Batia Weiss

Effects of ethylene and 1-methylcyclopropene on the postharvest qualities of ‘Shamouti’ oranges

Abstract Citrus fruits are nonclimacteric and produce only low amounts of ethylene. However, exogenous applied, and possibly endogenous ethylene, may be involved in the regulation of fruit maturation and senescence. We examined the effects of ethylene and 1-methylcyclopropene (1-MCP), an ethylene action inhibitor, on the postharvest qualities of ‘Shamouti’ ( Citrus sinensi s L. Osbeck) oranges. Neither ethylene nor 1-MCP had any effects on the loss of fruit weight and firmness. However, ethylene had disadvantageous effects on most other postharvest parameters. It increased the appearance of chilling injury (CI) symptoms, stem-end rot decay, and the content of volatile off-flavours in the juice head space and fruit internal atmosphere. The only protective effect of ethylene during postharvest storage was in reducing the amount of decay caused by mold rots. 1-MCP treatment effectively inhibited the ethylene effects on ‘Shamouti’ oranges, as indicated by blocking of the degreening process, but was ineffective in restoring the negative effects of ethylene during storage; it even weakened the tissue and increased CI symptoms, decay development, and the accumulation of volatile off-flavours. It is concluded that while high concentrations of exogenous applied ethylene are undesirable during storage, and enhance fruit deterioration, the small amounts of endogenous ethylene produced by the fruits may be required to maintain their natural resistance against various environmental and pathological stresses. In future, we suggest that 1-MCP may be applied as a postharvest treatment to inhibit the degreening process in citrus fruits which are preferably marketed green, but special care must be taken to avoid CI and decay development.

Reduction of postharvest decay in organic citrus fruit by a short hot water brushing treatment

Abstract The marketing of organic citrus fruit has markedly increased during the last few years. However, these fruits are not treated with chemical fungicides and suffer from relatively high rates of decay. In this study, we examined the possible use of a new hot water brushing (HWB) treatment, to disinfect the fruits and reduce decay development during postharvest storage. Preliminary observations have shown that a minimum exposure period of 20 s at 56°C was required to inhibit Penicillium digitatum (Pers.: Fr.) Sacc spore germination in vitro. In vivo studies, carried out by rinsing and brushing the fruit 24 h after artificial inoculation with a P. digitatum spore suspension, indicated that HWB at 56, 59 and 62°C for 20 s, reduced decay development in the infected wounds to only 20, 5 and less than 1%, respectively, of that in untreated control fruits or fruits treated with tap water. The effects of HWB at 56, 59 and 62°C for 20 s on epiphytic microflora, was further confirmed by the reduction of the microbial counts (CFU) on the fruit surface to only 24, 12, and less than 1%, respectively, of those on fruit that had been rinsed and brushed with tap water. Scanning electron microscopy showed that HWB at 56°C for 20 s had smoothed the fruit epicuticular waxes and thus covered and sealed stomata and cracks on the fruit surface, which could have served as potential pathogen invasion sites. Postharvest storage experiments using various organically grown citrus cultivars such as ‘Minneola’ tangerines, ‘Shamouti’ oranges and ‘Star Ruby’ red grapefruit, showed that HWB at 56°C for 20 s reduced decay development by 45–55%. The HWB treatment at 56°C did not cause surface damage, and did not influence fruit weight loss or internal quality parameters.

Influence of CaCl2 on Penicillium digitatum, grapefruit peel tissue, and biocontrol activity of Pichia guilliermondii.

ABSTRACT Interactions between CaCl(2), grapefruit peel tissue, Penicillium digitatum, and the yeast antagonist Pichia guilliermondii strain US-7 were investigated. Application of 68 or 136 mM CaCl(2) to grapefruit surface wounds reduced the incidence of green mold caused by Penicillium digitatum by 43 to 52%. In laboratory tests, a cell suspension (10(7) cells/ml) of Pichia guilliermondii containing either 68 or 136 mM CaCl(2) reduced the incidence of green mold from 27 to 3%. In large scale tests, dip application of 136 mM CaCl(2) with US-7 (10(7) cells/ml) significantly decreased the number of wounds infected by Penicillium digitatum. CaCl(2), with or without yeast cells, stimulated ethylene production in grapefruit tissue. Increasing concentrations of CaCl(2) resulted in decreased spore germination and germ tube elongation of Penicillium digitatum. Pectinolytic activity of crude enzyme preparations of Penicillium digitatum was also inhibited by the presence of increasing concentrations of CaCl(2). US-7 exhibited a strong ability to maintain cytosolic Ca(2+) homeostasis at levels that did not exceed 1.4 muM when exposed to 150 mM CaCl(2). On the other hand, strain 114 of Debaryomyces hansenii, which failed to give any protection against infection by Penicillium digitatum, showed reduced capacity to maintain Ca(2+) homeostasis. The effect of calcium in reducing infection of grapefruit wounds by Penicillium digitatum could be due to direct effects on host tissue (making cell walls more resistant to enzymatic degradation) or the pathogen (interfering with spore germination, growth, and inhibition of fungal pectinolytic enzymes). Alternatively, the ability of US-7 to maintain calcium homeostasis may allow it to grow or assist in its competitive ability in a microenvironment that, because of high levels of calcium ions, is inhibitory to growth of the green mold pathogen.

Biological control of Botrytis, Aspergillus and Rhizopus rots on table and wine grapes in Israel

Abstract One hundred and twenty-nine strains of epiphytic micro-organisms, isolated from table and wine grapes in Israel, were screened for antagonistic activity against Botrytis cinerea on table grapes. Two isolates (Candida guilliermondii, strain A42 and Acremonium cephalosporium, strain B11) were further evaluated for the control of decay in grapes caused by Aspergillus niger and Rhizopus stolonifer. Decay incidence caused by Botrytis cinerea, Aspergillus niger and Rhizopus stolonifer on wounded detached berries was reduced to 8, 14 and 22% respectively, by A42 and to 16, 82 and 60%, respectively, by B11. On small clusters with intact berries, decay was reduced to 30, 22 and 22%, respectively, by A42 and to 48, 39 and 30% respectively, by isolate B11. Both strains survived well under local vineyard conditions and during storage at 0°C and maintained relatively high cell counts on the berries. Field experiments were conducted in 1996, 1997 and 1998, with both table and wine grapes. Vines were sprayed with yeast suspension 2–5 times at 7–10 day intervals and decay was evaluated before harvest (wine grapes) or after storage (table grapes). A42 reduced decay caused by Botrytis cinerea in two of the three seasons in both table and wine grapes, and rots caused by Aspergillus niger in wine grapes were reduced significantly in 1997 and 1998. B11 reduced Botrytis cinerea development in the two years it was tested in wine grapes but in table grapes only in 1996. Morever, it did not control decay caused by Aspergillus niger.

Induction of Resistance to Penicillium digitatum in Grapefruit by β-Aminobutyric Acid

Abstractβ-Aminobutyric acid (BABA), an inducer of pathogen resistance in plants, induced disease resistance in reproductive parts of the plant, such as grapefruit peel tissue. Application of BABA to specific wound sites on the fruit peel surface induced resistance to Penicillium digitatum, the main postharvest pathogen of citrus fruit, in a concentration-dependent manner, being most effective at 20mM, and rather less effective at either higher or lower concentrations. The effect of BABA in inducing resistance to P. digitatum in the fruit peel surface was local and limited to the vicinity (within 1–2cm) of the BABA-treated site. In addition to inducing pathogen resistance, increasing concentrations of BABA (from 1 to 100mM) also exhibited direct antifungal activity and inhibited P. digitatum spore germination and germ tube elongation in vitro. The induction of resistance to P. digitatum by BABA was accompanied by the activation of various pathogen defense responses in grapefruit peel tissue, including activation of chitinase gene expression and protein accumulation after 48h, and an increase in phenylalanine ammonia lyase (PAL) activity after 72h.