activity of oxidizing enzymes, especially phenol oxidase, are part of the mechanism of
disease resistance that would be realized by inhibiting the polygalacturonase of the
pathogen by oxidized phenols [47]. It is also possible that biochemical protections are
present all the time in healthy plants, although observed variations in sensitivity to age
seem to indicate that they can develop at certain stages [34, 47].
PAL is a rate-determining enzyme in the activation of the phenylpropanoid
pathway, and the increase in PAL activity is associated with the biosynthesis of
active metabolites such as phytoalexins, phenols, lignins, and salicylic acid in plant
protection pathways [51]. POD participates in cell wall building processes, such as
phenol oxidation, suberisation, and lignification, during the protective response
against pathogenic agents [52]. PPO participates in the oxidation of polyphenols in
quinones (antimicrobial compounds) and lignification of plant cells during the
microbial invasion [53]. In addition, the accumulation of phenolic compounds is
associated with disease resistance in a number of interactions between plants and a
pathogen. The high level of phenolic compounds at the site of pathogen invasion
may limit or slow down its growth [54, 55].
In a number of infectious diseases, the metabolism of the affected parts varies
considerably under the influence of the pathogen. In leaf curl disease caused by
Taphrina deformans (Berk.) Tul., it induces serious changes in the biochemical
status of the infected plants, which are detectable not only in the tissues with
observable symptoms but also in distally situated ones. These changes include the
elevation of the activity of antioxidant enzymes (peroxidases), reduced polyphenols
content and plastid pigments, alterations of antiradical activity, anthocyanin, and free
proline concentrations [56]. The metabolism of the peach leaves affected by the
pathogen resembles strongly the characteristic of the still immature leaves. A
reduction in photosynthetic function is observed, and the import of sugars into the
leaves is dominated by their exports. In addition, the content of both soluble
carbohydrates and the enzymes involved in their metabolism is similar to that of
young leaves, not mature (Fig. 8). Many of the effects of the disease on the
metabolism of peach leaves are similar to those caused by other plant diseases on
the metabolism of photosynthetic organs [57].
Like other crops, peach also is attacked by many plant pathogens such as fungi,
bacteria, and viruses. Such pathogen-associated infections in plant tissues, particularly local and resistant (hypersensitive) infections, show a general metabolic change
that involves the accumulation of amounts of secondary metabolites (phenols,
flavonoids, coumarins, terpenoids, steroids, etc.). This change in the spectrum of
secondary metabolites is mainly in response to the infectious agent or physiological
stimuli and stress.
Besides playing a vital role in the normal development of healthy plants, the
temperature is also a key factor in determining the nature of the interactions between
plants and pathogens. Any major change in environmental conditions, especially
temperature, will affect not only plants but also pathogens and therefore plant
diseases [58]. Different temperature regimes are expected to have a direct impact
on biochemical compounds in both healthy and infected plants and the most
pronounced effect can be visualized in the total phenolic content (TPC). Polyphenols
16 Deranged Physiology of Peach
391
disease resistance that would be realized by inhibiting the polygalacturonase of the
pathogen by oxidized phenols [47]. It is also possible that biochemical protections are
present all the time in healthy plants, although observed variations in sensitivity to age
seem to indicate that they can develop at certain stages [34, 47].
PAL is a rate-determining enzyme in the activation of the phenylpropanoid
pathway, and the increase in PAL activity is associated with the biosynthesis of
active metabolites such as phytoalexins, phenols, lignins, and salicylic acid in plant
protection pathways [51]. POD participates in cell wall building processes, such as
phenol oxidation, suberisation, and lignification, during the protective response
against pathogenic agents [52]. PPO participates in the oxidation of polyphenols in
quinones (antimicrobial compounds) and lignification of plant cells during the
microbial invasion [53]. In addition, the accumulation of phenolic compounds is
associated with disease resistance in a number of interactions between plants and a
pathogen. The high level of phenolic compounds at the site of pathogen invasion
may limit or slow down its growth [54, 55].
In a number of infectious diseases, the metabolism of the affected parts varies
considerably under the influence of the pathogen. In leaf curl disease caused by
Taphrina deformans (Berk.) Tul., it induces serious changes in the biochemical
status of the infected plants, which are detectable not only in the tissues with
observable symptoms but also in distally situated ones. These changes include the
elevation of the activity of antioxidant enzymes (peroxidases), reduced polyphenols
content and plastid pigments, alterations of antiradical activity, anthocyanin, and free
proline concentrations [56]. The metabolism of the peach leaves affected by the
pathogen resembles strongly the characteristic of the still immature leaves. A
reduction in photosynthetic function is observed, and the import of sugars into the
leaves is dominated by their exports. In addition, the content of both soluble
carbohydrates and the enzymes involved in their metabolism is similar to that of
young leaves, not mature (Fig. 8). Many of the effects of the disease on the
metabolism of peach leaves are similar to those caused by other plant diseases on
the metabolism of photosynthetic organs [57].
Like other crops, peach also is attacked by many plant pathogens such as fungi,
bacteria, and viruses. Such pathogen-associated infections in plant tissues, particularly local and resistant (hypersensitive) infections, show a general metabolic change
that involves the accumulation of amounts of secondary metabolites (phenols,
flavonoids, coumarins, terpenoids, steroids, etc.). This change in the spectrum of
secondary metabolites is mainly in response to the infectious agent or physiological
stimuli and stress.
Besides playing a vital role in the normal development of healthy plants, the
temperature is also a key factor in determining the nature of the interactions between
plants and pathogens. Any major change in environmental conditions, especially
temperature, will affect not only plants but also pathogens and therefore plant
diseases [58]. Different temperature regimes are expected to have a direct impact
on biochemical compounds in both healthy and infected plants and the most
pronounced effect can be visualized in the total phenolic content (TPC). Polyphenols
16 Deranged Physiology of Peach
391