J. Agric. Food Chem. 2001, 49, 5817−5821
5817
Low Temperature Metabolism of Apple Phenolics and Quiescence of
Phlyctaena vagabunda
Vincenzo Lattanzio,*,† Donato Di Venere,‡ Vito Linsalata,‡ Paolo Bertolini,§
Antonio Ippolito,# and Mario Salerno#
Istituto di Orticoltura e Colture Industriali-CNR, Via S. Loja - Zona Industriale,
85050-Tito Scalo (PZ), Italy, Istituto sull’Orticoltura Industriale-CNR, Via Amendola 165/A,
70126-Bari, Italy, Dipartimento di Protezione e Valorizzazione Agro-Alimentare, Università,
Via Filippo Re 8, 40126 Bologna, Italy, and Dipartimento di Protezione delle Piante dalle Malattie,
Università, Via Amendola 165/A, 70126 Bari, Italy
The content of chlorogenic acid, (+)-catechin, (-)-epicatechin, phloretin glycosides, and quercetin
glycosides in fresh and stored Golden Delicious apples (Malus domestica Borkh) was determined.
The relative amount of phenolics in the peel, with the exception of chlorogenic acid and
(-)-epicatechin, was higher than that in the flesh. In addition, quercetin glycosides were detected
only in the skin. These compounds were tested for fungicidal activity against Phlyctaena vagabunda
Desm., the causal agent of a postharvest rot. Chlorogenic acid only inhibited P. vagabunda spore
germination and mycelial growth in vitro. Changes of apple phenolics and polyphenol oxidase activity
during cold storage and the biological activity of these phenolics have also been analyzed with
reference to the development of quiescent infections during cold storage plus shelf life at room
temperature. The results obtained suggested that phloridzin and chlorogenic acid in combination
with polyphenol oxidase activity could function to arrest P. vagabunda in quiescent infections
associated with immature and ripening apple fruit.
Keywords: Postharvest rot; phenolics; polyphenol oxidase; quiescent infections; disease resistance;
storage
INTRODUCTION
Rotting of stored apples (Malus domestica Borkh) by
Phlyctaena vagabunda Desm. (syn. Gloeosporium album
Osterw.) is an important source of wastage at the
present time. Postharvest losses of fruits, depending
upon the varietal degree of susceptibility to attack by
the pathogen, the length of storage, and the efficiency
of the marketing systems, have been estimated to be
more than 30% when environmental conditions promote
microorganism diffusion (1). Conidia of P. vagabunda
are produced by small infections on the wood of the tree
throughout the year and are spread by rain and dew
on the fruit, which is thus exposed to infection during
the whole of the growing season. There are conditions,
depending on the fungus and the nature of vegetable
tissue, in which infections, which take place in lenticels,
can develop in storage to produce lesions. Normally, the
susceptibility to attack by P. vagabunda is directly
related to the length of the storage period (2-4).
The term “quiescent infection” is used to describe a
limited development of the infection. No lesions are
produced while the fruit is on the tree. When picked,
however, the fruit, depending on the physiological stage
* To whom correspondence should be addressed (telephone
+0390971427249; fax +390971427249; e-mail lattanzio@
ioci.pz.cnr.it).
†
Istituto di Orticoltura e Colture Industriali.
‡ Istituto sull’Orticoltura Industriale.
§ Dipartimento di Protezione e Valorizzazione dei Prodotti
Agroalimentari.
#
Dipartimento di Protezione delle Piante dalle Malattie.
and the storage conditions, can undergo decay quite
quickly. To be successful, a pathogen must also be able
to overcome host defenses and initiate attack under
prevailing physiological and environmental conditions
(4). The problem of the nature of the resistance then
lies in determining which factors are responsible for
arresting the development of the fungus at the time of
the initial infection and, in addition, finding which
changes taking place prior to the onset of rotting enable
the fungus to resume growth (5).
In view of the research dealing with the fungistatic
properties of phenolics (6-8) and, in particular, with
the role of these substances against fungal pathogens
such as Venturia sp., Gloeosporium sp., Sclerotinia
fructigena, and Botrytis cinerea in apple (3, 9, 10), it was
decided to examine the phenolics of apples in relation
to susceptibility to attack by P. vagabunda. It is, in fact,
well-known that apple fruits contain, besides quercetin
glycosides, catechins and chlorogenic acid, a dihydrochalcon-O-glucoside, phloridzin, whose oxidation products may be involved in the defense mechanism of apple
leaves against the scab fungus Venturia inaequalis (1117). During the maturation of Golden Delicious apples,
an increase of internal ethylene levels and phenylalanine ammonia lyase (PAL), the key enzyme of phenolic
biosynthesis, has been observed: the onset of ethylene
production did occur at approximately the same time
as an increase in PAL activity (18, 19). It has been
suggested that ethylene produced by the host specifically at ripening may act as a signal to terminate
pathogen quiescence on the fruit surface (20). Finally,
it has been observed that low storage temperature
10.1021/jf010255b CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/30/2001
5818 J. Agric. Food Chem., Vol. 49, No. 12, 2001
stimulates PAL activity during the storage of fruits and
vegetables, thus increasing their antibiotic phenolic
content (21-23). When developmental changes in the
phenolic content of apple skin are considered, there is
a general agreement that phenolic levels decrease
during the fruit development and remain relatively
constant during fruit maturation (24, 25). On the
contrary, literature data concerning changes in phenolic
compound during cold storage are much more contradictory (12, 24, 26, 27).
The objectives of this study were (i) to describe the
effect of low nonfreezing temperatures on the fate of
apple phenolics during cold storage, (ii) to evaluate the
antifungal activity of preformed and oxidized phenolics
against P. vagabunda, and, finally, (iii) to discuss these
results with reference to the development of infection
during storage, occurring after a latent infection period,
to improve the knowledge of defense mechanisms against
fungal pathogens, and, consequently, to improve the
postharvest management of apple fruits.
MATERIALS AND METHODS
Plant Material. Apple fruits of the cv. Golden Delicious
(Malus domestica Borkh), which is considered a cultivar
susceptible to Phlyctaena postharvest rot, harvested at the
beginning of October, were stored at 2 °C. Analysis for
cinnamic acid derivatives and flavonoids were carried out on
fresh and stored samples. Stored samples comparable with the
ones analyzed were inoculated with spores of P. vagabunda
95 days after the harvest, a period long enough to increase
the susceptibility to rotting, and then analyzed for phenolics
from time to time. While stored inoculated samples were
analyzed, at the same time a comparable sample was transferred at 24 °C for 8-11 days and then analyzed.
Analysis of Phenolic Compounds. Chromatographic
analyses were carried out on apple “peel” and flesh. The term
peel in this context means tissue prepared from the fruit peeled
with a household potato peeler. The strips of peel were about
1 mm thick and consisted of cuticle, epidermis, and subepidermal chlorenchyma. Thirty grams of plant material were
homogenized with hot MeOH-EtOH (1:1) and then refluxed
under nitrogen for 1 h. After centrifugation, the solution was
first concentrated under vacuum, depigmented with petroleum
ether (bp 40-70 °C), and then qualitatively and quantitatively
analyzed for phenolics by HPLC.
HPLC analyses were performed with a Perkin-Elmer series
4 liquid chromatograph, which was equipped with a fluorimetric detector PE-LS3 and a computer aided Hewlett-Packard
spectrophotometric photodiode array detector 1040-A, following the method of Lattanzio and Van Sumere (22). In all cases,
phenolics were subjected to UV spectroscopy and chromatographic comparison against authentic samples by means of
an HP-K3 software postrun analysis coupled with PE-Chromatographics 2 software. The pilot signal to the spectrophotometric detector was set at 280 nm and the fluorimetric
detector was set at 285 nm (excitation) and 320 nm (emission).
An analytical Waters (Milford, MA) column (300 × 4 mm i.d.)
packed with µBondapak C18 (10 µm) was used throughout this
work. The solvent system consisted of (A) MeOH and (B) acetic
acid-water (5/95; v/v). The elution profile was 15-40% A in
B for 0-25 min; 40% A in B (isocratic) for 25-30 min; 4063% A in B for 30-45 min; 63% A in B (isocratic) for 45-47
min; 63-99% A in B for 47-51 min. The flow rate was always
1 mL/min. Samples of 6 µL were applied to the column by
means of a 6 µL loop valve.
Apple Polyphenol Oxidase. A crude extract of apple
polyphenol oxidase (PPO) was prepared by macerating apple
tissues in a blender for 10 min with (2.5 mL/g) cold acetone
(-15 °C). The suspension was centrifuged, and the residue was
twice homogenized with cold acetone. The acetone powder was
extracted, in the presence of insoluble poly(vinylpyrrolidone),
with Mc Ilvaine’s buffer pH 4.5 (5 mL/g of original tissue wt).
Lattanzio et al.
Enzyme activity was determined polarographically according
to Lattanzio et al. (23).
Bioassays. A virulent strain of P. vagabunda isolated from
infected apple fruits was maintained on potato-dextrose agar
(PDA: 2% dextrose, 1.5% agar) under light at 21 °C. Phenolic
compounds used for bioassays were purchased from Extrasynthèse (Genay, France).
The quantification of mycelium growth in agar-glucose
culture was used to determine antifungal activity of phenolics.
The nutrient solution contained 1.5% agar, 2% glucose, and
solvent for phenolics (MeOH 0.5%). Medium (10 mL), containing an appropriate amount (10-3 M) of the particular phenolic
compound, was “filter sterilized” using a Millipore filter (0.45
µm), transferred to a Petri dish, and then inoculated with a
small mycelial plug (6 mm in diameter), obtained from the
actively growing margin of the fungal culture on PDA medium,
and placed in the center of the test plate. Plates were incubated
in the dark at 21 °C until the mycelium of the control plate
(containing no test phenolics) had almost filled the Petri dish
(about 10-15 days). Growth rate was evaluated as the average
diameter of the mycelial mat following incubation.
For spore germination studies, 5-day-old cultures of P.
vagabunda maintained on PDA plates was used to prepare a
conidial suspension (6 × 105 conidia/mL) in sterile 0.1 M
sodium citrate buffer, pH 5. One milliliter of the conidial
suspension was diluted in a 200-mL Erlenmeyer flask with
20 mL of 0.1 M citrate buffer, pH 5, containing 1% glucose
and 10-3 M of phenolic compound to be tested. Samples were
incubated in the dark at 21 °C for 48 h. After 24 h of
incubation, 1 mL of the crude apple polyphenol oxidase was
added to the sample (1 mL of buffer in the control flask).
Following incubation, 100 µL of spore suspension was seeded
in PDA plates to evaluate the percentage of germinated spores.
The bioassays were repeated twice with three duplicates for
each test compound, and the results were averaged.
RESULTS AND DISCUSSION
The phenolics in apple flesh, in good agreement with
previous literature data (24, 25, 27), consisted of (+)catechin, chlorogenic acid, (-)-epicatechin, phloridzin
(phloretin 2′-O-glucoside), and an unidentified phloretin
2′-O-glycoside. This latter phloretin derivative was
provisionally identified because of its spectra characteristics. In addition, their lower retention time indicated a more polar structure, which was identified as
phloretin 2′-O-xylosylglucoside. Apple peel contained an
additional five or six quercetin glycosides (3-O-galactoside (hyperin), 3-O-glucoside (isoquercitrin), and/or 3-Orutinoside (rutin), 3-O-xyloside, 3-O-arabinoside, and
3-O-rhamnoside (quercitrin)). Apple peel also contains
ferulic acid and p-coumaric acid derivatives.
During the first 60 days of cold storage, when fruits
are rot-free, the concentrations of phenolics in the skin
of Golden Delicious apples showed a relevant biosynthetic increase (Figure 1). Afterward, fruit stored for a
longer period had a gradually decreasing content of
phenolics, particularly phloridzin. This decrease was
more pronounced after 10 days of shelf life at room
temperature, and these quantitative differences are due
to the effect of the temperature on the enzyme systems
of the phenolic metabolism. However, at the end of the
storage period the content of phenolic compounds in
apple skin was comparable or higher than that found
at the harvest. From these results, it is clear that even
with fruit changed in storage, thus becoming quite
susceptible to fungal attack, a significant decrease in
phenolic content in the skin of the fruit was not
recorded. In addition, among the phenolic compounds
identified in apple peel, only chlorogenic acid was found
to be an in vitro inhibitor of spore germination or
Phenolics and Apple Quiescent Infections
J. Agric. Food Chem., Vol. 49, No. 12, 2001 5819
Figure 1. Changes in preexisting phenolics and decay losses in Golden Delicious apples stored at 2 °C plus shelf life at room
temperature.
Table 1. Changes in Phenolic Compounds (mg/100 g
d.w.) in Infected Tissues Surrounding the Rotten Zone of
Apple Peel
compound
(+)-catechin
chlorogenic acid
(-)-epicatechin
phloretin 2′-O-xylosylglucoside
quercetin 3-O-galactoside
quercetin 3-O-glucoside +
quercetin 3-O-rutinoside
quercetin 3-O-xyloside
phloridzin
quercetin 3-O-arabinoside
quercetin 3-O-rhamnoside
healthy
tissue
infected
tissue
31.1
22.5
39.7
27.1
26.7
11.6
49.4
47.2
66.6
44.4
42.6
18.0
8.1
22.6
17.2
12.4
13.0
37.9
27.6
22.1
mycelial growth of P. vagabunda, while (+)-catechin,
(-)-epicatechin, phloridzin, and quercetin glycosides
showed no activity. It has been suggested (16, 28) that
phloridzin can be hydrolyzed in vivo by various fungi
such as Venturia inaequalis, Pennicillium spp., Fusarium spp., and Aspergillus spp. to give phloretin, which,
in turn, is degraded to phloroglucinol, phloretic acid, and
p-hydroxybenzoic acid which inhibits the development
of the fungus. However, in vitro bioassays of these
catabolic phloridzin derivatives showed no inhibitory
activity on fungal mycelial growth of P. vagabunda.
These data concerning changes in phenolic compounds and rotting of cold stored apples are in good
agreement with that of Nevoroske and co-workers (14,
15) concerning the relationships between phloridzin
content and resistance to Venturia inaequalis in apple
leaves. These authors suggested the hypothesis that
phloridzin oxidation in infected tissues, instead of
preexisting phenols, plays an important role in host
resistance and that the mechanism of resistance may
be governed by the hypersensitive responses linked to
the host-pathogen interaction. Therefore, it was decided to examine the phenolic metabolism in cold stored
Figure 2. In vivo changes of PPO activity in healthy and
infected tissues during cold storage of apple fruits.
apples when rotting became evident. For phenolic and
polyphenol oxidase activity analysis, the skin was
separated into two samples: infected tissues around (2
mm) the rotten zone and healthy tissues on the opposite
side of the apple.
Table 1 shows that when rot appears in infected
tissues surrounding the rotten zone a general increase
in phenolic levels was observed, as compared to healthy
tissue of the same fruit. PPO activity also increased in
these tissues, 2-3 times that in healthy tissues (Figure
2). These changes in phenolic content and PPO activity
may be considered as a part of the so-called hypersensitive reaction of the host cells to pathogen that is useful
for arresting the development of fungus without causing
further damage to the surrounding tissue (18, 29, 30).
Postinfectional accumulation of preexisting phenolics,
especially phloridzin and chlorogenic acid which are the
better substrates of apple PPO (KM ) 1.351 and 1.139
mM, respectively), provides an adequate substrate to
the increased PPO activity. Polyphenol oxidase, consuming oxygen and producing fungitoxic quinones,
5820 J. Agric. Food Chem., Vol. 49, No. 12, 2001
Figure 3. In vitro inhibition of fungal spore germination of
P. vagabunda by apple oxidized phenolics.
makes the medium unfavorable to further development
of pathogens.
In fact, in vitro bioassays show that, when a crude
extract of apple PPO was added to a spore suspension
of P. vagabunda that contains 10-3 M of each apple
phenolics, an inhibition of fungal spore germination was
observed. Figure 3 shows in vitro inhibition of fungal
spore germination of P. vagabunda by some oxidized
apple phenolics. This figure also shows a potential
synergistic effect of phloridzin and chlorogenic acid, in
good agreement with the results of Oszmianski and Lee
(31), demonstrating that the simultaneous presence of
chlorogenic acid and catechin in a model system increases the oxidation rate of phloridzin in the presence
of polyphenol oxidase. This synergistic effect should
probably be considered in the overall defensive strategy
of apple against fungal attack. Finally, when in long
stored apples these primary responses (increased phenolic level and activation of apple PPO in infected
tissues), which counteract the further development of
the pathogen, fall during the storage, a simultaneous
increase in apple rot is observed.
We can conclude that over the past few years the
importance of phenylpropanoid metabolism in fruit and
vegetable tissue under biotic and abiotic stress has been
well studied. In this connection, it is a well-known fact
that exposure of some plant tissues to low nonfreezing
temperatures can stimulate phenolic metabolism (21,
23, 32). However, there is relatively little information
on the effect of storage conditions on the fate of inducible
and preformed phenolic compounds frequently associated with postharvest disease resistance. Optimizing
storage conditions for maintaining this biochemical
potential can help improve the shelf life of plant
commodities. These conditions (pretreatments, refrigeration, controlled/modified atmospheres) must be selected with the aim of inducing an increase of phenylpropanoid/flavonoid levels in apple fruits during cold
storage.
Data presented in this research showed that a low
temperature of 2 °C caused a metabolic increase of
phenolics, especially chlorogenic acid and phloretin
glycosides, in stored apples. When phenolic levels
decreased during long-term storage, a concomitant
decline in resistance to latent infections was observed.
This drop is associated with the appearance of symptoms caused by P. vagabunda. As far as the resistance
mechanism to decay development is concerned, it must
be remembered that, among the phenolic compounds
identified in apple peel, chlorogenic acid only inhibited
Lattanzio et al.
P. vagabunda spore germination or mycelial growth in
vitro. Therefore, changes in phenolic content and PPO
activity in infected tissues surrounding the rotten zone,
i.e., an efficient hypersensitive response, must be considered in the overall defensive mechanism of apple
against fungal attack.
The results obtained suggested that phloridzin and
chlorogenic acid in combination with polyphenol oxidase
activity could function to arrest P. vagabunda in quiescent infections associated with immature and ripening
apple fruit. According to Noveroske et al. (15) and
Oszmianski and Lee (31), it appears that infection of
apple tissue elicited an active glycosidase and polyphenol oxidase capable of converting phloridzin to phloretin,
which was subsequently oxidized. This metabolism of
phloridzin initiated by cell decompartmentation at the
site of fungal infection and the synergistic effect of
chlorogenic acid that accelerated the oxidation of phloridzin in the presence of polyphenol oxidase could play
a prominent role in host resistance to further pathogen
development.
LITERATURE CITED
(1) Pratella, G. C.; Gallerani, G.; Biondi, G. La qualità della
frutta in rapporto al grado di maturazione alla raccolta.
Atti del Convegno Influenza dello stato di maturazione
degli ortofrutticoli alla raccolta sulle malattie postraccolta; MACFRUT-Cesena, 9 Maggio 1991.
(2) Biondi, G.; Linsalata, V.; Neri, F.; Cardinali, A.; Di
Venere, D. Indagine sui metaboliti che inducono la
quiescenza della Phlyctaena vagabonda nelle mele. In
La Qualità degli Ortofrutticoli freschi nella fase di
postraccolta; Anelli, G., Mencarelli, F., Eds; CNRRAISA Publisher: Roma, 1994.
(3) Hulme, A. C.; Edney, K. L. Phenolic substances in the
peel of Cox’s Orange Pippin apples with reference to
infection by Gloeosporium perennas. In Phenolics in
Plant and Disease; Pridham, J. B., Ed.; Pergamon
Press: Oxford, 1960.
(4) Prusky, D. Pathogen quiescence in postharvest diseases.
Annu. Rev. Phytopathol. 1996, 34, 413-434.
(5) Jersch, S.; Scherer, C.; Huth, G.; Schlösser, E. Proanthocyanidins as basis for quiescence of Botrytis cinerea
in immature strawberry fruits. Z. PflKrankh. PflSchutz.
1989, 96, 365-378.
(6) Laks, P. E.; Pruner, M. S. Flavonoid biocides: Structureactivity relations of flavonoids phytoalexins analogues.
Phytochemistry 1989, 28, 87-91.
(7) Lattanzio, V.; Cardinali, A.; Palmieri, S. The role of
phenolics in the postharvest physiology of fruits and
vegetables: browning reactions and fungal diseases. A
Review. Ital. J. Food Sci. 1994, 6, 3-22.
(8) Nicholson, R. L.; Hammerschmidt, R. Phenolic compounds and their role in disease resistance. Annu. Rev.
Phytopathol. 1992, 30, 369-389.
(9) Flood, A. E.; Kirkham, D. S. The effect of some phenolic
compounds on the growth and sporulation of two Venturia species. In Phenolics in Plants and Disease;
Pridham, J. B. Ed.; Pergamon Press: Oxford, 1960.
(10) Piano, S.; Neyrotti, V.; Migheli, Q.; Gullino, M. L.
Biocontrol capability of Metschnikowia pulcherrima
against Botrytis postharvest rot of apple. Postharvest
Biol. Technol. 1997, 11, 131-140.
(11) Byrde, R. J. W.; Fielding, A. H.; Williams, A. H. The
role of oxidized polyphenols in the varietal resistance
of apple to brown rot. In Phenolics in Plants and Disease;
Pridham, J. B., Ed.; Pergamon Press: Oxford, 1960.
(12) Ju, Z.; Yuan, Y.; Liu, C.; Zhan, S.; Wang, M. Relationships among simple phenol, flavonoid and anthocyanin
in apple fruit peel at harvest and scald susceptibility.
Postharvest Biol. Technol. 1996, 8, 83-93.
Phenolics and Apple Quiescent Infections
(13) Lister, C. E.; Lancaster, J. E.; Walzer, J. R. L. Developmental changes in enzymes of flavonoid biosynthesis
in the skin of red and green apple cultivars. J. Sci. Food
Agric. 1996, 71, 313-320.
(14) Noveroske, R. L.; Kuæ, J.; Williams, E. B. Oxidation of
phloridzin and phloretin related to resistance of Malus
to Venturia inaequalis. Phytopathology 1964, 54, 9297.
(15) Noveroske, R. L.; Williams, E. B.; Kuæ, J. β-Glycosidase
and phenoloxidase in apple leaves and their possible
relation to resistance to Venturia inaequalis. Phytopathology 1964, 54, 98-103.
(16) Overeem, J. C. Preexisting antimicrobial substances in
plants and their role in disease resistance. In Biochemical Aspects of Plant Parasite Relationships; Friend, J.,
Threlfall, D. R., Eds; Academic Press: London, 1976.
(17) Raa, J.; Overeem, J. C. Transformation reactions of
phloridzin in the presence of apple leaf enzymes. Phytochemistry 1968, 7, 721-731.
(18) Blankenship, S. M.; Richardson, D. G. Changes in
phenolic acids and internal ethylene during long-term
cold storage of pear. J. Am. Soc. Hortic. Sci. 1985, 110,
336-339.
(19) Blankenship, S. M.; Unrath, C. R. PAL and ethylene
content during maturation of Red and Golden Delicious
apples. Phytochemistry 1988, 27, 1001-1003.
(20) Flaishman, M. A.; Kolattukudy, P. E. Timing of fungal
invasion using host’s ripening hormone as a signal. Proc.
Natl. Acad. Sci. U.S.A. 1994, 91, 6579-7583.
(21) Engelsma, G. Low-temperature effects on phenylalanine
ammonia lyase in gherkin seedlings. Planta 1970, 91,
246-254.
(22) Lattanzio, V.; Van Sumere, C. F. Changes in phenolic
compounds during development and cold storage of
artichoke (Cynara scolymus L.) heads. Food Chem. 1987,
24, 37-50.
(23) Lattanzio, V.; Linsalata, V.; Palmieri, S.; Van Sumere,
C. F. The beneficial effect of citric and ascorbic acid on
the phenolic browning reaction in stored artichoke
J. Agric. Food Chem., Vol. 49, No. 12, 2001 5821
(24)
(25)
(26)
(27)
(28)
(29)
(30)
(31)
(32)
(Cynara scolymus L.) heads. Food Chem. 1989, 33, 93106.
Burda, S.; Oleszek, W.; Lee, C. Y. Phenolic compounds
and their changes in apples during maturation and cold
storage. J. Agric. Food Chem. 1990, 38, 945-948.
Mayr, U.; Treutter, D.; Santos-Buelga, C.; Bauer, H.;
Feucht, W. Developmental changes in the phenol concentrations of ‘Golden Delicious’ apple fruits and leaves.
Phytochemistry 1995, 38, 1151-1155.
Awad, M. A.; de Jager, A. Flavonoid and chlorogenic acid
concentrations in skin of ‘Jonagold’ and ‘Elstar’ apples
during and after regular and ultralow oxygen storage.
Postharvest Biol. Technol. 2000, 20, 15-24.
Golding, J B.; McGlasson, W. B.; Wyllie, W. B.; Leach,
D. M. Fate of apple peel phenolics during cool storage.
J. Agric. Food Chem. 2001, 49, 2283-2289.
Barz, W.; Hosel, W. Metabolism of flavonoids. In The
Flavonoids; Harborne, J.B., Mabry, T. J., Mabry, H.,
Eds; Chapman and Hall: London, 1975.
Dixon, R. A.; Paiva, N. L. Stress-induced phenylpropanoid metabolism. Plant Cell 1995, 7, 1085-1097.
He, X.-Z.; Dixon, R. A. Genetic manipulation of isoflavone 7-O-methyltransferase enhances biosynthesis of 4′O-methylated isoflavonoid phytoalexins and disease
resistance in alfalfa. Plant Cell 2000, 12, 1689-1702.
Oszmianski, J.; Lee, C. Y. Enzymatic oxidation of
phloretin glucoside in model system. J. Agric. Food
Chem. 1991, 39, 10050-1052.
Rhodes, M. J. C.; Wooltorton, L. S. C.; Hill, A. C.
Changes in phenolic metabolism in fruit and vegetable
tissue under stress. In Recent Advances in the Biochemistry of Fruit and Vegetables; Friend, J., Rhodes, M. J.
C., Eds; Academic Press: London, 1981.
Received for review February 26, 2001. Revised manuscript
received October 2, 2001. Accepted October 3, 2001.
JF010255B