Chaim Frenkel

Respiratory Upsurge in Blueberries and Strawberries as Influenced by Ethylene and Acetaldehyde

The effects of acetaldehyde and ethylene were compared in the induction of a respiratory rise in blueberry and strawberry fruits. The increase in CO2 production in blueberries caused by acetaldehyde was more pronounced than that caused by ethylene. Acetaldehyde induced a climacteric-like respiration in straw-berries, while ethylene had no effect. The results suggest that the action of acetaldehyde and ethylene involves different mechanisms.

Activity of Enzymes Involved in the Turnover of Hydrogen Peroxide during Fruit Senescence

Changes in activity of four enzymes associated with the turnover of hydrogen peroxide were monitored during ripening of several fruits. Glycolate oxidase activity increased during ripening in the pulp of pear (Pyrus communis L. var. Bosc), avocado (Persea americana Mill. var. Fuerte), and tomato (Lycopersicon esculentum Mill. var. Rutgers), declined in pear peel, and was not detectable in avocado peel. The changes in glycolate oxidase activity did not correspond to the changes in chlorophyll levels which occurred during ripening. Xanthine oxidase and peroxidase activity also increased during ripening in pear pulp. These changes in enzyme activity followed similar patterns whether expressed in terms of protein content or fresh weight. Catalase in pear pulp showed a continuous decline in activity per gram fresh weight but a slight increase followed by a decline in activity per milligram protein. Previously observed increases in hydrogen peroxide levels in senescent fruit tissue may be related to these changes in enzyme activity.

Interaction of Oxygen and Ethylene in the Release of Ragweed Seeds from Dormancy

Ragweed (Ambrosia artemisiifolia L.) seeds were stratified at 5 C in air or nitrogen. Stratification in air led to a progressive increase in germination with time of treatment. The seeds stratified in nitrogen failed to germinate and required an additional stratification in air to induce germination. In a separate experiment, dormant seeds were treated with 10 ppm ethylene in air or in O2. After 2 wk of treatment, germination was 5.5% in air, 13.3% in O2, 41.3% in air plus ethylene, 71.4% in oxygen plus ethylene, and 31.3% in ethephon. Application of thiourea, serving to inhibit catalase activity, also stimulated germination.

Regulation of Ripening in Bartlett Pears with Sulfhydryl Reagents

Sulfhydryl compounds and sulfhydryl reagents were applied to Bartlett pears to test their effect on the onset of ripening. The ripening parameters tested were softening and the evolution of CO2 and ethylene. Application of the sulfhydryl compounds cysteine and dithiothreitol resulted in the inhibition of ripening. By contrast, sulfhydryl reagents, including alloxan and N-ethylmaleimide, promoted the ripening process. The results suggest that the onset of ripening is influenced by a decline of sulfhydryl gradients in fruit.

Disruption of macromolecular hydration — a possible origin of chilling destabilization of biopolymers

Abstract Chilling temperatures (0–15°C) disrupt the metabolism of plants, resulting in abnormal growth and development and, often, in the death of the plant. Sensitivity to chilling is a major constraint in the production and cold preservation of crops, but the metabolic origin of sensitivity to chilling is not clear and, consequently, the phenomenon is not easily controlled. However, recent data suggest that chilling may destabilize the assembly and function of biopolymers by disrupting macromolecule-water interactions. A similar explanation may account for the altered properties and quality of food crops that have been chilled.

Development of cyanide-resistant respiration in mitochondria from potato tubers treated with ethanol, acetaldehyde, and acetic acid

Treatment of intact potato (Solanum tuberosum L.) tubers with acetaldehyde, ethanol or acetic-acid vapors led to a respiratory upsurge which was further increased when the volatiles were applied in 100% O2. Mitochondria from tubers held in 100% O2 (O2 control) displayed a substrate state, state 3, and state 4 in respiration, whereas in mitochondria from the volatile-treated tubers the respiratory rate of the different states was virtually indistinguishable. This respiratory pattern was companied by the development of a cyanide-resistant respiration since these mitochondria exhibited resistance to CN and sensitivity to CN+salicylhydroxamic acid. Acetaldehyde-treated potatoes showed a time-course development (up to 36 h) of cyanide resistance and concomitant sensitivity to salicylhydroxamic acid, indicating the onset of synthetic processes leading to the observed changes in mitochondrial respiration.

Decrease in Fruit Moisture Content Heralds and Might Launch the Onset of Ripening Processes

UNLABELLED It is known that fruit ripening is a genetically programmed event but it is not entirely clear what metabolic cue(s) stimulate the onset of ripening, ethylene action notwithstanding. Here, we examined the conjecture that fruit ripening might be evoked by an autonomously induced decrease in tissue water status. We found decline in water content occurring at the onset of ripening in climacteric and nonclimacteric fruit, suggesting that this phenomenon might be universal. This decline in water content persisted throughout the ripening process in some fruit, whereas in others it reversed during the progression of the ripening process. Applied ethylene also induced a decrease in water content in potato (Solanum tuberosum) tubers. In ethylene-mutant tomato (Solanum lycopersicum) fruit (antisense to1-aminocyclopropane carboxylate synthase), cold-induced decline in water content stimulated onset of ripening processes apparently independently of ethylene action, suggesting cause-and-effect relationship between decreasing water content and onset of ripening. The decline in tissue water content, occurring naturally or induced by ethylene, was strongly correlated with a decrease in hydration (swelling) efficacy of cell wall preparations suggesting that hydration dynamics of cell walls might account for changes in tissue moisture content. Extent of cell wall swelling was, in turn, related to the degree of oxidative cross-linking of wall-bound phenolic acids, suggesting that oxidant-induced wall restructuring might mediate cell wall and, thus, fruit tissue hydration status. We propose that oxidant-induced cell wall remodeling and consequent wall dehydration might evoke stress signaling for the onset of ripening processes. PRACTICAL APPLICATION This study suggests that decline in fruit water content is an early event in fruit ripening. This information may be used to gauge fruit maturity for appropriate harvest date and for processing. Control of fruit hydration state might be used to regulate the onset of fruit ripening.

Nonenzymatic Oxidation of Indole-3-Acetic Acid by H2O2 and Fe2+ Ions

Indole oxidation products detected by dimethylaminocinnamaldehyde were formed nonenzymatically from indole-3-acetic acid (IAA) in the presence of low concentrations of H2O2 Decarboxylation of IAA-[1-14C] was promoted by H2O2 concentrations as low as 5 μM, and this effect was greatly increased in the presence of Fe2+ ions Incubation of IAA-[1-14C] with H2O2 did not yield any radioactive compounds that were separated from free IAA by paper or thin-layer chromatography in four solvent systems These results indicate that IAA is nonenzymatically decomposed by H2O2 and do not support the hypothesis that the nonenzymatic inactivation of IAA by H2O2 is caused by the reversible formation of an IAA-H2O2 complex

Rapid method for preparation of pure veratryl alcohol for the assay of lignin peroxidase

We present a simple method for the rapid preparation of veratryl alcohol by reducing commercially available veratraldehyde with sodium borohydride resulting in high-purity veratryl alcohol. The lag period in the activity of lignin peroxidase from Phanerochaete chrysosporium that is associated with use of commercial preparations of the substrate is eliminated with the use of pure veratryl alcohol. The compound affords also estimation of low enzymatic activity.

Effect of Mannose on Ethylene Evolution and Ripening of Pear Fruit

As part of an investigation into the role of glycosidases in fruit ripening, we applied D(+) glucose, galactose and mannose as competitive inhibitors of their corresponding glycosidases. Preliminary experiments showed that mannose application inhibited softening and ethylene production. There are at least two possible explanations for this effect: (1) α-Mannosidase activity is inhibited by mannose, thus implicating this enzyme as having a controlling function in fruit ripening involved perhaps in glycoprotein processing; (2) Mannose is phosphosylated and not further metabolized, consequently lowering cellular inorganic phosphate levels and decreasing ATP synthesis. It is this mechanism by which the inhibitory effects of mannose (and its analogues, glucosamine and 2-deoxyglucose) on many metabolic processes have been explained (Herold and Lewis 1977, New Phytol 79:1–110).