells, cuticle, evaporation, vapour pressure difference     

Re-evaluation of Conditions for Plant Regeneration and Agrobacterium-Mediated Transformation from Tomato (Lycopersicon esculentum)

Conditions for plant regeneration from explants of tomato (Lycopersiconesculentum) cv. UC82B were studied for optimizing transformationprocedure. The best regeneration rate was obtained from cotyledonexplants from 8–10-d-old seedlings on a modified Murashigeand Skoog medium (1962) with 0·5 mg dm^–3 zeatinand 0·5 mg dm^–3 indolylacetic acid. Tomato cultivars(UC82B, Castone, Fl Ferline, Monalbo) and a Lycopersicon peruvkmum‘CMV sel. INRA’ were studied. The cultivarUC82Band the wild Lycopersicon species showed an efficient shootregeneration potential. Early events in the transformation of tomato cotyledons wereanalysed using an Agrobacterium tumefaciens strain carryinga binary vector with an nptII (pnos) gene and a reporter GUS-intron(p35S) chimeric gene. Two days after infection, GUS activityappeared specifically at the cut surface. Subepidermal cellswere more susceptible to transformation than epidermal cells.When selection for kanamycin resistance was applied 2 d afterinoculation, transformed cells were efficiently recovered. Preculturewith feeder cells stimulated cell transformation, but reducedregenerationcapacity from transformed cells. The optimal transformationrate was observed witha time of preculture of 1 and 2 d. Transformationevents for two tomato cultivars (UC82B and Monalbo) occurredat the same rate as 55% of the inoculated explants developedkanamycin resistant calli. However, transformed plants wereobtained at different rates of 8% and 14% for cv. Monalbo andcv. UC82B. Key words: Agrobacterium tumefaciens, ß-glucuronid, Lycopersicon esculentum, plant regeneration, transformation     

Intracellular Localization of 3H-IAA in the Apical Bud of Lycopersicon esculentum

Driss-Ecole, D. and Perbal, G. 1987. Intracellular localizationof ³H-IAA in the apical bud of Lycopersicon esculentum.—J.exp. Bot. 38: 1362–1372. High resolution autoradiography of ³H-IAA was performed on ultra-thinsections of the apical bud of Lycopersicon esculentum treatedby DCC or glutaraldehyde. A quantitative study of the localizationof the labelling in the compartments (cell wall, cytoplasm,vacuoles and nucleus) of four types of cells (cells of the lateralzone of the apex, cells of the pith meristem, proximal and distalcells of the upper part of the pith) was made. The statisticalanalysis of the results has proved that the variability of thelabelling of the cell compartments in the four cell types wassimilar after treatment by DCC or glutaraldehyde. The densityof labelling is higher in each compartment of the meristematiccells compared with those of the differentiating cells. Thecells of the pith meristem can be distinguished from the meristematiccells of the lateral zone of the apex by a greater density oflabelling in the cytoplasm and the nucleus. These two cellulartypes contain a high density of radioactivity in vacuoles. Byconsidering the percentage of labelling of each compartmentrelative to the total labelling in the cell, it can be shownthat the meristematic cells are characterized by a high percentagein the nucleus whereas the vacuoles of the differentiating cellscontain the highest percentage of radioactivity. Key words: ³H-IAA, tomato shoot     

The Effect of Temperature on Development and Dry-matter Accumulation of Vicia faba Seeds

Vicia faba plants were grown at 16 °C and the temperatureraised to 21 or to 26 °C, 51 d post anthesis (p.a.). Temperatureincrease accelerated pod and seed development, stimulated dry-matteraccumulation, starch and protein synthesis. However, the durationof dry-matter accumulation of seeds was shortened, resultingin a decrease of final seed weight. The effect of temperature on storage capacity (determined bycell number and cell size) was also investigated. Formationof new cells in the layer under the epidermis continued fora much longer period than previously has been described in theliterature. At all stages of development cotyledons containedyoung and small cells with small nuclei at the outside and theybecame larger and older towards the centre of the cotyledon.Cells at the centre were only able to divide at an early stageof seed development up to day 59 p.a. Thereafter cell divisionoccurred mainly in the first cell layer under the epidermisup to 63–67 d p.a. Nuclear counts of macerated cotyledoncells did not show an effect of temperature on the number ofcells. It has, however, been questioned if this method was suitableto measure accurately the number of cells at the last stagesof seed development. The rate of cell expansion was stimulated by higher temperaturesbut the duration of expansion was shortened and the final sizeof cells was not affected by the different temperature treatments.Senescence of the pod wall was accelerated at higher temperaturesand it is possible that transport of sucrose from the pod wallto the seeds terminated earlier than at lower temperatures.This may have resulted in an inhibition of cell formation atthe periphery of the cotyledons and in an inhibition of starchand protein synthesis. Vicia faba, protein, starch, cell size, cell number, temperature     

Involvement of Endomannanase in the Control of Tomato Seed Germination under Low Temperature Conditions

The cause of differences in germination rates in a cold-toleranttomato line (PI341988), a control line (UC82B), and six progenylines stemming from crosses and backcrosses between the twoparent lines was investigated. Pursuant to earlier work showingthat differences in germination ability at 12°C are dueto the barrier imposed by the endosperm layer, we analysed theactivity of cell-wall-hydrolysing enzymes extracted from theselines. A significant increase in endomannanase activity wasfound in plant line PI341988 prior to germination at 12°C.Extracts of PI341988 seeds that had imbibed at either 12 or25°C exhibited higher endomannanase activity than theircounterparts from plant line UC82B. Moreover, a positive relationshipwas found between germination ability at low temperature andendomannanase activity in the six progeny lines. Analysis ofendomannanase activity in sub-regions of the seed indicatedthat the increase in activity prior to germination was higherin the micropylar endosperm cap than in the rest of the seed.Exogenous application of mannanase originating from soil-bornebacteria increased germination rates under both moderate andlow temperature conditions. Cellulase (endo-1,4-ß-glucanase)activity was also found to be higher in plant line PI341988.However, the activity of this enzyme probably increases aftergermination and it is therefore not considered as a key enzymecontrolling germination at low temperatures.Copyright 1995,1999 Academic Press Lycopersicon esculentum, tomato, seed germination, cell wall     

The Structure and Development of the Cell-Wall in the Valoniaceae as Revealed by the Electron Microscope

The cell walls of Valonia ventricosa, V. macrophysa, V. ocellata,and Dictyosphaeria favulosa have been investigated with theelectron microscope. The pattern of wall structure and developmentappears to be similar in all. The firat wall consists of a tangle of cellulose fibrils embeddedin amorphous material, possibly pectin. Later, cellulose islaid down in successive larnellae in which the fibril directionshifts abruptly from layer to layer apparently through 120°,so that the direction is repeated on every fourth layer. Duringcell-extension, tearing of the lamellae occurs. The implicationof the observed facts is discussed.