[Effects of root zone hypoxia on respiratory metabolism of cucumber seedlings roots]

With the seedlings of Lübachun No. 4, a hypoxia-resistant cultivar, and Zhongnong No. 8, a hypoxia-sensitive cultivar, as test materials, and by the method of solution culture, this paper studied the effects of root zone hypoxia on their roots' respiratory metabolism. The results showed that root zone hypoxia inhibited the tricarboxylic acid (TCA) cycle significantly, But accelerated the anaerobic respiration of cucumber roots. Under root zone hypoxia stress, the decrement of succinic dehydrogenase (SDH) and isocitric dehydrogenase (IDH) activities and the increment of lactate dehydrogenase (LDH) activity and lactate and pyruvate contents were lesser in Lübachun No. 4 than in Zhongnong No. 8 seedlings roots, but conversely, the increment of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) activities and alcohol and alanine contents in Lübachun No. 4 seedlings roots were higher than those in Zhongnong No. 8 seedlings roots. On the 8th day of hypoxia stress, the ADH activity and alcohol and alanine contents increased by 409.30%, 112.13% and 30.64% in Lübachun No. 4 roots and by 110.42%, 31.84% and 4.78% in Zhongnong No. 8 roots, respectively, compared with the control. No significant differences in the alanine aminotransferase (AlaAT) activity and acetaldehyde content were observed between the two cultivars. It was concluded that the acceleration of alcohol fermentation and the accumulation of alanine were in favor of the enhancement of root zone hypoxia tolerance of cucumber roots.

Comparison of heavy metal effect on the proton pumps of plasma membrane and tonoplast in cucumber root cells

The effects of 10 microM cadmium, copper and nickel on the activities of vacuolar membrane and plasma membrane (PM) ATP-dependent proton pumps was investigated in Cucumis sativus L. root cells. It was demonstrated that vacuolar H+-ATPase (EC 3.6.3.14) and PM H+-ATPase (EC 3.6.3.6) differed in sensitivity to heavy metals. Exposure of cucumber seedlings to Cd, Cu and Ni had no significant effect on the activity of the vacuolar proton pump and, in the case of Ni, also on the activity of the PM proton pump. In contrast, Cd and Cu ions diminished both ATP hydrolysis and proton transport in plasma membranes. Transcript levels of genes encoding PM enzyme as well as the subunit A of tonoplast enzyme in roots stressed with heavy metals were similar to the control. Cd, Cu and Ni were accumulated in cucumber roots with similar efficiency, but their relative distribution between the symplast and apoplast differed. To explain the mechanism of heavy metal action on the plasma membranes of cucumber roots, the MDA content, as a lipid peroxidation product, and fatty acid composition were analyzed. It was shown that exposure of plants to Cd, Cu and Ni did not enhance the lipid peroxidation in the PM fraction. However, all metals caused an increase in the saturation of PM fatty acids and a decrease in the length of the fatty acid chain.

Continuous UV-B irradiation induces morphological changes and the accumulation of polyphenolic compounds on the surface of cucumber cotyledons

Sharp-headed and globular-headed trichomes are found on the surface of cucumber (Cucumis sativus L.) cotyledons. Most sharp-headed trichomes consist of three cells. Toluidine blue O stains sharp-headed but not globular-headed trichomes. The effect of continuous ultraviolet-B (UV-B; 290-320 nm) irradiation on the surface of cucumber cotyledons was examined with respect to the two trichome types. Continuous UV-B irradiation induced cell division at or under the basal part of sharp-headed trichomes, resulting in an increase in the number of cell layers from three to six. In parallel, the area stained by toluidine blue O expanded to include epidermal cells surrounding sharp-headed trichomes. Regions of alkali-induced fluorescence due to the presence of polyphenolic compounds coincided with areas stained by toluidine blue O. In contrast, continuous UV-B irradiation did not cause morphological changes in globular-headed trichomes. Thus, continuous UV-B irradiation causes the accumulation of polyphenolic compounds in cucumber cotyledons and induces specific morphological changes in or around sharp-headed trichomes. UV-B exposure also increases lignin content in this tissue. Therefore, continuous UV-B irradiation may induce the specific accumulation of polyphenolic compounds, especially stress lignins, in and near sharp-headed trichomes.

A nitrite transporter associated with nitrite uptake by higher plant chloroplasts

Chloroplasts take up cytosolic nitrite during nitrate assimilation. In this study we identified a nitrite transporter located in the chloroplasts of higher plants. The transporter, CsNitr1-L, a member of the proton-dependent oligopeptide transporter (POT) family, was detected during light-induced chloroplast development in de-etiolating cucumber seedlings. We detected a CsNitr1-L-green fluorescent protein (GFP) fusion protein in the chloroplasts of leaf cells and found that an immunoreactive 51 kDa protein was present in the isolated inner envelope membrane of chloroplasts. CsNitr1-L has an isoform, CsNitr1-S, with an identical 484 amino acid core sequence; however, in CsNitr1-S the 120 amino acid N-terminal extension is missing. Saccharomyces cerevisiae cells expressing CsNitr1-S absorbed nitrite from an acidic medium at a slower rate than mock-transformed control cells, and accumulated nitrite to only one-sixth the concentration of the control cells, suggesting that CsNitr1-S enhances the efflux of nitrite from the cell. Insertion of T-DNA in a single CsNitr1-L homolog (At1g68570) in Arabidopsis resulted in nitrite accumulation in leaves to more than five times the concentration found in the wild type. These results show that it is possible that both CsNitr1-L and CsNitr1-S encode efflux-type nitrite transporters, but with different subcellular localizations. CsNitr1-L may possibly load cytosolic nitrite into chloroplast stroma in the chloroplast envelope during nitrate assimilation. The presence of genes homologous to CsNitr1-L in the genomes of Arabidopsis and rice indicates that facilitated nitrite transport is of general physiological importance in plant nutrition.

The selection of mosaic (MSC) phenotype after passage of cucumber (Cucumis sativus L.) through cell culture — a method to obtain plant mitochondrial mutants

Mosaic (MSC) mutants of cucumber (Cucumis sativus L.) appear after passage through cell cultures. The MSC phenotype shows paternal transmission and is associated with mitochondrial DNA rearrangements. This review describes the origins and phenotypes of independently produced MSC mutants of cucumber, including current knowledge on their mitochondrial DNA rearrangements, and similarities of MSC with other plant mitochondrial mutants. Finally we propose that passage of cucumber through cell culture can be used as a unique and efficient method to generate mitochondrial mutants of a higher plant in a highly homozygous nuclear background.

In situ reverse transcription PCR on plant tissues

In situ detection techniques allow specific nucleic acid sequences to be exposed in morphologically preserved tissue sections. In combination with immunocytochemistry, in situ detection can relate microscopic topological information to gene activity at the transcript or protein levels in specific tissues. The advantage of in situ methods over the conventional techniques (e.g., Northern blot, reverse transcription polymerase chain reaction [RT-PCR], or real-time PCR) is that they allow the investigation of the putative spatial distribution of nucleic acid products activity in a heterogeneous cell population. In this chapter, we describe a protocol for in situ RT-PCR detection of specific messenger RNA in cucumber (Cucumis sativus), although this protocol can be used for any plant species, floral buds, and somatic embryo tissue sections on glass microscope slides. A successful in situ RT-PCR procedure requires the optimization of many conditions related to the tissue types used, for example, a cell's age, size, and composition, which may influence the detection of RT-PCR products, as well as specific transcript availability. Moreover, parameters, such as the fixation time, thermal cycling set-up, and the time of detection of RT-PCR products, also should be optimized. The importance of the other factors also is estimated in the protocol. In addition several types of controls that are necessary for a trustworthy in situ RT-PCR method are being discussed.

Complete sequence and organization of the cucumber (Cucumis sativus L. cv. Baekmibaekdadagi) chloroplast genome

The nucleotide sequence of the cucumber (Cucumis sativus L. cv. Baekmibaekdadagi) chloroplast genome was completed. The circular double-stranded DNA, consisting of 155,527 bp, contained a pair of inverted repeat regions (IRa and IRb) of 25,187 bp each, which were separated by small and large single copy regions of 86,879 and 18,274 bp, respectively. The presence and relative positions of 113 genes (76 peptide-encoding genes, 30 tRNA genes, four rRNA genes, and three conserved open reading frames) were identified. The major portion (55.76%) of the C. sativus chloroplast genome consisted of gene-coding regions (49.13% protein coding and 6.63% RNA regions; 27.81% LSC, 9.46% SSC and 18.49% IR regions), while intergenic spacers (including 20 introns) made up 44.24%. The overall G-C content of C. sativus chloroplast genome was 36.95%. Sixteen genes contained one intron, while two genes had two introns. The expansion/contraction manner of IR at IRb/LSC and IR/SSC border in Cucumis was similar to that of Lotus and Arabidopsis, and the manner at IRa/LSC was similar to Lotus and Nicotiana. In total, 56 simple sequence repeats (more than 10 bases) were identified in the C. sativus chloroplast genome.

Cucumber (Cucumis sativus L.)

We describe two novel Agrobacterium tumefaciens-based methods of cucumber transformation. The first involves direct regeneration from leaf microexplants selected on kanamycin-containing medium. The second involves regeneration from a long-term established embryogenic suspension culture emitting green autofluorescence (GAF) and selection on medium containing hygromycin. In the latter method, GAF was used as a reporter, thereby allowing a simple and reliable identification of transgenic cells with a high regeneration capacity. (No false positives were observed.) The transformation efficiency in the leaf microexplants fluctuated from 0.8 to 6.5% of the primary explants, whereas in the embryogenic suspension-cultured cells it varied from 6.4 to 17.9% of the aggregates. In the GAF method, the step involving the elimination of the Agrobacterium cells by antibiotics could be omitted; however, this reduced the transformation efficiency to about 3%. The time required from inoculation to regenerated plant in the greenhouse was the same for both methods, but the GAF method required more preinoculation time than the leaf microexplant method.

Preferential localization of abscisic acid in primordial and nursing cells of reproductive organs of Arabidopsis and cucumber

Abscisic acid (ABA) is known to function in plant stress responses and seed dormancy, and much is known about its detailed mechanisms of signal transduction. Recent studies suggest that this hormone may also play important roles in sugar signaling and assimilate distribution during fruit development. However, little is known about the role of ABA in actively growing or differentiating fruits and other plant organs or tissues. To explore whether ABA functions during the early development of reproductive organs, we carried out ABA immunolocalization using monoclonal antibodies. The specific ABA accumulation pattern was verified by gas chromatography-mass spectrometry (GC-MS). ABA was not only detected in primordial cells of flower organs, but was also detected in nursing cells (e.g. tapetum and integuments), which function in supplying nutrition for germ cell development. These findings suggest that, in addition to its well-known function as a 'negative hormone', ABA may play some 'positive' roles during plant development, including possible involvement in the regulation of assimilate distribution.

Cucurbit phloem serpins are graft-transmissible and appear to be resistant to turnover in the sieve element-companion cell complex

Serpins are unique inhibitors of serine proteases that are located in various plant tissues and organs. An orthologue of the pumpkin (Cucurbita maxima) phloem serpin CmPS-1 was amplified from cucumber (Cucumis sativus) RNA by RT-PCR, cloned, and designated as CsPS-1 (GenBank accession no. AJ866989). Alternative amino acid sequences in the reactive centre loop suggest distinct inhibitory specificity between CmPS-1 and CsPS-1. A difference in the electrophoretic mobility of these serpins was used in heterografts to establish that serpins are phloem-mobile. Immuno light microscopy revealed that the phloem serpins are localized exclusively to sieve elements (SE), while the phloem filament protein CmPP1, used as a reference, is localized to both SEs and companion cells (CCs). Similar to CmPS-1, CsPS-1 accumulates over time in phloem exudates, indicating that serpins differ from other phloem-mobile proteins whose concentrations appear to be stable in phloem exudates. These differences could reflect alternative mechanisms regulating protein turnover and/or inaccessibility of protein degradation. The functionality of the pore/plasmodesma units connecting SEs and CCs was tested with graft-transmitted CmPP1 as a transport marker. The occurrence of CmPP1 in the CCs of the Cucumis graft partner shows that translocated 88 kDa phloem filament protein monomers can symplasmically exit the SE and accumulate in the CC. By contrast, serial sections probed with the serpin antibody demonstrate that the 43 kDa serpin does not enter CCs. Collectively, these data indicate that CCs play a decisive role in homeostasis of exudate proteins; proteins not accessing the CCs accumulate in SEs and display a time-dependent increase in concentration.

The perisperm-endosperm envelope in Cucumis: structure, proton diffusion and cell wall hydrolysing activity

BACKGROUND

and Aims The envelope surrounding the embryo in cucurbit seed, which consists of a single layer of live endosperm cells covered by lipid- and callose-rich layers, is reported to show semi-permeability and also to act as the primary barrier to radicle emergence. Structure, development and permeability of the envelope and activity of cell wall hydrolases during germination of cucumber and muskmelon seeds were investigated.

METHODS

Sections of seeds were stained with aniline blue and Sudan III. Proton diffusion and endo-beta-mannanase activity were detected by tissue printing. A gel-diffusion assay was performed to quantify endo-beta-mannanase activity, while the activity of beta-glucanase was determined with laminarin as the substrate and glucose formation measured using the GOD-POD method.

KEY RESULTS

The lipid layer differentiated during seed development in cucumber in the epidermis of a multilayered nucellus, whereas the callose layer appeared to develop outside the endosperm cell layer. Accordingly, the envelope has been called the perisperm-endosperm (PE) envelope. Chloroform treatment of seeds, which resulted in a substantial reduction in Sudan staining of the lipid layer, also enhanced the permeability of the PE envelope to 2,3,5-triphenyltetrazolium chloride. Proton diffusion occurred when the PE envelopes from seeds had their inner surface in contact with bromocresol purple-containing agarose gels, but not when their outer surface was in contact. Substantial endo-beta-mannanase activity was present in the caps of the PE envelopes, whereas a marked increase in beta-glucanase activity was observed in radicles prior to germination.

CONCLUSIONS

The lipid layer seems to contribute to the semi-permeability of the PE envelope. The diffusion of protons might create an acidic environment conducive to the activity of cell wall hydrolases, namely endo-beta-mannanase (EC 3.2.1.78) and beta-glucanase [beta(1-->3)glucanohydrolase; EC 3.2.1.6], which, in turn, may play a role in the weakening of the PE envelope necessary for the protrusion of the radicle in cucumber and muskmelon seeds.

Development of a simple algorithm for the detection of chilling injury in cucumbers from visible/near-infrared hyperspectral imaging

Hyperspectral images of cucumbers under a variety of conditions were acquired to explore the potential for the detection of chilling-induced damage in whole cucumbers. Region of interest (ROI) spectral features of chilling injured areas, resulting from chilling treatment at 0 degrees C, showed the reduction of reflectance intensity over the period at post-chilling room temperature (RT) storage. A large spectral difference between good, smooth skins and chilling-injured skins occurred in the 700-850 nm visible/near-infrared (NIR) region. Both simple band ratio algorithms and principal component analysis (PCA) were attempted to discriminate the ROI spectra of good cucumber skins from those of chilling injured ones. Results revealed that both the dual-band ratio algorithm (R(811nm)/R(756nm)) and a PCA model from a narrow spectral region of 733-848 nm can detect chilling-injured skins with a success rate of over 90%. The results also suggested that chilling injury is relatively difficult to detect at the initial post-chilling RT stage, especially during the first 0-2 days in storage, due to insignificant manifestation of chilling induced symptoms.

[Expression of CFL gene during differentiation of floral and vegetative buds in cucumber cotyledonary nodes cultured in vitro]

CFL gene, a LFY homologue, was cloned from cucumber (Cucumis sativus L.). In this paper, in situ hybridization was performed to analyze the expression pattern of CFL gene at the stage of floral and vegetative buds differentiation in cucumber cotyledonary nodes cultured in vitro. The results showed that at the stage of floral differentiation, CFL gene was strongly expressed in primordia, floral organ primordia, and each whirl of floral organs at the early stage of their formation, but weakly expressed or not expressed in floral organs after their formation (Fig. 2). At the stage of vegetative bud differentiation, CFL gene was strongly expressed in meristem, leaf primordium and young leaves, and no apparent expression signal was detected in mature tissues (Fig. 3). The results suggest that the expression of CFL gene be necessary for the differentiation and formation of floral and vegetative primordias, and it plays an important role in floral and vegetative development in cucumber. The results also indicate that CFL gene involving in mitosis initiation, mitosis controlling, and transformation of vegetative meristem to floral meristem.

A systemic small RNA signaling system in plants

Systemic translocation of RNA exerts non-cell-autonomous control over plant development and defense. Long-distance delivery of mRNA has been proven, but transport of small interfering RNA and microRNA remains to be demonstrated. Analyses performed on phloem sap collected from a range of plants identified populations of small RNA species. The dynamic nature of this population was reflected in its response to growth conditions and viral infection. The authenticity of these phloem small RNA molecules was confirmed by bioinformatic analysis; potential targets for a set of phloem small RNA species were identified. Heterografting studies, using spontaneously silencing coat protein (CP) plant lines, also established that transgene-derived siRNA move in the long-distance phloem and initiate CP gene silencing in the scion. Biochemical analysis of pumpkin (Cucurbita maxima) phloem sap led to the characterization of C. maxima Phloem SMALL RNA BINDING PROTEIN1 (CmPSRP1), a unique component of the protein machinery probably involved in small RNA trafficking. Equivalently sized small RNA binding proteins were detected in phloem sap from cucumber (Cucumis sativus) and lupin (Lupinus albus). PSRP1 binds selectively to 25-nucleotide single-stranded RNA species. Microinjection studies provided direct evidence that PSRP1 could mediate the cell-to-cell trafficking of 25-nucleotide single-stranded, but not double-stranded, RNA molecules. The potential role played by PSRP1 in long-distance transmission of silencing signals is discussed with respect to the pathways and mechanisms used by plants to exert systemic control over developmental and physiological processes.

Breakage of resistance to Cucumber mosaic virus by co-infection with Zucchini yellow mosaic virus : enhancement of CMV accumulation independent of symptom expression

Resistance to the cucumovirus Cucumber mosaic virus (CMV) in cucumber cv. Delila was manifested as a very low level of accumulation of viral RNA and capsid protein, and an absence of CMV-induced symptoms. In addition, resistance was observed at the single cell level, with a reduction in accumulation of CMV RNAs, compared to accumulation in cells of the susceptible cucumber cv. Bet Alpha. Resistance to CMV in cv. Delila was broken by co-infection with the potyvirus Zucchini yellow mosaic virus (ZYMV). Resistance breakage in cv. Delila plants was manifested by an increase in the accumulation of (+) and (-) CMV RNA as well as CMV capsid protein, with no increase in the level of accumulation of ZYMV. Resistance breakage in the resistant cultivar by ZYMV also occurred at the single cell level. Thus, synergistic interactions known to occur between a potyvirus and a cucumovirus led to resistance breakage during a double infection. However, resistance breakage was not accompanied by an increase in disease symptoms beyond those induced by ZYMV itself. On co-inoculation with an asymptomatic variant of ZYMV-AG an enhancement of CMV infection occurred without disease manifestation. Consequently, intensification of viral RNA and capsid protein accumulation can occur without a corresponding increase in disease development, suggesting that different host genes regulate viral accumulation and disease development in the CMV-resistant cucumber plants.

[Carpel development in male flowers of cucumber (Cucumis sativus. L)]

Carpel development in cucumber male flowers was studied by morphological, histochemical and isoenzyme electrophoretic analyses. The results showed that: (1) cell number of the carpel in male flowers increased continuously during the development of male flowers, and the carpel in male flowers was abundant in RNA content; (2) the carpel in male flowers at latter developmental stage was differentiated, and a placenta-like structure was formed in the carpel of male flowers during anthesis, while the ovule did not appear as that of mature female flowers; (3) The POD and esterase isoenzyme electrophoretic profiles of carpel varied from the development of male flowers, which further indicated that the carpel in mature male flowers was no longer at the stage of primordium. It could be deduced from the results that carpel of cucumber male flowers develops continuously in a distinct pattern with that of female flowers during sex expression process.

Ethylene stimulates endoreduplication but inhibits cytokinesis in cucumber hypocotyl epidermis

The effects of ethylene on cell division are generally considered inhibitory. In this study, we demonstrate that transient ethylene exposure, while suppressing cytokinesis, stimulates DNA synthesis. We monitored DNA synthesis and cytokinesis in the epidermis of cucumber (Cucumis sativus) hypocotyls, an organ whose post-germination development involves strictly limited cell division. During exposure to ethylene, DNA synthesis, assessed by the incorporation of the thymidine homolog 5-bromo-2'-deoxyuridine, was detected in 20% of the epidermal cells, whereas DNA synthesis was nearly undetectable in normal air. Cytofluorometric analysis of nuclei in affected cells showed an up to 8-fold increase in DNA content. During this time, new cell plate formation was not detected. However, shortly after ethylene was removed, DNA content was rapidly restored to 2C (diploid) levels in all cells, and new cell plate formation dramatically increased. These results demonstrate that ethylene promotes DNA synthesis and its endoreduplication but inhibits cytokinesis, thereby maintaining some cells in G2 phase.

Mosaic (MSC) cucumbers regenerated from independent cell cultures possess different mitochondrial rearrangements

Passage of the highly inbred cucumber ( Cucumis sativus L.) line B through cell culture produces progenies with paternally transmitted, mosaic (MSC) phenotypes. Because the mitochondrial genome of cucumber shows paternal transmission, we evaluated for structural polymorphisms by hybridizing cosmids spanning the entire mitochondrial genome of Arabidopsis thaliana L. to DNA-gel blots of four independently generated MSC and four wild-type cucumbers. Polymorphisms were identified by cosmids carrying rrn18, nad5-exon2, rpl5, and the previously described JLV5 deletion. Polymorphisms revealed by rrn18 and nad5-exon2 were due to one rearrangement bringing together these two coding regions. The polymorphism revealed by rpl5 was unique to MSC16 and was due to rearrangement(s) placing the rpl5 region next to the forward junction of the JLV5 deletion. The rearrangement near rpl5 existed as a sublimon in wild-type inbred B, but was not detected in the cultivar Calypso. Although RNA-gel blots revealed reduced transcription of rpl5 in MSC16 relative to wild-type cucumber, Western analyses revealed no differences for the RPL5 protein and the genetic basis of the MSC16 phenotype remains enigmatic. We evaluated 17 MSC and wild-type lines regenerated from independent cell-culture experiments for these structural polymorphisms and identified eight different patterns, indicating that the passage of cucumber through cell culture may be a unique mechanism to induce or select for novel rearrangements affecting mitochondrial gene expression.

DNA damage in the early primordial anther is closely correlated with stamen arrest in the female flower of cucumber (Cucumis sativus L.)

To investigate the regulatory mechanisms of sex expression in cucumber, morphological observations and biochemical analyses were carried out on inappropriate stamen development of female flowers of cucumber. It was found that developmental arrest of the inappropriate stamen mainly occurs at the anther primordium. This arrest is closely correlated with DNA damage, as detected by TUNEL assay, and might result from anther-specific DNase activation. It was also found that the DNA damage does not lead to cell degeneration, although chromatin condensation is observed in the anther primordia.

Diffusion of an organic cation into root cell walls

Uptake of a cationic dye (methylene blue) by isolated root cell walls, roots of whole transpiring seedlings, and excised roots was investigated using 7-day-old seedlings of cucumber, maize, and wheat. The number of ionogenic groups per 1 g dry and wet weight of the root cell walls, their swelling capacity (K(cw)), time-dependence of methylene blue (M(cw)) ion exchange capacity, and diffusion coefficients of the cation diffusion in the polymer matrix of the cell walls (D(cw)) were determined. The M(cw) value depended on pH (or carboxyl group dissociation); it changed in accordance with the number of carboxyl groups per 1 g cell wall dry weight. This parameter decreased in the order: cucumber > wheat > maize. For description of experimental kinetic curves and calculation of cation diffusion coefficients, the equation for ion diffusion into a cylinder of infinite length was used. The chosen model adequately described cation diffusion in cell walls and roots. Diffusion coefficient values for cucumber, wheat, and maize were 3.1*10(-8), 1.3*10(-8), and 8.4*10(-8) cm(2)/sec, respectively. There was a statistically significant linear dependence between K(cw) and D(cw) values, which characterize the same property of the polymer matrix, rigidity of its polymer structure or the degree of cross-linkage or permeability. This also confirms the right choice of the model selected for calculation of methylene blue diffusion coefficients, because K(cw) and D(cw) values were obtained in independent experiments. The coefficients determined for methylene blue diffusion in transpiring seedling roots (D(ts)) and excised roots (D(er)) depended on the plant species. The rate of methylene blue diffusion into the excised roots was either 1.5-fold lower (cucumber) or 3-4-times lower (maize, wheat) than in cell walls. The values of diffusion coefficients in roots of whole seedlings were comparable which those for the cell walls. On the basis of the experimental data and results of calculations, it is concluded that the mechanism of methylene blue uptake by plant roots involves ion exchange reactions between the organic cation and cell wall carboxyl groups, and the uptake rate is determined by the cation diffusion in the polymer matrix of the cell walls.

Nitrate transport across the tonoplast of Cucumis sativus L. root cells

Nitrate transport across the tonoplast has been studied using vacuole membranes isolated from cucumber roots grown in nitrate. The addition of NO3- ions into the tonoplast with ATP-generated transmembrane proton gradient caused the dissipation of delta pH, indicating the NO3(-)-induced proton efflux from vesicles. NO3(-)-dependent H+ efflux was almost insensitive to the transmembrane electrical potential difference, suggesting the presence of an electroneutral NO3-/H+ antiporter in the tonoplast. Apart from saturation kinetics, with respect to nitrate ions, NO3(-)-linked H+ efflux from the tonoplast of cucumber roots showed other characteristics expected of substrate-specific transporters. Experiments employing protein modifying reagents (NEM, pCMBS, PGO and SITS) indicated that a crucial role in the activity of tonoplast nitrate/proton antiporter is played by lysine residues (strong inhibition of NO3-/H+ antiport by SITS). None of the ion-channel inhibitors (NIF, ZnSO4 and TEA-Cl) used in the experiments had a direct effect on the nitrate transport into tonoplast membranes. On the other hand, every protein reagent, as well as NIF and ZnSO4, significantly affected the ATP-dependent proton transport in vesicles. Only TEA-Cl, the potassium channel blocker, had no effect on the vacuolar proton pumping activity.

[Studies on calcium sensitive period during floral differentiation of cucumber cotyledonary node cultures in vitro]

Cotyledonary nodes of cucumber cultured on calcium-free medium for 0, 1, 2, 3, 4, 5, 6d respectively, were transferred to medium with 6.0 mmol/L CaCl2 for 24h, then returned to calcium-free medium. Cotyledonary nodes cultured on calcium-free or 6.0 mmol/L CaCl2 medium for all time, were taken as controls. Results showed that cotyledonary nodes were transferred to 6.0 mmol/L CaCl2 medium for 24h during 0-3d after the beginning of culture, percentage of floral bud formation at cotyledonary nodes was increased significantly. Transferring cotyledonary nodes on the 3d day after the beginning of culture was achieved best effect, percentage of floral bud formation was up to 34.3%. We deduced that the calcium sensitive period during floral differentiation of cucumber cotyleddonary node cultured in vitro may be 0-4d after the beginning of culture.

Determination of element distribution between the symplasm and apoplasm of cucumber plant parts by total reflection X-ray fluorescence spectrometry

The distribution of Cd, Ni, Pb and Fe between the symplasm (cytoplasm) and apoplasm (cell wall) of cucumber roots and leaves was determined by total reflection X-ray fluorescence spectrometry following a special sample preparation procedure. The plants were grown in modified Hoagland nutrient solution containing Fe in chemical form of Fe(III)-citrate or Fe(III)-EDTA, as well as the heavy metal contaminants, each in concentration of 10 microM. In the roots the larger part of Pb was found in the apoplasm, while Ni and Cd were mostly in the symplasm. In the leaves however, about 50-60% of the Pb content and practically the total amount of Cd were detected in the symplasm. About 30-40% of the translocated Ni remained in the apoplasm of the leaves. The Cd-, Ni- and Pb-treatments resulted in higher total concentration of Fe in the roots, however, the relative amount of Fe in the symplasm decreased in all cases. In the leaves of the control plants the larger part (60-80%) of Fe occurs in the symplasm. Due to the heavy metal effects, the relative amount of Fe in the symplasm decreased except in the Pb-contaminated plants, where in the presence of Fe(III)-EDTA, the Pb treatment resulted in a moderate increment of Fe concentration in the symplasm.

Extracellular freezing in leaves of freezing-sensitive species

Low-temperature scanning-electron microscopy was used to study the freezing of leaves of five species that have no resistance to freezing: bean (Phaseolus vulgaris L.), tobacco (Nicotiana tabacum L.), tomato (Lycopersicon esculentum L.), cucumber (Cucumis sativus L.), and corn (Zea mays L.). In the leaves of the four dicotyledonous species, ice was extracellular and the cells of all tissues were collapsed. In contrast, in maize leaves ice was extracellular in the mesophyll, and these cells were collapsed, but the epidermal and bundle-sheath cells apparently retained their original shapes and volume. It is concluded that the leaves of the freezing-sensitive dicotyledonous species tested were killed by cellular dehydration induced by extracellular freezing, and not by intracellular freezing. Freezing injury in maize leaves apparently resulted from a combination of freezing-induced cellular dehydration of some cells and intracellular ice formation in epidermal and bundle-sheath cells.

Effect of Mentha x piperita essential oil and monoterpenes on cucumber root membrane potential

Peppermint (Mentha piperita L.) essential oil and its main components were assessed for their ability to interfere with plant plasma membrane potentials. Tests were conducted on root segments isolated from etiolated seedlings of cucumber (Cucumis sativus L.). Increasing the concentration of peppermint essential oil from 5 to 50 ppm caused a decrease in membrane potential (Vm) hyperpolarization of 10-3 mV, whereas concentrations from 100 up to 900 ppm caused an increasing depolarization of Vm (from 5 to 110 mV). When tested at 300 ppm, (+)-menthyl acetate, (-)-limonene and 1,8-cineole did not exert any significant effect on V(m), whereas (+)-menthofuran (73 mV), (+)-pulegone (85 mV), (+)-neomenthol (96 mV), (-)-menthol (105 mV) and (-)-menthone (111 mV) showed increased ability to depolarize V(m). A plot of log of octanol-water partition coefficient (K(ow)) against their depolarizing effect showed a significant negative correlation, suggesting that among all monoterpenoids increased membrane depolarization depends on lower K(ow). However, among monoterpene ketones, alcohols and furans, increased membrane depolarization is associated with a decline in water solubility. The possible effect of monoterpenoids on membrane ion fluxes is also discussed, since changes in the bioelectric potential of cells imply changes in the flux of ions across the plasma membrane