vigor with different frequencies of 1, 5, 10, 20, and 50 Hz at the voltage of 16 and
20 kV, and the treatment time was 40 s, finding that when the frequency of the
electric field increased, the effects increased and reached the maximum at 10 Hz,
and after 10 Hz, as the electric field frequency increased, the effects began to
decrease. This last experiment exemplifies the hormetic curve in a vigor treatment
where there is a maximal dose at which a maximum response value is reached and
then vigor starts decreasing.
In several studies, PEF pretreatments in plants, whole fruits, or other food
sources result in an increase in the natural bioactive compounds. When biological
cells are exposed to an EF, the charge accumulates along the plasma membrane
causing electroporation, a transmembrane potential difference which causes
porosity, and thus the diffusion of intracellular components in cellular juice
increasing the extractability of natural bioactive compounds by the release of
solutes into the solvent (El Kantar et al. 2018; Vicaş et al. 2017; Barba et al. 2015;
Hendrawan et al. 2019). The time exposure and intensity of the EF are critical since
a lower EF may form smaller pores allowing the ions to pass through, but large
molecules may not get out of the cell, however, higher EF are suspected to damage
antioxidant compounds due to long exposure to high-voltage electric current
(Hendrawan et al. 2019). Table 5.2 summarizes some examples of the application
of magnetic or electric fields on plants or foods with a commercial interest and
presents the effect on the production of natural bioactive compounds.
5.2.2 Acoustic Emissions
Acoustic emissions (AE) stimulus is one of the recent physical abiotic factors
whose beneficial effects on plant growth, development, and health have been discussed. AE from ecological conditions or artificially applied can initiate diverse
signals that trigger transduction cascades, similar to other abiotic stress factors
(Alvarado et al. 2019). From a bioacoustics perspective, chewing serves as an alarm
signal to plants and has been demonstrated that applying recorded insect chewing
sounds caused an increase of phytochemical production (Appel and Cocroft 2014).
In the same way, Jeong et al. (2014), reported an improvement of natural protection
responses in rice plants caused by amplification at 100 decibels of a wide range of
frequencies between 0 and 1.5 kHz. Moreover, Hassanien et al. (2014), found a
higher disease resistance in pepper, cucumber, and tomato after AE treatments.
The biological mechanism of how sound affects plants is still under discussion.
A mechano-stimuli perception of waves has been proposed, but a reliable explanation of sound-specific structure for recognition by plants has not been completely
elucidated (Alvarado et al. 2019). This mechanism consists of the second messenger of calcium ion (Ca
2+ ) signals. The channels that mediate Ca
2+
flux are
possibly located in the plasmatic membrane where Ca
2+ is sensed possibly through
various Ca
2+ sensors and/or CDPKs (Calcium-dependent protein kinase), which
pass the message through phosphorylation/dephosphorylation to different signaling
160
H. Aguirre-Becerra et al.
20 kV, and the treatment time was 40 s, finding that when the frequency of the
electric field increased, the effects increased and reached the maximum at 10 Hz,
and after 10 Hz, as the electric field frequency increased, the effects began to
decrease. This last experiment exemplifies the hormetic curve in a vigor treatment
where there is a maximal dose at which a maximum response value is reached and
then vigor starts decreasing.
In several studies, PEF pretreatments in plants, whole fruits, or other food
sources result in an increase in the natural bioactive compounds. When biological
cells are exposed to an EF, the charge accumulates along the plasma membrane
causing electroporation, a transmembrane potential difference which causes
porosity, and thus the diffusion of intracellular components in cellular juice
increasing the extractability of natural bioactive compounds by the release of
solutes into the solvent (El Kantar et al. 2018; Vicaş et al. 2017; Barba et al. 2015;
Hendrawan et al. 2019). The time exposure and intensity of the EF are critical since
a lower EF may form smaller pores allowing the ions to pass through, but large
molecules may not get out of the cell, however, higher EF are suspected to damage
antioxidant compounds due to long exposure to high-voltage electric current
(Hendrawan et al. 2019). Table 5.2 summarizes some examples of the application
of magnetic or electric fields on plants or foods with a commercial interest and
presents the effect on the production of natural bioactive compounds.
5.2.2 Acoustic Emissions
Acoustic emissions (AE) stimulus is one of the recent physical abiotic factors
whose beneficial effects on plant growth, development, and health have been discussed. AE from ecological conditions or artificially applied can initiate diverse
signals that trigger transduction cascades, similar to other abiotic stress factors
(Alvarado et al. 2019). From a bioacoustics perspective, chewing serves as an alarm
signal to plants and has been demonstrated that applying recorded insect chewing
sounds caused an increase of phytochemical production (Appel and Cocroft 2014).
In the same way, Jeong et al. (2014), reported an improvement of natural protection
responses in rice plants caused by amplification at 100 decibels of a wide range of
frequencies between 0 and 1.5 kHz. Moreover, Hassanien et al. (2014), found a
higher disease resistance in pepper, cucumber, and tomato after AE treatments.
The biological mechanism of how sound affects plants is still under discussion.
A mechano-stimuli perception of waves has been proposed, but a reliable explanation of sound-specific structure for recognition by plants has not been completely
elucidated (Alvarado et al. 2019). This mechanism consists of the second messenger of calcium ion (Ca
2+ ) signals. The channels that mediate Ca
2+
flux are
possibly located in the plasmatic membrane where Ca
2+ is sensed possibly through
various Ca
2+ sensors and/or CDPKs (Calcium-dependent protein kinase), which
pass the message through phosphorylation/dephosphorylation to different signaling
160
H. Aguirre-Becerra et al.