The role of phytohormones in alleviating the adverse effects of both abiotic and
biotic stress factors is well known. Among plant herbs, salicylic acid (SA) acts as a
signaling and regulatory molecule in plant environmental stress responses by
SA-mediated control of metabolic and molecular processes [61, 62].
Horsakova et al. [63] found that in the Plum pox virus infection in two peach
varieties (“Symphony” and “Royal Glory”), the antioxidant activity (expressed by
DPPH, ABTS, FRAP, DMPD, and Free Radicals) of all polyphenol compounds
increases significantly. The increased antioxidant activity in the fruit of PPV-infected
peach trees is probably due to the function of protective systems that regulate the
production of reactive oxygen species and thus protect cells from oxidative damage.
Peach fruits contain a whole range of natural substances that have a positive effect on
human health. Carotenoids, vitamin C, and polyphenol compounds [64–67] are
considered to be major antioxidants.
Free radicals are reactive oxygen species (ROS), namely atoms or molecules that,
due to the absence of an electron, show high reactivity. Under normal circumstances,
the production of ROS in the cell is low; however, the oxidative stress caused by
PPV infection may lead to an increase in ROS [68], which in turn leads to a distortion
of the balance between production and the elimination of ROS [69]. However, plants
have developed very good protective mechanisms to neutralize ROS, thus protecting
cells from oxidative damage. Protective antioxidant systems prevent the initiation of
chain oxidation by removing partially reduced oxygen species such as superoxide
and hydrogen peroxide [70]. Superoxide dismutase (SOD) catalyzes the conversion
of the superoxide radical into hydrogen peroxide, which is subsequently converted
by catalase (CAT) or ascorbate peroxidase (APX) into water [68]. Other processes
occur in the so-called ascorbate-glutathione cycle when, during the ascorbate peroxidase catalysis, the hydrogen peroxide reacts with the ascorbate to form two
molecules of water. At the same time, MDHA is formed which either is
disproportionated to dehydroascorbate (DHA) and ascorbate or is reduced from
NAD (P) H to ascorbate by dependent MDND. Dehydroascorbate is transformed
into ascorbate during reduction with glutathione in a dehydroascorbate reductase
(DHAR) catalyzed reaction. Oxidation of glutathione leads to the formation of
disulfide (GSSG) between the cysteine residues of two glutathione molecules
[71]. Oxidized glutathione is reduced by glutathione reductase using NADPH [72].
7
Influence of Biostimulants on the Content of Secondary
Metabolites and Increase of Plant Tolerance in Stress
Factors
Based on the biochemical mechanisms of plant cell protection and the importance of
a number of metabolites for the detoxification of active oxygen species and radicals,
a study has been developed to study the impact of biostimulants (substances without
nutritional effect but affecting different processes) to increase plant tolerance to
stress factors. Ascorbic acid and the ascorbate-glutathione cycle play an important
role in the detoxification of ROS and the modulation of other fundamental functions
16 Deranged Physiology of Peach
393
biotic stress factors is well known. Among plant herbs, salicylic acid (SA) acts as a
signaling and regulatory molecule in plant environmental stress responses by
SA-mediated control of metabolic and molecular processes [61, 62].
Horsakova et al. [63] found that in the Plum pox virus infection in two peach
varieties (“Symphony” and “Royal Glory”), the antioxidant activity (expressed by
DPPH, ABTS, FRAP, DMPD, and Free Radicals) of all polyphenol compounds
increases significantly. The increased antioxidant activity in the fruit of PPV-infected
peach trees is probably due to the function of protective systems that regulate the
production of reactive oxygen species and thus protect cells from oxidative damage.
Peach fruits contain a whole range of natural substances that have a positive effect on
human health. Carotenoids, vitamin C, and polyphenol compounds [64–67] are
considered to be major antioxidants.
Free radicals are reactive oxygen species (ROS), namely atoms or molecules that,
due to the absence of an electron, show high reactivity. Under normal circumstances,
the production of ROS in the cell is low; however, the oxidative stress caused by
PPV infection may lead to an increase in ROS [68], which in turn leads to a distortion
of the balance between production and the elimination of ROS [69]. However, plants
have developed very good protective mechanisms to neutralize ROS, thus protecting
cells from oxidative damage. Protective antioxidant systems prevent the initiation of
chain oxidation by removing partially reduced oxygen species such as superoxide
and hydrogen peroxide [70]. Superoxide dismutase (SOD) catalyzes the conversion
of the superoxide radical into hydrogen peroxide, which is subsequently converted
by catalase (CAT) or ascorbate peroxidase (APX) into water [68]. Other processes
occur in the so-called ascorbate-glutathione cycle when, during the ascorbate peroxidase catalysis, the hydrogen peroxide reacts with the ascorbate to form two
molecules of water. At the same time, MDHA is formed which either is
disproportionated to dehydroascorbate (DHA) and ascorbate or is reduced from
NAD (P) H to ascorbate by dependent MDND. Dehydroascorbate is transformed
into ascorbate during reduction with glutathione in a dehydroascorbate reductase
(DHAR) catalyzed reaction. Oxidation of glutathione leads to the formation of
disulfide (GSSG) between the cysteine residues of two glutathione molecules
[71]. Oxidized glutathione is reduced by glutathione reductase using NADPH [72].
7
Influence of Biostimulants on the Content of Secondary
Metabolites and Increase of Plant Tolerance in Stress
Factors
Based on the biochemical mechanisms of plant cell protection and the importance of
a number of metabolites for the detoxification of active oxygen species and radicals,
a study has been developed to study the impact of biostimulants (substances without
nutritional effect but affecting different processes) to increase plant tolerance to
stress factors. Ascorbic acid and the ascorbate-glutathione cycle play an important
role in the detoxification of ROS and the modulation of other fundamental functions
16 Deranged Physiology of Peach
393