On the dependence of salt tolerance of beans (Phaseolus vulgaris L.) on soil water matric potentials

Summary Bean plants (Kora cv) were grown in potted soil artificially salinized by adding NaCl and CaCl₂ to the irrigation water to obtain an electrical conductivity of the soil saturation extract (EC_e) thirty days after emergence of 0.1, 0.3, 0.5 and 0.7 S/m at 25°C and a sodium adsorption ratio (SAR) of 4 (mmol/l)². Thereafter, plants were irrigated when soil water matric potential (_M) was in the range of –20 to –30 kPa (wet treatment) and when _M was in the range of –40 to –60 kPa (dry treatment).Transpiration rates (Tr) and leaf extension rates (LER) per plant or per unit of leaf area were decreased by increasing soil salinity and by decreasing soil moisture. However, a given decrement of _M produced a considerable larger decrement in Tr of LER than an equivalent decrement of soil water osmotic potential (₀). Absolute yields of green pods under wet treatments were from twice to one and a half time as large under the wet than under the dry treatment at equivalent values of ₀. Relative yields were reduced by 25% when EC_e were about 0.5 S/m and 0.7 S/m in the dry and wet treatment respectively. Salt tolerance data of crops may not have a quantitative interest when soil irrigation regimes under which they were obtained are not specified.     

Characterization of a Gy4 glycinin gene from soybean Glycine max cv. Forrest

The glycinin gene family encoding the glycinin subunits in soybean plants is composed of at least five gene members. A genomic clone S312 containing the Gy4 gene from a genomic library of cv. Forrest was isolated and partially characterized. The organization of this gene was found to be similar to that of a null allele from cv. Raiden, but different from the Gy4 gene from cv. Dare. The complete nucleotide sequence of this gene has been determined. It is 2599 bp long consisting of four exons and three introns. Comparing the DNA sequences between this gene and the gene from Dare and a null allele from Raiden, the difference found in the coding region was 5-GCAGTGCAAG-3 (nt 824 to 833) in the former case versus 5-TGGAGTTGCAATT-3 (nt 1314 to 1326) in the latter case in the exon 2 domain, resulting in three amino acid differences and one amino acid absence. Some other differences were also found in the non-coding region. The coding sequence and 5-flanking region of the Gy4 gene, when compared with that of other legumin genes as well as group 1 glycinin subunit genes, revealed some interesting features: (1) a transposable element-like sequence was found in the hypervariable region (HVR) of the exon 3 domain, which was lacking in the legumin and the glycinin group 1 genes; (2) in the 5-flanking region from nt –145 to –1, two high-homology sequences were found: one from nt –141 to nt –132, the other from nt –118 to nt –92 which includes the legumin box and the RY repeat element.     

Seasonal variation of carrageenans in tetrasporic,cystocarpic and ‘sterile’ stages of Gigartina skottsbergii S. et G. (Rhodophyta,Gigartinales)

Cystocarpic and sterile plants of Gigartina skottsbergii produce -carrageenans, while tetrasporophytes produce -carrageenans. No seasonal variation in the carrageenan yields in the three stages was evident. Yields were clearly higher in cystocarpic (34.1–64.7%) and sterile samples (32.3–71.8%) than in tetrasporic plants (10.1–34.0%). Chemical characteristics of the polysaccharides for any stage are the same throughout the year.The authors are Research Members of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina.The authors are Research Members of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina.     

Fish populations in the Paraná River: Lentic environments of Diamante and San Pedro Areas (Argentine Republic)

Data on the fish populations of lentic water bodies of the Lower Paraná River in the Diamante and San Pedro areas (33°40–31°55 S, 59°40'–60°40 W), sampled during 1971 are given. Limnological characteristics such as surface area and depth of ponds, water turbidity, and plant cover are included. Differences in fish population size, structure, specific diversity, dominance, and biomass of particular species are established between two areas in a dry season.     

Structure and orientation of Apo B-100 peptides into a lipid bilayer

Peptides corresponding to lipid binding domains of Apo B-100 were synthesized, purified, and incubated with dimyristoylphosphatidylcholine (DMPC) liposomes. The secondary structure of the apo B-100 peptide-lipid complexes was evaluated by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Those peptides belonging to the hydrophobic core domain of apo B-100 when associated with phospholipids were rich in sheet structure; a predominant helical conformation was shown to be associated with one peptide located in a surface region of apo B-100. IR dichroic spectra revealed, in the case of the core peptides, that the sheet component is the only oriented structure with respect to the phospholipid acyl chains. This orientation of the sheet was recently found in LDL particles after proteolytic digestion by trypsin (Goormaghtigh, E., Cabiaux, V., De Meutter, J., Rosseneu, M., and Ruysschaert, J. M., 1993,Biochemistry 32, 6104–6110). Altogether, the data suggest that sheet, present in a high proportion in the native apo B-100, is probably another protein structure in addition to the amphipathic helix which strongly interacts with the lipid outer layer surrounding the LDL particle.Abbreviations used Apo apolipoprotein - ATR attenuated total reflection - CD circular dichroism - DMPC dimyris-toylphosphatidylcholine - DMSO dimethylsulfoxide - FTIR Fourier transform infrared spectroscopy - HPLC high-performance liquid chromatography - LDL low density lipoprotein - TFA trifluoroacetic acid - C_x apoB-100 core peptide corresponding to the following residues: C₂, 2462–2482; C₃, 4208–4231; C₅, 4493–4513; and C₇, 4271–4288 - S₆ apoB-100 surface Peptide Corresponding to Residues 374–400     

Some aspects of the growth of Gracilaria tenuistipitata in pond culture

The effect of temperature, salinity, nitrogen, culture density and depth on the growth of Gracilaria tenuistipitata were investigated between April 1985 and March 1986 in outdoor ponds in Guangxi Province, South China. The mean annual growth rate was 2.4% per day. Under favourable temperatures of 20–30 °C, daily growth rate may reach as high as 3.3%. Salinity had an obvious effect on growth and photosynthesis and growth peaked at 21, with a broad plateau between 7–27. Growth experiments showed that a total nitrogen (NH₄-N plus NO₃-N) concentration of 4 M was sufficient to enable the plants to maintain a daily growth rate of 2.7%. The best growth of the plant was obtained at a culture density of 0.5–1 kg m^–2 and a culture depth of 30 cm in the pond.     

An efficient cryopreservation procedure for potato (Solanum tuberosum L.) utilizing the new ice blocking agent, Supercool X1000

Cryopreservation has been recognized as a practical and efficient tool for long-term storage of vegetatively propagated plants. This study was conducted to investigate effects of modified vitrification techniques on cryopreservation of potato. In vitro plants of potato cultivars Superior and Atlantic were cold acclimated, and axillary buds were precultured, osmoprotected, exposed to PVS-2 solution, plunged into liquid nitrogen, thawed, and finally planted in the regeneration medium. In the modified vitrification technique an ice-blocking agent, Supercool X1000, was added with PVS-2 solution. Cold acclimation affected survival of cryopreserved shoot tips, and the highest survival (46.7%) was obtained after 3 weeks of acclimation at 10°C. Shoot tips exposed to 2M glycerol plus 0.6M sucrose for 40 min gave 51.5% and 11.7% survival in Atlantic and Superior at 10°C, respectively. Cold acclimated and osmoprotected shoot tips were dehydrated with PVS-2 containing different concentrations of Supercool X1000 prior to a plunge into liquid nitrogen. Treatments with 0.1% and 1% of Supercool X1000 significantly improved survival by 55% in Superior and 71.3% in Atlantic, respectively. After cryopreservation, vitrified shoot tips resumed growth within a week in a medium (1 mg l^–1 GA₃, 0.5 mg l^–1 zeatin, and 0.1 mg l^–1 IAA) with a low level of Pluronic F-68 (0.005%) and survival was 33.7% higher in Atlantic and 14.7% higher in Superior than the control (without Pluronic F-68).     

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Our objectives were to quantify and compare soil CO₂ efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO₂ release. We measured soil CO₂ efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). At the same time we measured soil CO₂ concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between old alluvium sites, the two-year average CO₂ flux rates ranged from 117.3 to 128.9 mg C m^–2 h^–1. Significantly higher soil CO₂ flux occurred on the residual sites (141.1 to 184.2 mg C m^–2 h^–1). Spatial differences in CO₂ efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO₂ storage was high and the amount of CO₂ stored in the upper and lower soil profile was different between old alluvial and residual sites. The major factor identified for explaining temporal variations in soil CO₂ efflux was soil water content. During periods of high soil water content CO₂ emission decreased, probably due to lower diffusion and CO₂ production rates. During the 2-year study period inter-annual variation in soil CO₂ efflux was not detected.     

Dry miombo – ecology of its major plant species and their potential use as bio-indicators

Results of an extensive vegetation survey of 214 miombo woodland sitesare reported, encompassing the full spectrum of site types in the northernregion of Zimbabwe. At each sample site the vascular species (altogether640) were recorded, as well as 38 environmental variables (climatic,topographic, edaphic factors; geology, vegetation features, exploitation andland tenure). The sampling design was orientated along the environmentalgradients: altitude, slope, precipitation and geology. A reduced species set(286 species; frequency > 3%)was analysed by Canonical Correspondence Analysis (CCA), complementedby the Monte Carlo Permutation Test. The first axis represents acombination of the major environmental variables, altitude, precipitation andsoil texture, and the second – being of marginal importance – thePiriwiri and Lomagundi sediments. The matrix–consisting of the species and sites arranged according to the firstordination-axis – shows a diagonal feature embodying the moistureavailability gradient of the study area, subsequently divided into sevenordinalmoisture classes. The weighted averages of the species scores of therelevés – the scores being fixed by the membership of the singlespecies to one of the respective moisture classes – permits ranking thesites due to moisture availability, thereby assessing their land use capacity.     

Conflicting priorities in site management in England

There are two main designations of protected areas in the UK-nature reserves, of various kinds, and special nature conservation areas, known as Sites of Special Scientific Interest. General approaches to choices of priorities and the resolution of conflict in the management of these areas are described and difficulties identified. Similar problems arise when considering the wider role of protected areas in national nature conservation policies, e.g. biodiversity targets and European Habitats and Species Directive objectives. Because choices and priorities stem from underlying values, the public must be involved in the identification of objectives for both protected areas and the rest of the land surface. The Natural Areas programme being developed by English Nature provides a mechanism whereby people may be involved in characterising the wildlife of their area and in identifying targets for its maintenance and enhancement. The use of land and the management practices associated with it are the major factors influencing the nature conservation value of protected areas and their wider context.English Nature (the Nature Conservancy Council for England) is the statutory advisor to the government on nature conservation in England and promotes the conservation of England's wildlife and natural features.     

Rapid plant regeneration of pea using thidiazuron

A simple and rapid pea regeneration procedure was developed. An average of up to 20 shoots formed from hypocotyl explants of cvs. Sugar Ann and Patriot cultured on Murashige and Skoog basal medium supplemented with 0.5 or 1.0 M thidiazuron (N-phenyl-N-1,2,3-thiadiazol-5-ylurea). Hypocotyls of Puget and Sugar Daddy did not respond. Regenerated shoots rooted rapidly when cultured on Murashige and Skoog basal medium containing either 2.0 M -naphthaleneacetic acid or 1.0–2.0 M indole-3-butyric acid. Seeds were harvested from regenerated plants after only 9–11 weeks.Abbreviations BAP 6-benzyladenine - 2,4-d 2,4-dichlorophenoxy acetic acid - GA₃ gibberellic acid - IBA indole-3-butyric acid - MS Murashige and Skoog (1962) medium - NAA -naphthaleneacetic acid - TDZ thidiazuron (N-phenyl-N-1,2,3-thiadiazol-5-ylurea)     

Cross-linking of the endogenous inhibitor protein (IF1) with rotor (gamma,epsilon) and stator (alpha) subunits of the mitochondrial ATP synthase

The location of the endogenous inhibitor protein ( IF₁) in the rotor/stator architecture of the bovine mitochondrial ATP synthase was studied by reversible cross-linking with dithiobis(succinimidylpropionate) in soluble F₁I and intact F₁F₀I complexes of submitochondrial particles. Reducing two-dimensional electrophoresis, Western blotting, and fluorescent cysteine labeling showed formation of –IF₁, IF₁–IF₁, –IF₁, and –IF₁ cross-linkages in soluble F₁I and in native F₁F₀I complexes. Cross-linking blocked the release of IF₁ from its inhibitory site and therefore the activation of F₁I and F₁F₀I complexes in a dithiothreitol-sensitive process. These results show that the endogenous IF₁ is at a distance 12 Å,to and subunits of the central rotor of the native mitochondrial ATP synthase. This finding strongly suggests that, without excluding the classical assumption that IF₁ inhibits conformational changes of the catalytic subunits, the inhibitory mechanism of IF₁ may involve the interference with rotation of the central stalk.     

Ecosystem structure and soil-surface conditions drive the variability in the foliar δ13C and δ15N of Stipa tenacissima in semiarid Mediterranean steppes

We evaluated the effects of ecosystem composition and structure (species richness and diversity, cover and spatial attributes of vegetation), abiotic factors (climate and topographical features) and the condition of the bare-ground areas (evaluated using soil-surface indicators) on the performance of Stipa tenacissima [evaluated using foliar ¹³C, ¹⁵N, nitrogen concentration and the carbon-to-nitrogen (C:N) ratio] in 15 steppes of SE Spain. Foliar ¹³C values of S. tenacissima showed a low degree of variation in the studied steppes, with average values ranging from –24.1 to –22.9. Higher variation was found in the ¹⁵N values, which ranged from –5.5 to –2.4. The nitrogen concentration and the C:N ratio varied between 5.0 and 8.0 mg g^–1, and between 55.4 and 85.3, respectively. The ¹³C values became less negative with increasing spatial aggregation of perennial vegetation, while the C:N values increased with increasing perennial vegetation cover. The ¹⁵N values became more negative with increasing infiltration in the bare-ground areas, but the nitrogen concentration was not related to any of the environmental variables measured. Our results suggest that the relative importance of ecosystem structure and soil-surface conditions in the bare ground areas was higher than that of abiotic factors as determinants of the performance of S. tenacissima. The results also show that even subtle changes in these ecosystem features may lead to modifications in plant performance in semiarid S. tenacissima steppes, and thus to modifications in the associated ecosystem functions in the mid- to long-term.     

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with “antisense” rbcS

Tobacco (Nicotiana tabacum L.) plants transformed with antisense rbcS to decrease the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO₂ at 20°C at 100, 300 and 750 mol·m^–2·s^–1 irradiance, and at 28°C at 100, 300 and 1000 mol·m^–2·s^–1 irradiance. (i) Measurement of photosynthesis under ambient conditions showed that the flux control coefficient of Rubisco (C _infRubisco ^supA ) was very low (0.01–0.03) at low growth irradiance, and still fairly low (0.24–0.27) at higher irradiance. (ii) Short-term changes in the irradiance used to measure photosynthesis showed that C _infRubisco ^supA increases as incident irradiance rises, (iii) When low-light (100 mol·m^–2·s^–1)-grown plants are exposed to high (750–1000 mol·m^–2·s^–1) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. However, when high-light-grown leaves (750–1000 mol·m^–2·s^–1) are suddenly exposed to high and saturating irradiance (1500–2000 mol·m^–2·s^–1), C _infRubisco ^supA remained relatively low (0.23–0.33), showing that in saturating light Rubisco only exerts partial control over the light-saturated rate of photosynthesis in sun leaves; apparently additional factors are co-limiting photosynthetic performance, (iv) Growth of plants at high irradiance led to a small decrease in the percentage of total protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminated at growth irradiance avoided an inbalance between the light reactions and Rubisco; this was shown by the low value of C _infRubisco ^supA (see above) and by measurements showing that non-photochemical quenching was low, photochemical quenching high, and NADP-malate dehydrogenase activation was low at the growth irradiance. In contrast, when a leaf adapted to low irradiance was illuminated at a higher irradiance, Rubisco exerted more control, non-photochemical quenching was higher, photochemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conclude that changes in leaf composition allow the leaf to avoid a one-sided limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions, (v) Antisense plants with less Rubisco contained a higher content of insoluble (thylakoid) protein and chlorophyll, compared to total protein or structural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at which the plant had grown. We propose that N-allocation in low light is not optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in low light.Abbreviations A rate of photosynthesis - C _infRubisco ^supA flux control coefficient of Rubisco for photosynthesis - c_i internal CO₂ concentration - qE energy-dependent quenching of chlorophyll fluorescense - qQ photochemical quenching of chlorophyll fluorescence - NADP-MDH NADP-dependent malate dehydrogenase - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose-1,5-bisphosphate This work was supported by the Deutsche Forschungsgemeinschaft (SFB 137).     

Diatoms on herbarium macrophytes as indicators for water quality

To describe the reference situation for water quality in the eutrophicated broad area De Nieuwkoopse Plassen (Holland), attached diatoms from aquatic macrophytes, collected between 1934 and 1958 have been used. Recent samples for comparison were taken in 1989.The diatoms do not indicate significant changes in salinity. Eutrophication has reduced the number of rare species and increased the abundance of meso- to hyper-eutraphentic species, particularly Fragilaria berolinensis. Reduction of total phosphorus to concentrations below 0.02 mg 1^–1 might be sufficient to restore the diatom assemblages of the 1930s.     

Changes in brain components during the development of mice homozygous for the locus “dwarf” (dw)1,5

Brain composition and developmental changes were investigated in mice homozygous for the locus dwarf, and characterized by a reduced level of growth hormone, thyroid stimulating hormone, and prolactin, and by secondary hypothyroidism. The difference in adult brain weight (–32%) between the dwarf and the normal mice was not found to parallel the difference in body weight (–71%), whereas the differences in the weight of the liver (–79%) and that of the kidney (–75%) did. Several biochemical parameters of brain development were assayed in dwarf and normal mice between the ages of 15 and 210 days. Levels of cerebrosides, sulfatides, gangliosides, phospholipids, cholesterol, protein, and RNA (per gram wet weight) were the same for the dwarf and the controls, but the net difference in total brain DNA was less than the net total brain RNA difference (–11% vs. –27%). Total brain lipids (absolute quantities) were the same at 15 days. The difference was –37% by the 50th day, and remained constant thereafter. No change in the specific activity of 2,3-cyclic nucleotide 3-phosphohydrolase or 3-phosphoadenosine-5-phosphosulfate: galactocerebroside sulfotransferase was observed. These data suggest that the regulation of the development of brain structures is maintained, but the level of the synthesis of the various brain constituents is reduced in proportion to the brain weight. The development of the dwarf brain seems to proceed harmoniously.Abbreviations used PAPS 3-phosphoadenosine-5-phosphosulfate - PAPS-CST 3-phosphoadenosine-5-phosphosulfate:galactocerebroside sulfotransferase - CNP 2, 3-cyclic nucleotide 3-phosphohydrolase - Neu NAc N-acetylneuraminic acid This paper is part of the Doctorat d'Etat thesis of L. L. Sarliève.     

Status and habitat preferences of Uganda cheetahs: an attempt to predict carnivore occurrence based on vegetation structure

In this paper we examine whether the occurrence of cheetahs (Acinonyx jubatus) in Uganda can be predicted from habitat characteristics extracted from a vegetation map. We first established the status of the cheetah in Uganda through field-interviews that Gros conducted in 1990. Cheetahs occurred almost exclusively in the Karamoja region where we estimated 53–310 individuals. Based on 216 sightings, the average number of adults in all-adults sightings was 1.65 + SD 0.95 and the average number of cubs in family groups 2.5 + SD 1.65. Compared to Graham and Parker's 1965 East African survey, average adult group size was slightly smaller in 1990 and large family groups were rarer. Comparison with Gros 1990 survey showed considerably lower cub-to-adult ratio and percent of observations with cubs in Uganda than in Kenya. A Geographic Information System (GIS) analysis of vegetation structure in areas where cheetahs were observed and in those where none were reported suggested that cheetahs might favor habitats with 25–50% woody cover and grasses of medium height (50–100 cm). A discriminant analysis correctly classified 72.1% of used habitats and 70.4% of no-report habitats. A logistic regression analysis improved the correct allocation of used habitats by 2.2%. Either the discriminant function or the logistic regression, which require only four easily obtainable vegetation characteristics, may help to pinpoint suitable cheetah habitats for conservation purposes. Our approach could be adapted for analyzing habitat suitability for other species of carnivores.     

Thermodynamics of Mn2+-binding to goat α-lactalbumin

By means of reaction calorimetry we measured the apparent enthalpy change, H^app, of the binding of Mn²⁺-ions to goat -lactalbumin as a function of temperature. The observed H^app can be written as the sum of contributions resulting from a conformational and a binding process. In combination with the thermal unfolding curve of goat -lactalbumin, we succeeded in separating the complete set of thermodynamic parameters (H, G, S, C_p) into the binding and conformational contributions. By circular dichroism we showed that NH ₄ ⁺ -ions, upon binding to bovine a-lactalbumin, induce the same conformational change as do Na⁺ and K⁺: the binding constant equals 98 ± 9 M^–1.Abbreviations BLA bovine -lactalbumin - GLA goat -lactalbumin - HLA human -lactalbumin - CD circular dichroism Offprint requests to: H. Van DaelDeceased     

Structural study of fucoidan from Cladosiphon okamuranus tokida

A structural study was carried out on a fucoidan isolated from the brown seaweed Cladosiphon okamuranus. The polysaccharide contained fucose, glucuronic acid and sulfate in a molar ratio of about 6.1 : 1.0 : 2.9. The results of Smith degradation showed that this polysaccharide has a linear backbone of 13-linked -fucopyranose with a half sulfate substitution at the 4-positions, and a portion of the fucose residues was O-acetylated. The data obtained from partial acid hydrolysis, a methylation analysis and NMR spectra indicated that the -glucuronic acid residue is linked to the 2-positions of the fucose residues, which were not substituted by a sulfate group. These results indicated that the average structure of this fucoidan is as follows: -[(3Fuc-4(±OSO₃-)1–)₅3[GlcA12]Fuc1–]_n–. (Half of each fucose residue was sulfated. One O-acetyl ester was present in every 6 fucose residues.)     

Actin associated with plasmodesmata

Summary We have used several methods to localise actin associated with plasmodesmata. In meristematic plant material fixed in 0.1% glutaraldehyde/1% paraformaldehyde and embedded in LR White resin, actin was localised (in TEM using 5 nm gold-labelled secondary antibody to C₄ anti-actin primary antibody) in the neck region by the plasma membrane and endoplasmic reticulum, and also down the length of the plasmodesma, deep in the cell wall. When the chemical fixation was replaced by rapid freezing in liquid propane (without cryoprotectants) and substitution in acetone, the plasmodesmata were labelled in similar positions, but with less background label on sections. While only 8–20% of plasmodesmata were labelled, the label was 10 to 100 fold denser over plasmodesmata than over the surrounding wall indicating specific association with plasmodesmata. We presume the apparent extracellular location of some label was due to the size of the antibodies between the site of attachment and the observed position of the gold particle. Gold label was found in similar locations in material fixed in 3% paraformaldehyde, infiltrated with sucrose, frozen, sectioned (10–12 m thick), then labelled with antibodies before resin embedding. Furthermore, cell walls in epidermal peels stained with rhodamine-phalloidin showed localised patches of fluorescence, presumably at the site of plasmodesmata (or primary pit-fields), which were connected on either side to fluorescent strands of actin in the cytoplasm. Suspension cultured cells ofNicotiana plumbaginifolia similarly stained showed very faint, narrow fluorescent strands crossing the walls of sister cells, which may indicate actin associated with individual plasmodesmata, shown in TEM to be sparsely distributed in these walls. In addition, the neck regions of cytochalasin-treated plasmodesmata were greatly enlarged and lacked the normal extracellular ring of particles. We propose that actin associated with plasmodesmata stabilizes the neck region and possibly also the cytoplasmic sleeve, and may be actively involved in regulating cell-to-cell transport.Abbreviations BSA bovine serum albumin - EDTA ethylenediaminetetraacetic acid - EGTA ethyleneglycol bis-(-aminoethyl ether)-N,N,N,N-tetraacetic acid - PAGE polyacrylamide gel electrophoresis - PBS phosphate buffered saline - Pipes piperazine-N,N-bis(2-ethanesulphonic acid) - Mes 2(N-morpholino)ethanesulfonic acid - SDS sodium dodecyl sulphate - Tris tris-(hydroxymethyl)aminomethane