Mode of action of abscisic acid in triggering ethylene biosynthesis and softening during ripening in mango fruit

The role of abscisic acid (ABA) in triggering ethylene biosynthesis and ripening of mango fruit was investigated by applying ABA [S-(+)-cis,trans-abscisic acid] and an inhibitor of its biosynthesis [nordihydroguaiaretic acid (NDGA)]. Application of 1 mM ABA accelerated ethylene biosynthesis through promoting the activities of ethylene biosynthesis enzymes (1-aminocyclopropane-1-carboxylic acid synthase, ACS; 1-aminocyclopropane-1-carboxylic acid oxidase, ACO) and accumulation of 1-aminocyclopropane-1-carboxylic acid (ACC), enhanced fruit softening and activity of endo-polygalacturonase and reduced pectin esterase activity in the pulp. The activities of ethylene biosynthesis and softening enzymes were significantly delayed and/or suppressed in the pulp of NDGA-treated fruit. The ABA-treated fruit had higher total sugars and sucrose as well as degradation of total organic acids, and citric and fumaric acids compared with NDGA treatment. These results suggest that ABA is involved in regulating mango fruit ripening and its effects are, at least in part, mediated by changes in ethylene production.     

Mode of action of nitric oxide in inhibiting ethylene biosynthesis and fruit softening during ripening and cool storage of ‘Kensington Pride’ mango

The mode of action of nitric oxide (NO) in inhibiting ethylene biosynthesis and fruit softening during ripening and cool storage of mango fruit was investigated. Hard mature green mango (Mangifera indica L. cv. ‘Kensington Pride’) fruit were fumigated with 20 μL L⁻¹ NO for 2 h at 21 °C and allowed to ripen at 21 ± 1 °C for 10 d, or stored at 13 ± 1 °C for 21 d. During ripening and cool storage, ethylene production and respiration rate from whole fruit were determined daily. The 1-aminocyclopropane-1-carboxylic acid (ACC) content, activities of ACC synthase (ACS), ACC oxidase (ACO), and fruit softening enzymes such as pectin esterase (PE), endo-1,4-β-d-glucanase (EGase), exo- and endo-polygalacturonase (exo-PG, endo-PG) as well as firmness and rheological properties of pulp were determined at two- and seven-day intervals during ripening and cool storage, respectively. NO fumigation inhibited ethylene biosynthesis and respiration rate, and maintained higher pulp firmness, springiness, cohesiveness, chewiness, adhesiveness, and stiffness. NO-fumigated fruit during cool storage and ripening had lower ACC contents through inhibiting the activities of both ACS and ACO in the fruit pulp. NO-fumigated fruit showed decreased activities of exo-PG, endo-PG, EGase, but maintained higher PE activity in pulp tissues during ripening and cool storage. In conclusion, NO fumigation inhibited ethylene biosynthesis through inhibition of ACS and ACO activities leading to reduced ACC content in the fruit pulp which consequently, reduced the activities of fruit softening enzymes during ripening and cool storage.     

Metabolic profiling of ‘Conference’ pears under low oxygen stress

Pears (Pyrus communis L. cv. Conference) may develop core breakdown when stored under low oxygen or elevated carbon dioxide conditions. This physiological disorder is characterized by the development of brown spots due to oxidation of phenolic compounds, and eventually, cavities in the center of the fruit. Based on metabolic profiling of brown and sound tissue using GC-EI-TOF-MS, the hypothesis that this disorder is due to an imbalance between oxidative and reductive processes at the cellular level was investigated. Brown tissue was clearly characterized by a distinctive pattern in changes which included a decrease of malic acid and an increase in fumaric acid and gamma aminobutyric acid (GABA), which indicated a reduced metabolic activity at the level of the Krebs cycle and a putative block of the GABA shunt pathway. Increased gluconic acid concentration might be related to ascorbic acid degradation due to insufficient reducing equivalents or to an impaired pentose phosphate pathway. For the first time, GABA and gluconic acid have been shown to be metabolic markers for core breakdown. The concentrations of other compounds which are believed to be related to hypoxic stress response such as trehalose and putrescine were also considerably higher in brown tissue than in sound tissue. The concentration of some sugars which are typically found in xyloglucans also increased during brown development, possibly indicating cell wall breakdown due to enzymatic processes or chemical reactions of hydroxyl radicals.     

Methods to analyze physico-chemical changes during mango ripening: A multivariate approach

Canonical discriminant analysis (CDA) was used to identify the best method to discriminate between maturity and ripening stages, assessed in terms of dry matter content, firmness, color (peel and flesh), total soluble solids content attributes, before and during ‘Keitt’ mango ripening at 20 °C. Dry matter content was determined by hot-air oven and microwave oven methods. Fruit firmness was determined non-destructively by hand squeezing, with a durometer, using acoustic resonance and low-mass elastic impact methods (AWETA), as well as destructively by the penetrometer. Peel and flesh color were expressed as L*, a*, b*, h⁰ and C* values. Total soluble solids content was analyzed from filtered juice from whole fruit tissue and from unfiltered juice squeezed out by hand. Canonical discriminant analysis indicated that the durometer and the penetrometer were better methods to assess firmness than hand firmness, acoustic resonance or impact methods. The best color attributes to follow changes during early stage of ‘Keitt’ mango ripening were a* and b* values of the flesh, whereas b* value of the peel was considered better during later stages of ripening. Results of total soluble solids content in filtered juice from whole fruit tissue were less variable compared to unfiltered juice squeezed out by hand. Dry matter content was better assessed by drying the sample in a microwave oven than in a hot-air oven. A combined CDA including the best methods to assess each ripening attribute, as well as titratable acidity, showed that the best tools to assess changes in fruit during ripening were the penetrometer, followed by flesh a* value and total soluble solids content (from filtered juice from whole fruit).     

Superoxide anion and hydrogen peroxide in the yeast antagonist–fruit interaction: A new role for reactive oxygen species in postharvest biocontrol?

The importance of reactive oxygen species (ROS) in plant defense responses against certain pathogens is well documented. There is some evidence that microbial biocontrol agents also induce a transient production of ROS in a host plant which triggers local and systemic defense responses to pathogens. The ability of biocontrol agents used to control postharvest diseases to induce defense-related oxidative responses in fruits, however, has not been explored. Here we show that the yeast antagonists, Metschnikowia fructicola (strain 277) and Candida oleophila (strain 182) generate greater levels of super oxide anion (O₂⁻) on intact fruit surfaces (poor in nutrients) then those applied on a nutrient-poor agar medium. Even though yeast antagonists show a high level of O₂⁻ on nutrient-rich media, when applied on fruits around wounds (areas abundant in nutrients) accumulation of O₂⁻, as detected by nitro blue tetrazolium staining, occurred much more rapidly on the latter. Using laser scanning confocal microscopy we observed that the application of M. fructicola and C. oleophila into citrus and apple fruit wounds correlated with an increase in H₂O₂ accumulation in host tissue. In citrus fruit, the level of H₂O₂ around inoculated wounds increased by 4-fold compared to controls (wounds inoculated with water) as early as 18 h after inoculation. Yeast continued to stimulate H₂O₂ production in citrus fruit up to 66 h after inoculation and H₂O₂ levels were still 3-fold above the control. Living yeast cells were detected in fruit wounds at this time point indicating the ability of M. fructicola to tolerate host ROS, which has been reported to be an intrinsic characteristic of efficient yeast antagonists. Similar increase in H₂O₂ accumulation around yeast-inoculated wounds was observed in apple fruit exocarp. The present data, together with our earlier discovery of the importance of H₂O₂ production in the defense response of citrus flavedo to postharvest pathogens, indicate that the yeast-induced oxidative response in fruit exocarp may be associated with the ability of specific yeast species to serve as biocontrol agents for the management of postharvest diseases.     

Fungicidal activity of compounds extracted from the pericarp of Areca catechu against Colletotrichum gloeosporioides in vitro and in mango fruit

Areca catechu L., a member of the Palmaceae family, is one of the most commonly used drugs in the world. Compounds obtained from the hexane, ethyl acetate and methanol extracts of the pericarp of A. catechu L. were assessed in vitro and in mango fruit for antifungal activity against Colletotrichum gloeosporioides Penz. In vitro studies also indicated that three triterpenes, namely fernenol (fern-9(11)-en-3α-ol), arundoin (fern-9(11)-en-3α-ol ME), and a mixture of stigmasterol and β-sitosterol, and one fatty acid, lauric acid, could inhibit the mycelial growth of C. gloeosporioides with EC₅₀ values of 36.7, 47.5, 56.7 and 111.5 mg L^?1, respectively. Furthermore, fernenol, arundoin, and the mixture of stigmasterol and β-sitosterol highly inhibited spore germination and germtube elongation. Mango fruit studies suggested that fernenol, arundoin and the mixture of stigmasterol and β-sitosterol were significantly more effective than benomyl for controlling postharvest anthracnose disease when used at 100 and 200 mg L^?1.     

Studies on classification models to discriminate ‘Braeburn’ apples affected by internal browning using the optical properties measured by time-resolved reflectance spectroscopy

This work aimed at studying the feasibility of time-resolved reflectance spectroscopy (TRS) to nondestructively detect internal browning (IB) in ‘Braeburn’ apples through the development of classification models based on absorption (μ_a) and scattering (μ_s′) properties of the pulp.This research was carried out in two seasons: in 2009, apples were measured by TRS at 670 nm and in the 740–1040 nm spectral range on four equidistant points around the equator, whereas in 2010 apples were measured by TRS at 670 nm and at 780 nm on eight equidistant points.The values of the absorption coefficients measured in the 670–940 nm range increased with IB development. On the contrary, μ_s′780 was higher in healthy fruit than in IB ones. The μ_a780 also significantly increased with IB severity, showing high values when IB affected the pulp tissues compared to the core ones. Also μ_a670 changed with IB development, but it was not able to clearly discriminate healthy fruit from IB ones because its value was also affected by the chlorophyll content of the pulp.The absorption and scattering coefficients were used as explanatory variables in the linear discriminant analysis in order to classify each apple tissue as healthy or IB; then the models obtained were used for fruit classification. The best classification performance was obtained in 2010 using μ_a780 and μ_s′780 and considering the IB position within the fruit: 90% of healthy fruit and 71% of IB fruit were correctly classified. By using all the μ_a measured in the 670–1040 nm range plus the μ_s′780, IB fruit classification was slightly better while healthy fruit classification was worse. The better result of 2010 was due to the increased number of TRS measurement points that allowed better exploration of the fruit tissues. However, the asymmetric nature of this disorder makes detection difficult, especially when the disorder is localized in the inner part of the fruit (core) or when it occurs in spots. A different TRS set-up (position and distance of fibers, time resolution) should be studied in order to reach the deeper tissue within the fruit in order to improve browning detection.     

Current status of tropical fruit breeding and genetics for three tropical fruit species cultivated in Japan: pineapple,mango, and papaya

Tropical fruit crops are predominantly produced in tropical and subtropical developing countries, but some are now grown in southern Japan. Pineapple (Ananas comosus), mango (Mangifera indica) and papaya (Carica papaya) are major tropical fruits cultivated in Japan. Modern, well-organized breeding systems have not yet been developed for most tropical fruit species. Most parts of Japan are in the temperate climate zone, but some southern areas such as the Ryukyu Islands, which stretch from Kyushu to Taiwan, are at the northern limits for tropical fruit production without artificial heating. In this review, we describe the current status of tropical fruit breeding, genetics, genomics, and biotechnology of three main tropical fruits (pineapple, mango, and papaya) that are cultivated and consumed in Japan. More than ten new elite cultivars of pineapple have been released with improved fruit quality and suitability for consumption as fresh fruit. New challenges and perspectives for obtaining high fruit quality are discussed in the context of breeding programs for pineapple.     

Impact of atmospheric ozone-enrichment on quality-related attributes of tomato fruit

Tomato fruit (Lycopersicon esculentum L. cv. Carousel) were exposed to ozone concentrations ranging between 0.005 (controls) and 1.0 μmol mol⁻¹ at 13 °C and 95% RH. Quality-related attributes and organoleptic characteristics were examined during and following ozone treatment. Levels of soluble sugars (glucose, fructose) were maintained in ozone-treated fruit following transfer to ‘clean air’, and a transient increase in β-carotene, lutein and lycopene content was observed in ozone-treated fruit, though the effect was not sustained. Ozone-enrichment also maintained fruit firmness in comparison with fruit stored in ‘clean air’. Ozone-treatment did not affect fruit weight loss, antioxidant status, CO₂/H₂O exchange, ethylene production or organic acid, vitamin C (pulp and seed) and total phenolic content. Panel trials (employing choice tests, based on both appearance and sensory evaluation) revealed an overwhelming preference for fruit subject to low-level ozone-enrichment (0.15 μmol mol⁻¹), with the effect persisting following packaging.     

Effect of high pressure processing on postharvest physiology of ‘Keitt’ mango

High pressure processing (HPP) has been proposed as a potential quarantine method against the Mexican fruit fly (Anastrepha ludens), which parasitizes mango (Mangifera indica) fruit. It is efficient against this pest in conditions of high pressure and moderate temperatures. However, further studies are required to evaluate the effect of this quarantine treatment on fruit quality. The aim of this study was to evaluate the effect of HPP on the postharvest physiology of ‘Keitt’ mango during storage at 25 °C. Mangoes at physiological maturity were pressurized at 50, 70, and 90 MPa for 9 min (a control was obtained with no pressurization). The fruit were stored for 14 d and changes in physiological and physicochemical variables associated with ripening were assessed. The pressure level affected the respiration rate resulting in changes in other variables, such as total soluble solids. High pressure did not inhibit fruit ripening and the quality of the treated fruit was similar to that of the control.     

Preformed antifungal compounds in strawberry fruit and flower tissues

Antifungal activity against the pathogen, Botrytis cinerea, and a bioassay organism, Cladosporium cladosporioides, declined with advancing strawberry fruit maturity as shown by thin layer chromatography (TLC) bioassays. Preformed antifungal activity was also present in flower tissue. The fall in fruit antifungal compounds was correlated with a decline in natural disease resistance (NDR) against B. cinerea in planta. Crude extracts of green stage I fruit (7 days after anthesis) contained at least two preformed antifungal compounds (R_f=0.44 and 0.37) that were not present in white and red stage fruit. These compounds were shown with TLC reagent sprays to be neither phenolics nor alkaloids. Positive reactions to Ehrlich’s reagent suggested that R_f=0.37 was a terpene. Most antifungal activity was found in the achenes of green stage I fruit. However, antifungal activity was found in all tissue types (viz. pith, cortex, epidermis) of green stage I fruit. TLC bioassays revealed that all fruit stages yielded antifungal activity at the origin (R_f=0.00). The approximate area of fungal inhibition at the origin in green stage 1 fruit extracts was 1.87- and 1.73-fold greater than in white and red stages, respectively. TLC reagent sprays showed that the antifungal compound(s) at origin included phenolics. This observation is consistent with previous reports that phenolic compounds in strawberry fruit are inhibitory to B. cinerea.     

Distribution,stability and fate of phenolic compounds in white and purple eggplants (Solanum melongena L.)

Eggplants rank among the vegetables richest in antioxidants, but little is known about the allocation, stability, and turnover of these metabolites. The distribution, accumulation and degradation of phenolic antioxidants in the inner and outer pulp of two commercially important eggplant types (white and dark purple), at harvest and after 14 and 30 d of refrigerated storage under non-chilling conditions (10 °C and 90% RH) were determined in this study. Chlorogenic acid (ChA) was histolocalized by fluorescence with 2-aminoethyl-diphenylborinate and the activity of phenolic compounds oxidizing enzymes (polyphenoloxidase, PPO and peroxidase, POD) as well as H₂O₂ concentration in both fruit regions was determined. During storage, dark purple fruit were more susceptible to dehydration and showed greater deterioration than white eggplants. Both genotypes accumulated higher sugar content in the inner pulp as opposed to acids, which were more concentrated in the outer region. At harvest, pulp antioxidant capacity was similar in both eggplant types. TEAC and DPPH assays and in situ localization, showed greater total antioxidants and ChA content in the core than in the outer pulp in both white and dark purple fruit. The stability of ChA was markedly different between genotypes. In white fruit, antioxidants increased during the first two weeks of storage, remaining stable afterwards. In contrast, in dark purple eggplants, phenolic compounds declined after an initial stage at which they accumulated. PPO and POD in vitro activities, associated mainly with fruit seeds, fibers, and vascular bundles did not correlate with pulp browning or loss of phenolic antioxidants. Instead, the reduction of ChA in the core of dark purple fruit was associated with increased production of H₂O₂. Results indicate that antioxidants are predominantly located in the inner pulp of eggplants regardless of the genotype, but are more stable in white fruit. Rather than being the result of browning reactions, substantial losses of phenolic antioxidants in whole eggplants under the recommended storage conditions likely result from seed coat development and vascular lignification in the immature fruit.     

Postharvest physiology of ‘Aroma’ apples in relation to position on the tree

The effects of fruit position within the canopy on the onset of the respiratory climacteric and the rise in ethylene production as well as changes in peel colour and chemical composition were studied in apples (Malus x domestica Borkh. cv. Aroma) during ripening in normal air at 20 °C for 6–8 weeks over two crop seasons. The commencement of the rise in both CO₂ and ethylene production was equal independent of fruit position but the peak of ethylene was behind that of CO₂ with a lag of several days. While the climacteric ethylene peak was considerably higher in shaded inside apples, the internal ethylene concentration was at the same level independent of canopy position. During maturation on the tree outside fruit developed a red peel colour while inside fruit remained green. Outside fruit had a higher content of dry matter, soluble solids and soluble sugars but a somewhat lower amount of titratable acidity than inside fruit. High summer temperatures in the second year resulted in a significantly higher content of soluble solids and organic acids independent of fruit position but diminished the soluble solids difference between outside and inside fruit and increased the difference in malic and citric acid concentrations. High summer temperatures also increased the difference in peel colour between outside and inside fruit. Independent of canopy position, the soluble solids concentrations remained unchanged during ripening while the amounts of sucrose as well as malic acid and the titratable acidity decreased with a concomitant rise in the cell sap pH. The higher content of soluble sugars and a somewhat lower amount of titratable acidity in outside red-coloured apples probably contribute to improved fruit quality but the difference seems to be strongly dependent on the growing conditions, especially the sum of heat units.