Tissue Discs Research Articles

The effect of cell turgor on sugar uptake in strawberry fruit cortex tissue

A reduction in cell turgor has been shown to stimulate sugar uptake in several plant sink tissues and it may regulate the import of assimilate into the sink apoplast, as well as maintain cell turgor. To determine whether cell turgor influences sugar uptake by strawberry (Fragaria x ananassa Duch. cv. Brighton) fruit cortex tissue, disks were cut from greenhouse‐grown primary fruit at the green‐white stage of development and placed in buffered incubation solutions containing either mannitol or ethylene glycol as an osmoticum. Cell turgor of fruit disks was calculated from the difference between the water potential of bathing solution and tissue solute potential after incubation at various osmolarities. Cell turgor increased when tissue disks were placed into mannitol incubation solutions more dilute than the water potential of fresh tissue (about 415 mOsmol kg−1). The rate of uptake of [14C]‐sucrose or [14C]‐glucose decreased as osmolarity of the incubation solution increased, i.e. as cell turgor declined. Cell turgor and the rate of [14C]‐sucrose uptake were unaffected when rapidly permeating ethylene glycol was used as an osmoticum. A decrease in cell turgor reduced both the Vmax of the saturable (carrier mediated) kinetic component of sucrose uptake, and the slope of the linear (diffusional) component. The sulfhydryl binding reagent p‐chloromercuibenzenesulfonic acid, an inhibitor of the plasma membrane sucrose carrier, strongly inhibited only the saturable component of sucrose uptake. Increased uptake of the nonmetabolizable sugar, O‐methyl‐glucose, at high turgor was similar to that of glucose, indicating that carrier activity was influenced by cell turgor, not cell metabolism. Turgor did not influence efflux of [14C]‐sucrose from disks and had no effect on cell viability. Strawberry fruit cells do not possess a sugar uptake system that is stimulated by a reduction in turgor.

Differential effects of turgor on sucrose and potassium transport at the tonoplast and plasma membrane of sugar beet storage root tissue

ABSTRACTWhen turgor was increased, by decreasing the concentration of mannitol bathing discs of sugar beet storage root tissue, the rates of sucrose and potassium uptake into the vacuole were decreased. At all external mannitol concentrations the rate of sucrose and potassium uptake across the plasma membrane was an order of magnitude greater than the rate of quasi‐steady uptake into the vacuole, implying a very large efflux. Efflux of both sucrose and potassium was increased at high turgor. However, while increasing turgor decreased the rate of K+ uptake, the rate of sucrose uptake at the plasma membrane increased with time. Compartmental analysis of tracer exchange kinetics was used to determine unidirectional K+ fluxes. From these results, it was estimated that the increase in K+ efflux accompanying a 1.5 MPa increase in turgor could lead to a net increase of 140mol−3h−1 in the external potassium concentration. It is suggested that the turgor‐imposed increase in solute efflux is a means of regulating intracellular osmotic pressure and/or turgor in sugar beet storage roots, but that sucrose is preferentially retrieved from the apoplast, even under conditions of excessively high turgor. However, much of this sucrose is probably lost from the cell, implying a ‘futile’ sucrose transport cycle at the plasma membrane. The turgor‐stimulated leak of potassium could play a major role in the regulation of turgor pressure in sugar beet storage root tissue.

Altered aggrecan synthesis correlates with cell and nucleus structure in statically compressed cartilage.

Previous studies have shown that static equilibrium compression of cartilage tissue in vivo and in vitro decreases chondrocyte synthesis of aggrecan molecules. In order to identify mechanisms of cellular response to loading, we have investigated alterations in cell and nucleus structure and the accompanying changes in the synthesis of aggrecan in statically compressed cartilage explants. Using glutaraldehyde fixation and quantitative autoradiography of compressed and radiolabeled cartilage disks we spatially localized newly synthesized aggrecan. Using stereological tools to analyze these same specimens we estimated the cell and nucleus volume, surface area and directional radii. We found that aggrecan synthesis was reduced overall in compressed tissue disks. However, the compression induced a spatial (radial) inhomogeneity in aggrecan synthesis which was not present in uncompressed disks. This spatial inhomogeneity appeared to be directly related to mechanical boundary conditions and the manner in which the load was applied and, therefore, may represent a spatially specific functional adaptation to mechanical loading. Coincident with reduced aggrecan synthesis, we observed reductions in cell and nucleus volume and radii in the direction of compression which were in approximate proportion to the reduction in tissue thickness. Cell and nucleus dimensions perpendicular to the direction of compression did not change significantly. Therefore the observed deformation of the cell and nucleus in statically compressed cartilage approximately followed the dimensional changes imposed on external specimen surfaces. The strong correlation observed between local changes in aggrecan synthesis and alterations in cell and nucleus structure also lend support to certain hypotheses regarding the intracellular signal transduction pathways that may be important in the biosynthetic response of chondrocytes to mechanical loading.

Screening for Resistance toSclerotinia trifoliorumin Alfalfa by Inoculation of Excised Leaf Tissue

Excised leaf tissues of alfalfa were inoculated with Sclerotinia trifoliorum to determine whether leaf-inoculation techniques may be used to screen for heritable and effective resistance to the pathogen. Leaf tissues were placed on water agar in petri plates, inoculated with mycelia of S. trifoliorum, and incubated at 17°C. Host responses were evaluated according to the rate and extent of necrosis that developed during 1 week. One thousand plants of cultivar Delta were screened for resistance by successive inoculation of excised unifoliate leaves, first trifoliate leaflets, and disks of tissue from later leaves. The five plants considered most resistant were intercrossed, and progeny were compared with those of five plants selected for susceptibility, five selected at random, and the parent cultivar. Progeny of resistant plants had significantly (P = 0.05) less severe disease than did progenies of susceptible and random plants and the parent cultivar when evaluated by inoculation of unifoliate leaves, first trifoliate leaflets, leaf disks, excised stems, and whole plants. In field experiments with natural infection during two growing seasons, progeny of resistant plants had less severe disease and produced significantly (P = 0.05) higher yields of forage through the first or second harvests than did progenies of susceptible and random plants and the parent cultivar. These results establish that inoculation of excised leaf tissue is an effective and efficient means to screen for resistance to S. trifoliorum in alfalfa.

Mutagenesis of endopolygalacturonase from Fusarium moniliforme: histidine residue 234 is critical for enzymatic and macerating activities and not for binding to polygalacturonase-inhibiting protein (PGIP).

The sequence encoding the endopolygalacturonase (PG) of Fusarium moniliforme was cloned into the E. coli/yeast shuttle vector Yepsec1 for secretion in yeast. The recombinant plasmid (pCC6) was used to transform Saccharomyces cerevisiae strain S150-2B; transformed yeast cells were able to secrete PG activity into the culture medium. The enzyme (wtY-PG) was purified, characterized, and shown to possess biochemical properties similar to those of the PG purified from F. moniliforme. The wtY-PG was able to macerate potato medullary tissue disks and was inhibited by the polygalacturonase-inhibiting protein (PGIP) purified from Phaseolus vulgaris. The sequence encoding PG in pCC6 was subjected to site-directed mutagenesis. Three residues in a region highly conserved in all the sequences known to encode PGs were separately mutated: His 234 was mutated into Lys (H 234-->K), and Ser 237 and Ser 240 into Gly (S 237-->G and S 240-->G). Each of the mutated sequences was used to transform S. cerevisiae and the mutated enzymes were purified and characterized. Replacement of His 234 with Lys abolished the enzymatic activity, confirming the biochemical evidence that a His residue is critical for enzyme activity. Replacement of either Ser 237 or Ser 240 with Gly reduced the enzymatic activity to 48% and 6%, respectively, of the wtY-PG. When applied to potato medullary tissue, F. moniliforme PG and wtY-PG caused comparable maceration, while the variant PGs exhibited a limited (S 234-->G and S 240-->G) or null (H 234-->K) macerating activity. The interaction between the variant enzymes and the P. vulgaris PGIP was investigated using a biosensor based on surface plasmon resonance (BIAlite). The three variant enzymes were still able to interact and bind to PGIP with association constants comparable to that of the wild type enzyme.

Growth-regulator-induced Alleviation of Chilling Injury in Green and Red Bell Pepper Fruit during Storage

Bell pepper (Capsicum annuum L.) plants were sprayed at full bloom with paclobutrazol (50 and 100 mg·liter–1), uniconazole (20 and 50 mg·liter–1), or mefluidide (20 and 50 mg·liter–1). Mature-green and red fruit were harvested 4 and 8 weeks after spraying, respectively. Paclobutrazol and uniconazole, but not mefluidide, affected plant growth and fruit morphology. All three growth regulators alleviated chilling injury that developed on green and red peppers after 28 days at 2C. Potassium leakage was lower from tissue disks, and weight loss less, from treated fruit than from control fruit. Ethylene and CO2 production at 20C were similar in control and paclobutrazol-treated fruit after 28 days at 2C. These results are consistent with previous findings that showed these growth regulators are able to increase tolerance to low temperatures in whole plants. Chemical names used: N-[2,4-dimethyl-5-trifluoromethyl sulfonyl amino phenyl acetamide] (mefluidide); (2RS, 3RS) 1-(4-chlorophenyl)-4,4 dimethyl-2-(1,2,4-triazol-1-yl)-pentan-3-ol (paclobutrazol); (E)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4 triazol-1-yl)-pentan-3-ol (uniconazole).

Comparison of Sorbitol Transport in Excised Tissue Discs and Cortex Tissue of Intact Apple Fruit

Summary Transport of [ 14 C]sorbitol from a bathing solution into isolated tissue discs of apple fruit was compared with transport of sorbitol supplied to wells cut in tissue of intact fruit. After uptake of sorbitol, labelled tissue was excised from fruit and, as for isolated discs, the tissue was washed in a hypotonic solution to remove label from the apoplast. This treatment also released radioactive label from the cytoplasm. The procedure allowed the separation of radioactivity located in the apparent free space, comprising apoplast and cytoplasm, from the radioactivity remaining in the compartmented space of the tissue. Transport of sorbitol from an osmotically adjusted medium into the apparent free space of tissue discs was linear with respect to substrate concentration. Uptake was not affected by competitors such as sucrose or mannitol, and was not inhibited by the SH-reagent PCMBS and the uncoupler CCCP. Transport showed a pH-optimum around pH 7.0. Exchange diffusion of [ 14 C]sorbitol previously incorporated in tissue discs was not stimulated by external sorbitol. These findings indicate that in the presence of an osmoticum diffusion of sorbitol across the plasma membrane is the most likely form of transport. Under the same conditions uptake of sorbitol into the compartmented space of excised tissue discs showed a hiphasic response with a saturable component at low concentrations and a non-saturable, linear, diffusion-like component at higher concentrations (up to 100 mM), indicating carrier mediated transport at the tonoplast. In intact fruit, transport of sorbitol into the apparent free space was linear with respect to substrate concentration and had a pH-optimum of pH 7.0. These results were obtained using a hypotonic uptake medium and confirmed the observations made on tissue discs. However, unlike isolated tissue, uptake in intact fruit was not increased by a low osmotic solute concentration of the uptake medium. In intact fruit, transport of sorbitol under hypotonic conditions was further reduced 57% by sucrose, 49% by mannitol and 30% by PCMBS. These observations, made only on intact fruit, would point to the presence of a carrier for sorbitol located in the plasma membrane. It is assumed that in intact fruit the integrity of the plasma membrane is maintained, allowing sorbitol to interact with a carrier. This ability may be lost when tissue is excised from the fruit and kept in solutions of high osmotic activity. A likely location for such a carrier would be within channels of the plasma membrane and its activity might be controlled by pressure sensitive gating of such structures.

Three polygalacturonases constitutively synthesized by Aspergillus alliaceus

Of three molecular forms of polygalacturonases synthesized by Aspergillus alliaceus on glucose media, two were exopolygalacturonases (exoPG1 and exoPG2) and one was an endopolygalacturonase (endoPG). Low-methoxylated beet pectin was the preferred substrate for the endoPG and exoPG2 whereas pectic acid was the optimal substrate for exoPG1. The highest activities of endoPG, exoPG1 and exoPG2 were at pH 5.5, 3.5, and 6.0 and at 35, 45 to 50 and 35°C, respectively. Disks of potato-tuber tissue were macerated by endoPG, but not by exoPG1 or exoPG2.

Characterization of myotonic dystrophy kinase (DMK) protein in human and rodent muscle and central nervous tissue.

Myotonic dystrophy (DM) is the most common form of inherited neuromuscular disease in adults and is characterized by progressive muscle wasting and myotonia. The mutation responsible for DM has been identified as the amplification of a polymorphic (CTG)n repeat in the 3' untranslated region of a gene encoding a serine/threonine kinase (DMK). We have produced a polyclonal rabbit antibody preparation against a fusion protein encoding the C-terminal amino acids 471-629 of the human DMK gene. This antibody specifically detects products of both full length and truncated human DMK genes expressed in bacteria and in insect cells. On immunoblots, we observed protein species of approximately 74 and 82 kDa in cardiac muscle, skeletal muscle, ependyma and choroid plexus. By immunofluorescence, DMK was found to localize post-synaptically at the neuromuscular junction of skeletal muscle, at intercalated discs of cardiac tissue and at the apical membrane of the ependyma and choroid plexus. We have also detected two to three species (approximately 45-50 kDa) in other regions of the brain. Synaptic localization of DMK in the cerebellum, hippocampus, midbrain and medulla was noted. These results suggest that DMK plays a specialized role in intercellular communication.

Synthesis and Storage of Sucrose in Relation to Activities of Its Metabolizing Enzymes in Sugarcane Cultivars Differing in Maturity

Metabolic changes in the contents of sucrose and hexoses in relation to the activities of invertase, sucrose synthase and sucrose-phosphate synthase in early (CoJ 64) and late (Co 1148) maturing cultivars of sugarcane have been studied. During early stages of cane growth, lower activities of sucrose synthase and sucrose-phosphate synthase in leaf blade In CoJ 64 over Co 1148 were observed. However, sucrose content in sheath/blade was higher in CoJ 64 than in Co 1148. With the advancing age, the activity of soluble acid invertase (pH 5.4) in stem declined more rapidly in CoJ 64. This resulted in building up of high ratio of sucroselinvert sugars in stem tissue of this cultivar. Feeding uniformly-labelled sucrose and glucose to the cut discs of leaf sheath resulted in higher uptake of 14C in CoJ 64 than in Co 1148. Uptake by stem tissue discs of 14C from sucrose was less than that from hexoses. Based on these results, it is suggested that (i) the rapid fall in the activity of soluble acid invertase in stem concomitant with fast accumulation of sucrose in this tissue is an index of early maturity of the cane, and (ii) high content of sucrose in sheath is a reflection of an efficient translocation of this sugar in early maturing cultivars.

Physiological and Mechanical Properties of Kiwifruit Tissue Associated with Texture Change during Cool Storage

Kiwifruit [Actinidia deliciosa (A. Chev) C.F. Liang et A.R. Ferguson] flesh firmness can decline by as much as 94% during fruit ripening. This phenomenon was investigated at the cellular level, with the aim of characterizing changes in the physiological condition and mechanical properties of cells. The tensile strength of kiwifruit outer pericarp tissue was measured, and low-temperature scanning electron microscopy was used to examine the mode of cell failure at fracture surfaces. The propensity with which cells ruptured was determined by incubating tissue discs in hypertonic and hypotonic solutions, and water potentials, osmotic potentials, turgor pressures, and tissue density were measured. An initial rapid reduction in flesh firmness—from 80 to 27 N during 6 weeks of storage at 0C—was related to a reduction in the adhesion between neighboring cells. Following tensile tests, an examination of fracture surfaces indicated that cells from freshly harvested fruit had ruptured, exposing the cell interior. After 6 weeks of storage, neighboring cells separated from each other without breaking open. With 23 additional weeks of storage at 0C, flesh firmness decreased from 27 to 5 N. The final softening stage was associated with an increase in the proportion of cells that separated at the middle lamella and an increase in the plasticity of the cell wall.

Genetic manipulation of 6-phosphofructokinase in potato tubers

The aim of this work was to discover whether genetic manipulation of 6-phosphofructokinase [EC 2.7.1.11; PFK(ATP)] influenced the rate of respiration of tuber tissue of Solanum tuberosum L. Transgenic plants were produced that contained the coding sequence of the Escherichia coli pfkA gene linked to a patatin promoter. Expression of this chimaeric gene in tubers resulted in a 14to 21-fold increase in the maximum catalytic activity of PFK(ATP) without affecting the activities of the other glycolytic enzymes. Tubers, and ‘aged’ disks of tuber tissue, from transformed plants showed no more than a 30% fall in the content of hexose 6-monophosphates; the other intermediates of glycolysis increased threeto eightfold. Fructose-2,6-bisphosphate was barely detectable in aged disks of transformed tubers. The relative rates of 14CO2 production from [1-14C]-and [6-14C]-glucose supplied to disks of transformed and control tubers were similar. Oxygen uptake and CO2 production by aged disks of transformed tubers did not differ significantly from those from control tubers. The same was true of CO2 production, in air, and in nitrogen, for tuber tissue. It is concluded that PFK(ATP) does not dominate the control of respiration in potato tubers.

Modification of the Cellular Heat Sensitivity of Cucumber by Growth under Supplemental Ultraviolet-B Radiation.

The effect of ultraviolet B (UV-B) radiation on the thermal sensitivity of cucumber (Cucumis sativus L.) was studied using UV-B-sensitive cv Poinsett 76 and UV-B-resistant cv Ashley grown under control and elevated (300 mW m-2) UV-B radiation levels. Using both cotyledon and leaf discs, the ability of the tissue to reduce triphenyl tetrazolium chloride (TTC) was determined after treatment at 50[deg]C for various times. Semilogarithmic plots of TTC reduction as a function of time at 50[deg]C were curvilinear. They were monophasic for the control cucumber and biphasic for cucumber grown in the presence of elevated UV-B. Treatment of cucumber plants at 37[deg]C for 24 h or of tissue discs at acute UV-B levels for 1 h further modified their response to elevated temperature. These results suggest that growth of cucumber under enhanced UV-B radiation levels increased its ability to withstand elevated temperatures.

Activity of 1-aminocyclopropane-l-carboxylate (ACC) oxidase (ethylene-forming enzyme) in the pulp and peel of ripening bananas

SummaryActivity of 1-aminocyclopropane-l-carboxylate (ACC) oxidase (previously known as the ethylene-forming enzyme, EFE) in the pulp and peel of ripening bananas has been stud-ied in vitro, using tissue discs, and in vitro. The enzyme extracted from both pulp and peel resembles ACC oxidases from a variety of other fruits in its requirement for ascorbate, Fe2+ and CO2, when assayed in vitro, and in its Mr of about 41,000. When the enzyme was assayed in vivo, ascorbate, Fe2+ and CO2 stimulated activity in pulp tissue during the later stages of ripening. By contrast, the enzyme assayed in vivo from peel tissue was not stimulated by these factors. Changes in enzyme activity during ripening did not support a role for the pulp as a trigger for the ripening of the whole fruit.

Sucrose uptake by the phloem parenchyma of carrot storage root

The characteristics of sucrose uptake into the symplast of phloem tissue discs harvested from fresh, actively-growing carrot storage roots are described. Sucrose uptake exhibited a curvilinear response with increasing sucrose concentration. The inhibitors p-chloromercuribenzenesulphonic acid (PCMBS) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreased uptake and resulted in solely linear relationships between uptake and sucrose concentration. These results suggest that active carrier-mediated transport occurs at the plasmalemma in addition to a diffusive mechanism. The former saturates at a lower concentration (approximately 20 mM) than the latter which does not saturate below 100 mM

A Simple, Room-temperature Extraction Method for 14Carbon-labeled Red Raspberry Leaf Tissues

Labeled (`“C) compounds were recovered from tissue disks taken from 14CO2-fed leaves of l-year-old greenhouse-grown plants and l-month-old ex vitro transplants of red raspberry (Rubus idaeus L.) by hot (boiling in 80% ethanol immediately after 14C exposure), delayed-hot (boiling in 80% ethanol after a 2- to 3-day ethanol soak), and room-temperature (RT) (2-to 3-day soak in 80% ethanol) extraction methods. The RT extraction method was simple but as effective for extracting 14C-labeled compounds from red raspberry leaf tissues as hot and delayed-hot extraction methods.

Changes in Lipid Peroxidation and Lipolytic and Free-Radical Scavenging Enzyme Activities during Aging and Sprouting of Potato (Solanum tuberosum) Seed-Tubers.

Previous research has shown that cell membranes of potato (Solanum tuberosum L. cv Russet Burbank) seed-tubers lose integrity between 7 and 26 months of storage (4[deg]C, 95% relative humidity), and this loss coincides with a significant decrease in growth potential. The age-induced decline in membrane integrity is apparently due to increased peroxidative damage of membrane lipids. Malondialdehyde (MDA) and ethane concentrations (sensitive markers of lipid peroxidation and membrane damage) increased in seed-tuber tissues with advancing age. Moreover, in vivo ethane production from discs of cortex tissue from 13- and 25-month-old seed-tubers was 87% greater (on average) than that from discs from 1-month-old tubers. Calcium suppressed ethane production from all ages of tissue discs, and the effect was concentration dependent. Linoleic acid enhanced ethane production from 5- and 17-month-old tubers by 61 and 228%, respectively, suggesting that older tissue may contain a higher free-radical (FR) titer and/or lower free polyunsaturated fatty acid content. In addition, throughout plant establishment, the internal ethane concentration of older seed-tubers was 54% higher than that of younger seed-tubers. MDA concentration of tuber tissue declined by about 65% during the initial 7 months of storage and then increased 267% as tuber age advanced to 30 months. The age-induced trend in tuber reducing sugar concentration was similar to that of MDA, and the two were linearly correlated. The age-dependent increase in reducing sugars may thus reflect peroxidative degeneration of the amyloplast membrane, leading to increased starch hydrolysis. Compared with 5-month-old seed tubers, 17- and 29-month-old seed-tubers had significantly higher levels of lipofuscin-like fluorescent compounds (FCs), which are produced when MDA reacts with free amino acids. Age-dependent increases in MDA, ethane, and FCs were not associated with higher activities of phospholipase and lipoxygenase in tissue from older tubers. In fact, 8-month-old seed-tubers had significantly higher activities of these enzymes than 20-month-old seed-tubers. However, the activities of superoxide dismutase, peroxidase, and catalase in 20-month-old tubers were substantially higher out of storage, and increased at a faster rate during plant establishment, than in 8-month-old seed-tubers. Collectively, these results suggest that a gradual build-up of FRs leads to peroxidative damage of membrane lipids during aging of potato seed-tubers.

733 (PS 7) TEMPERATURE OPTIMUMS FOR ANTHOCYANIN SYNTHESIS IN `BRAEBURN', `DELICIOUS', `FUJI', AND `GALA' APPLE TISSUE

Color in apple skin is a composite of varying concentrations of chlorophyll, carotenoids, and anthocyanins/flavanols. Anthocyanin biosynthesis is influenced by a number of factors including rootstock, nitrogen status, wounding, certain plant growth regulators, light, and temperature. This study investigated changes in the optimum temperature for anthocyanin accumulation of `Braeburn', `Delicious', `Fuji', and `Gala' apple tissue disks. Preclimacteric (core ethylene < 1.0 ul·liter-1) apples with little color development were harvested in September, 1992 and immediately brought to the laboratory. Six 15mm disks about 3mm thick were removed from the shaded side of each fruit and transferred to moistened filter paper on temperature-controlled blocks preset at temperatures ranging from 15C to 40C at intervals of 5C and covered with plastic (CO2 permeable) film. The disks were illuminated for up to 48 hours with a high pressure sodium lamp at a distance of about 1m. Following the illumination period individual peel disks were extracted with acidic methanol for anthocyanins. The temperature optimum for maximum anthocyanin biosynthesis for each cultivar will be reported.

AREA‐ AND PERIMETER‐DEPENDENT PROPERTIES AND FAILURE OF MATURE‐GREEN AND RED‐RIPE TOMATO PERICARP TISSUE

ABSTRACTThe influence of tomato fruit ripeness on area‐ and perimeter‐dependent properties and dominant failure mechanisms of pericarp tissue were investigated. Tissue discs from mature‐green and red‐ripe fruit were punctured with a flat‐ended cylindrical probe and compressed with a flat plate at a constant rate of deformation. Approximately linear force‐deformation curves were obtained to tissue failure by both puncture and flat plate compression, interrupted by a region of pseudoplastic deformation at a relatively low initial bioyield force. Based on estimated area‐ and perimeter‐dependent coefficients and firmness (force/deformation) values, initial bioyielding of tissue appeared to be associated with an abrupt increase in cell‐to‐cell compaction. Puncture of mature‐green tissue led to premature failure induced by shearing or rupture of tissue at the probe perimeter. The contribution to puncture of perimeter‐dependent or shear‐associated forces and a putative “zone of influence” increased markedly with ripening, while the contribution of area‐dependent or compression associated forces generally decreased. A concomitant decrease in failure force and firmness with ripening reflected a general loss of both tissue compression and shear strengths. These results suggested that the dominant mode of tissue failure changed with ripening, from cell relaxation and rupture to cell debonding. The data obtained in this study emphasize the need to exercise caution in the interpretation of force‐deformation parameters derived from puncture tests alone.

THE EFFECTS OF TURGOR PRESSURE ON PUNCTURE AND VISCOELASTIC PROPERTIES OF TOMATO TISSUE

ABSTRACTThe effects of turgor pressure on puncture and viscoelastic properties of mature‐green tomato pericarp were examined using tissue discs soaked in a range of osmotica (0.0–0.6 M mannitol) for at least 36 h at 4C. Turgor pressure was estimated from the osmotic potential of soaking solutions that induced incipient plasmolysis. Based on volume changes, the osmotic potential and turgor pressure of fresh tissue were estimated to be −0.56 ± 0.08 MPa and 0.20 MPa, respectively. However, puncture and viscoelastic properties corresponded to a turgor pressure of 0.15 MPa. The discrepancy between calculated and actual turgor pressures was attributed to the presence of apoplastic solutes. The data from this study revealed a general increase in cell wall stress, strain and elasticity with increasing turgor. With increases in turgor above that of untreated tissue both wall extensibility and elasticity became limiting and thus cell wall stiffness increased. Conversely, a decrease in turgor below that of untreated tissue led to an increase in viscoelasticity. Increases in bioyield and pseudoplastic bioyield strains with a variation in turgor from that of untreated tissue were consistent with cell debonding as a dominant mechanism of tomato tissue bioyielding. The reduced failure force, deformation and firmness with increasing turgor were consistent with cell rupture as a predominant mechanism of failure of mature‐green tomato pericarp tissue.