Malondialdehyde (MDA) is a product of this lipid peroxidation and is used as a
stress indicator in some tissues. To deal with ROS, the plants have developed an
effective antioxidant defense system that reacts to oxidative stress and prevents the
buildup of ROS and restores the oxidative damage. This system includes both lipidsoluble antioxidants (tocopherol and carotene) and water-soluble reductants, including ascorbic acid (AsA), glutathione (GSH), and enzymes such as catalase (CAT),
ascorbate peroxidase (ARX), superoxide dismutase (SOD), and glutathione reductase (GR). The substance melatonin (N-acetyl-5-methoxytyptamine), which is a
neurohormone secreted from the pineal gland in mammals, is found in plant tissues
too. Melatonin has been reported to be involved in the growth, development, and
response to stress in plants [39].
5.2
Water Deficiency and Oxidative Stress
Another important factor for the development of plants and in particular peach is the
presence of sufficient water for irrigation. Water stress stimulates stinging and
reduces CO 2 fixation, which can significantly reduce photosynthetic electron transport [40]. If water stress is prolonged and/or severe, part of the energy supplied by
photons can be redirected to processes favoring the formation of reactive oxygen
species (ROS) such as hydrogen peroxide (H 2 O 2 ), which leads to oxidative damage
to plant tissues [41]. However, the plants may activate ROS neutralizing enzymes
and nonenzymatic systems including secondary metabolites such as phenolic compounds, alkaloids, isoprenoids, phenylpropanoids, and other antioxidants such as
glutathione and ascorbic acid (AsA) to reduce oxidative damage [42, 43]. In addition, these systems can also play a very important role in the protection of cell
membrane integrity [44, 45].
6
Biotic Stress
Except in abiotic stress, it has also been found that the level of phenolic compounds
in plants increases as a response to infection by phytopathogens [46], consistent with
the proposed role of these compounds in the protective plant mechanism. It has been
found that infected plant tissues and resistant tissues are characterized by a general
displacement of the metabolic model, which involves the activation of phenoloxidizing enzymes and peroxidases. In fact, the degree of resistance is related to
the number of phenolic compounds oxidized by phenolases [47].
The use of pesticides and fertilizers has been found to modulate the biosynthesis of
phenols in plants [48–50]. Consequently, the increase in the polyphenols content
observed in organically grown peaches and pears may support the hypothesis
[48–50] that protective mechanisms against infects are related to an increase of
endogenous polyphenols when there are no external pesticides that are widespread in
conventional agriculture. Many plants show that the regulation of phenolic metabolism
depends on several factors. Changes in the level of phenols and in the amount and
390
L. Koleva-Valkova and A. Harizanova
stress indicator in some tissues. To deal with ROS, the plants have developed an
effective antioxidant defense system that reacts to oxidative stress and prevents the
buildup of ROS and restores the oxidative damage. This system includes both lipidsoluble antioxidants (tocopherol and carotene) and water-soluble reductants, including ascorbic acid (AsA), glutathione (GSH), and enzymes such as catalase (CAT),
ascorbate peroxidase (ARX), superoxide dismutase (SOD), and glutathione reductase (GR). The substance melatonin (N-acetyl-5-methoxytyptamine), which is a
neurohormone secreted from the pineal gland in mammals, is found in plant tissues
too. Melatonin has been reported to be involved in the growth, development, and
response to stress in plants [39].
5.2
Water Deficiency and Oxidative Stress
Another important factor for the development of plants and in particular peach is the
presence of sufficient water for irrigation. Water stress stimulates stinging and
reduces CO 2 fixation, which can significantly reduce photosynthetic electron transport [40]. If water stress is prolonged and/or severe, part of the energy supplied by
photons can be redirected to processes favoring the formation of reactive oxygen
species (ROS) such as hydrogen peroxide (H 2 O 2 ), which leads to oxidative damage
to plant tissues [41]. However, the plants may activate ROS neutralizing enzymes
and nonenzymatic systems including secondary metabolites such as phenolic compounds, alkaloids, isoprenoids, phenylpropanoids, and other antioxidants such as
glutathione and ascorbic acid (AsA) to reduce oxidative damage [42, 43]. In addition, these systems can also play a very important role in the protection of cell
membrane integrity [44, 45].
6
Biotic Stress
Except in abiotic stress, it has also been found that the level of phenolic compounds
in plants increases as a response to infection by phytopathogens [46], consistent with
the proposed role of these compounds in the protective plant mechanism. It has been
found that infected plant tissues and resistant tissues are characterized by a general
displacement of the metabolic model, which involves the activation of phenoloxidizing enzymes and peroxidases. In fact, the degree of resistance is related to
the number of phenolic compounds oxidized by phenolases [47].
The use of pesticides and fertilizers has been found to modulate the biosynthesis of
phenols in plants [48–50]. Consequently, the increase in the polyphenols content
observed in organically grown peaches and pears may support the hypothesis
[48–50] that protective mechanisms against infects are related to an increase of
endogenous polyphenols when there are no external pesticides that are widespread in
conventional agriculture. Many plants show that the regulation of phenolic metabolism
depends on several factors. Changes in the level of phenols and in the amount and
390
L. Koleva-Valkova and A. Harizanova