debris and litter are probably to be a key pathway of allelopathy (Song et al. 2019).
Allelochemicals in dry environments bivouac in the soil for a protracted period in
comparison with humid environments (Einhellig 1999). The scientific pieces of
evidence showed that higher plants growing with minimal nutrients, moisture stress,
and temperature conditions are hypersensitive to allelochemicals than those grown in
normal environments (Rivoal et al. 2011). The soil system is an active and continually changing and dynamic environment that influences the fate and functions of
allelochemicals concerning time and space. The ecological conditions which foist
water and nutrient deficiency stress and severe temperatures would increase
allelopathy.
3 Mechanism of Allelochemical Action
Allelopathy has been well recognized for many years ago (Rice 1984), despite it
decoding the mechanisms of action for the allelochemicals is not documented
(Mohamadi and Rajaie 2009). The allelochemicals usually affect the nutrient uptake
and mineral absorption, cell division, membrane permeability, photosynthesis, respiration, growth and development, phytotoxicity, enzyme activity, and protein
synthesis, which are all influenced by allelochemicals. Sorgoleone, a lipophilic
benzoquinone, is a well-recognized compound for its allelopathic nature that is
synthesized by Sorghum bicolor, which acts as a potential allelochemical by
inhibiting the photosystem II (Weir et al. 2004). Many secondary metabolites
including allelochemicals will modify membrane permeability in suitable concentrations that caused leakage of cell contents, which leads to programmed cell death
(apoptosis) and necrosis (Li et al. 2010a, b; Chai et al. 2013).
Higher concentrations of rutin and quercetin have been exhibited to hinder the seed
germination of plant species by impairing the respiration process and ATP levels in
embryogenic callus through substrate oxidation or restricting phosphate ion uptake
that may unfasten the oxidative phosphorylation mechanism (Takahashi et al. 1998).
Allelochemicals disturb plant growth by persuading different phases of respiration,
such as electron transfer in the mitochondria, oxidative phosphorylation, CO 2 generation, and ATP enzyme activity. Hejl and Koster (2004) witnessed that juglone could
reach mitochondrial root cells of corn, thereby restricting root oxygen uptake.
Allelochemical monoterpenoids, viz., camphor, 1,8-cineole, beta-pinene, alphapinene, and camphene, reserved cell proliferation and DNA synthesis in plant meristems (Nishida et al. 2005). All phenolic acids can influence the structure of DNA and
RNA. Ferulic acid and cinnamic acid as well as many phenols and alkaloids can also
inhibit protein synthesis (Li et al. 2010a, b). The plant response to allelochemicals is
identical to biotic and abiotic stress. This signposted that allelochemicals might have
pertinent functions in the cross talk flanked by biotic and abiotic stress signaling
molecules as allelochemicals generate reactive oxygen species (ROS). None of the
allelochemicals is generic; they are specific in their mechanism of action. Additionally,
it should be reconnoitered in future research to discover their respective molecular
5 Allelochemicals as Natural Herbicides for Sustainable Agriculture to Promote. . .
103
Allelochemicals in dry environments bivouac in the soil for a protracted period in
comparison with humid environments (Einhellig 1999). The scientific pieces of
evidence showed that higher plants growing with minimal nutrients, moisture stress,
and temperature conditions are hypersensitive to allelochemicals than those grown in
normal environments (Rivoal et al. 2011). The soil system is an active and continually changing and dynamic environment that influences the fate and functions of
allelochemicals concerning time and space. The ecological conditions which foist
water and nutrient deficiency stress and severe temperatures would increase
allelopathy.
3 Mechanism of Allelochemical Action
Allelopathy has been well recognized for many years ago (Rice 1984), despite it
decoding the mechanisms of action for the allelochemicals is not documented
(Mohamadi and Rajaie 2009). The allelochemicals usually affect the nutrient uptake
and mineral absorption, cell division, membrane permeability, photosynthesis, respiration, growth and development, phytotoxicity, enzyme activity, and protein
synthesis, which are all influenced by allelochemicals. Sorgoleone, a lipophilic
benzoquinone, is a well-recognized compound for its allelopathic nature that is
synthesized by Sorghum bicolor, which acts as a potential allelochemical by
inhibiting the photosystem II (Weir et al. 2004). Many secondary metabolites
including allelochemicals will modify membrane permeability in suitable concentrations that caused leakage of cell contents, which leads to programmed cell death
(apoptosis) and necrosis (Li et al. 2010a, b; Chai et al. 2013).
Higher concentrations of rutin and quercetin have been exhibited to hinder the seed
germination of plant species by impairing the respiration process and ATP levels in
embryogenic callus through substrate oxidation or restricting phosphate ion uptake
that may unfasten the oxidative phosphorylation mechanism (Takahashi et al. 1998).
Allelochemicals disturb plant growth by persuading different phases of respiration,
such as electron transfer in the mitochondria, oxidative phosphorylation, CO 2 generation, and ATP enzyme activity. Hejl and Koster (2004) witnessed that juglone could
reach mitochondrial root cells of corn, thereby restricting root oxygen uptake.
Allelochemical monoterpenoids, viz., camphor, 1,8-cineole, beta-pinene, alphapinene, and camphene, reserved cell proliferation and DNA synthesis in plant meristems (Nishida et al. 2005). All phenolic acids can influence the structure of DNA and
RNA. Ferulic acid and cinnamic acid as well as many phenols and alkaloids can also
inhibit protein synthesis (Li et al. 2010a, b). The plant response to allelochemicals is
identical to biotic and abiotic stress. This signposted that allelochemicals might have
pertinent functions in the cross talk flanked by biotic and abiotic stress signaling
molecules as allelochemicals generate reactive oxygen species (ROS). None of the
allelochemicals is generic; they are specific in their mechanism of action. Additionally,
it should be reconnoitered in future research to discover their respective molecular
5 Allelochemicals as Natural Herbicides for Sustainable Agriculture to Promote. . .
103