monoterpenes, eucalyptol, and camphor), in addition to the nuclear abnormalities
within the vacuole [21, 22]. For instance, the activity of mitotic cells was reduced
by more than 50% in watermelon (Citrullus lanatus (Thumb.) Matsum. & Nakai.) by
the application of maize (Zea mays L.) pollen extract. It also increased the irregularities of nuclear and pyknotic nuclei and reduced the growth of radicle and
hypocotyl [23]. The allelochemicals from barley (Hordeum vulgare L.) roots (i.e.,
hordenine and gramine) caused damage in the tips of cell walls of radical in white
mustard (Sinapis alba L.) and enhanced the organelle disorganization and autophagy
of cell [24]. Similarly, the ultrastructure of chloroplasts and mitochondria in cucumber (Cucumis sativus L.) was distorted by an allelochemical “cinnamic acid” [25].
The random amplification of the polymorphic DNA profiles of various plants has
also been reported to be affected through the allelochemicals released from the
catmint (Nepeta meyeri Benth.) and field bindweed (Convolvulus arvensis L.) [26,
27]. The disturbance of microtubules in the roots of wheat (Triticum aestivum L.) and
Arabidopsis (Arabidopsis thaliana L.) is strongly influenced by an allelochemical
citral [28, 29]. The citral cell caused ultrastructure changes and cell-wall condensation and decreased the intercellular communication and root hair development in
Arabidopsis [30].
2.2
Inhibition of Cell Division and Cell Elongation
In plant meristems, DNA synthesis and cell proliferation are affected by
allelochemicals, e.g., monoterpenoids (i.e., camphene, beta-pinene, camphor,
alpha-pinene, and 1,8-cineole) [31]. Reduction in the process of mitosis particularly
in lettuce (Lactuca sativa L.) has also been reported in response to exposure
to allelochemicals [32]. In a study, the cell number in each period of cell division
was decreased by sorgoleone [an allelochemical in sorghum (Sorghum bicolor (L.)
Moench)] with subsequent damage in tubulins and polyploidy nuclei in bean plant
(Phaseolus vulgaris L.) [33]. A rye (Lolium rigidum Gaud.) allelochemical, i.e.,
DIBOA (2, 4 dihydroxy-1,4 (2H)-benzoxazin-3-one), has been reported to reduce
the renewal of root cap cells of cucumber with simultaneous reduction in growth
[34]. In soybean (Glycine max (L.) Merr.), the application of allelopathic water
extracts of jimson weed (Datura stramonium L.) decreased the primary/lateral root
elongation, inhibited the length and density of root hairs, suppressed the root tip cell
division, and increased the chromosomal aberration in micronucleus index especially at higher rates of extract application [35].
2.3
Increase in the Cell Membrane Permeability
The allelochemicals increase the production of free radicals, which results in higher
lipid peroxidation and alteration in the membrane permeability, which destruct
the biological membranes in the plant system [19, 36–39]. For example, the application of the allelopathic water extracts, prepared from the aerial plant parts of
546
A. Nawaz et al.
within the vacuole [21, 22]. For instance, the activity of mitotic cells was reduced
by more than 50% in watermelon (Citrullus lanatus (Thumb.) Matsum. & Nakai.) by
the application of maize (Zea mays L.) pollen extract. It also increased the irregularities of nuclear and pyknotic nuclei and reduced the growth of radicle and
hypocotyl [23]. The allelochemicals from barley (Hordeum vulgare L.) roots (i.e.,
hordenine and gramine) caused damage in the tips of cell walls of radical in white
mustard (Sinapis alba L.) and enhanced the organelle disorganization and autophagy
of cell [24]. Similarly, the ultrastructure of chloroplasts and mitochondria in cucumber (Cucumis sativus L.) was distorted by an allelochemical “cinnamic acid” [25].
The random amplification of the polymorphic DNA profiles of various plants has
also been reported to be affected through the allelochemicals released from the
catmint (Nepeta meyeri Benth.) and field bindweed (Convolvulus arvensis L.) [26,
27]. The disturbance of microtubules in the roots of wheat (Triticum aestivum L.) and
Arabidopsis (Arabidopsis thaliana L.) is strongly influenced by an allelochemical
citral [28, 29]. The citral cell caused ultrastructure changes and cell-wall condensation and decreased the intercellular communication and root hair development in
Arabidopsis [30].
2.2
Inhibition of Cell Division and Cell Elongation
In plant meristems, DNA synthesis and cell proliferation are affected by
allelochemicals, e.g., monoterpenoids (i.e., camphene, beta-pinene, camphor,
alpha-pinene, and 1,8-cineole) [31]. Reduction in the process of mitosis particularly
in lettuce (Lactuca sativa L.) has also been reported in response to exposure
to allelochemicals [32]. In a study, the cell number in each period of cell division
was decreased by sorgoleone [an allelochemical in sorghum (Sorghum bicolor (L.)
Moench)] with subsequent damage in tubulins and polyploidy nuclei in bean plant
(Phaseolus vulgaris L.) [33]. A rye (Lolium rigidum Gaud.) allelochemical, i.e.,
DIBOA (2, 4 dihydroxy-1,4 (2H)-benzoxazin-3-one), has been reported to reduce
the renewal of root cap cells of cucumber with simultaneous reduction in growth
[34]. In soybean (Glycine max (L.) Merr.), the application of allelopathic water
extracts of jimson weed (Datura stramonium L.) decreased the primary/lateral root
elongation, inhibited the length and density of root hairs, suppressed the root tip cell
division, and increased the chromosomal aberration in micronucleus index especially at higher rates of extract application [35].
2.3
Increase in the Cell Membrane Permeability
The allelochemicals increase the production of free radicals, which results in higher
lipid peroxidation and alteration in the membrane permeability, which destruct
the biological membranes in the plant system [19, 36–39]. For example, the application of the allelopathic water extracts, prepared from the aerial plant parts of
546
A. Nawaz et al.