Allelopathic effect of Datura stramonium was studied by many authors [38–41].
Effect of Datura stramonium on the growth and survival of various grass and legume
species was performed [39]. Seed and leaf extracts of Datura stramonium have
allelopathic effects on leaf chlorophyll content, root, and shoot length of Cenchrus
ciliaris L. (grass) and Notonia wightii Am (legume) species [38]. Other reports
support allelopathic effect of Datura stramonium [42, 43].
Research report is available [44] on allelopathic effects of Melia azedarach L.
leaf litter and leaf aqueous extracts on germination, growth, and yield of Vigna
mungo L. (black gram) and Cicer arietinum L. (chickpea). Both leaf aqueous
extract and leaf litter inhibited the germination, initial growth, and biomass
of black gram and chickpea. The chemicals responsible for allelopathic effect
of leaf litter were phenolic acids, alkaloids, methyl ketones (volatile allelochemical),
unsaturated fatty acids, benzofuran, propargyl acid, benzoxepine, fluorobenzoic
acid, silicyclobutane, and palmitic acid analyzed by GCMS. Crude alkaloidal fraction of Crotalaria retusa (leaf and stem) was isolated from its methanolic extract
and tested for its allelopathy in Phaseolus vulgaris (bean) at varying concentrations.
Germination of bean seeds was reduced with increasing concentrations of
alkaloid fraction. Study of mechanism for allelopathy shows crude alkaloid fraction
cause oxidative stress that generate reactive of reactive oxygen species that initiated
metabolic derangement in the bean seedlings [45].
2.3
Terpenoids
Terpenoids are known for their medicinal use since ancient times, but there are a number
of allelochemicals present in terpenoids (Fig. 3). 1,8-Cineole and camphor are volatile
monoterpenes which show inhibitory effect on plant growth [46]. One research reported
that extraction procedure can improve the allelopathic activity of Cynara cardunculus.
Different extracting solvents, water, methanol, ethanol, and ethyl acetate, were used, and
all show allelopathic effect more than 50% when tested against six weeds Amaranthus
retroflexus, Portulaca oleracea, Stellaria media, Anagallis arvensis, Echinochloa crusgalli, and Lolium perenne. Also four sesquiterpene lactones (cynaropicrin, cynaratriol,
desacylcynaropicrin, and 11,13-dihydro-desacylcynaropicrin) and a lignan
(pinoresinol) were isolated from the ethyl acetate fraction of the aqueous extract of
Cynara cardunculus. All of them showed allelopathic effects, with cynaropicrin,
desacylcynaropicrin, and pinoresinol having maximum activity [47].
One research reported ecological characteristics of terpenoids and their
allelopathic effects on plants and conclude that terpenoids can cause inhibition,
promotion, and autotoxic action on seed germination and seedling growth [48].
Toxicity of terpenoids may be due to different factors: (1) inhibition of ATP
formation, (2) disruption of hormonal activity, (3) alkylation of nucleophiles, (4)
complexation with protein, (5) binding with free sterols, and (5) inhibition
of respiration [49]. Some terpenoids (essential oil and monoterpenoids) affect seed
germination and plant growth [50, 51]. Allelopathic effects of leaf oil emulsion
of Eucalyptus grandis  Eucalyptus urophylla were tested against proliferation
19 Plant Allelochemicals and Their Various Applications
447
Effect of Datura stramonium on the growth and survival of various grass and legume
species was performed [39]. Seed and leaf extracts of Datura stramonium have
allelopathic effects on leaf chlorophyll content, root, and shoot length of Cenchrus
ciliaris L. (grass) and Notonia wightii Am (legume) species [38]. Other reports
support allelopathic effect of Datura stramonium [42, 43].
Research report is available [44] on allelopathic effects of Melia azedarach L.
leaf litter and leaf aqueous extracts on germination, growth, and yield of Vigna
mungo L. (black gram) and Cicer arietinum L. (chickpea). Both leaf aqueous
extract and leaf litter inhibited the germination, initial growth, and biomass
of black gram and chickpea. The chemicals responsible for allelopathic effect
of leaf litter were phenolic acids, alkaloids, methyl ketones (volatile allelochemical),
unsaturated fatty acids, benzofuran, propargyl acid, benzoxepine, fluorobenzoic
acid, silicyclobutane, and palmitic acid analyzed by GCMS. Crude alkaloidal fraction of Crotalaria retusa (leaf and stem) was isolated from its methanolic extract
and tested for its allelopathy in Phaseolus vulgaris (bean) at varying concentrations.
Germination of bean seeds was reduced with increasing concentrations of
alkaloid fraction. Study of mechanism for allelopathy shows crude alkaloid fraction
cause oxidative stress that generate reactive of reactive oxygen species that initiated
metabolic derangement in the bean seedlings [45].
2.3
Terpenoids
Terpenoids are known for their medicinal use since ancient times, but there are a number
of allelochemicals present in terpenoids (Fig. 3). 1,8-Cineole and camphor are volatile
monoterpenes which show inhibitory effect on plant growth [46]. One research reported
that extraction procedure can improve the allelopathic activity of Cynara cardunculus.
Different extracting solvents, water, methanol, ethanol, and ethyl acetate, were used, and
all show allelopathic effect more than 50% when tested against six weeds Amaranthus
retroflexus, Portulaca oleracea, Stellaria media, Anagallis arvensis, Echinochloa crusgalli, and Lolium perenne. Also four sesquiterpene lactones (cynaropicrin, cynaratriol,
desacylcynaropicrin, and 11,13-dihydro-desacylcynaropicrin) and a lignan
(pinoresinol) were isolated from the ethyl acetate fraction of the aqueous extract of
Cynara cardunculus. All of them showed allelopathic effects, with cynaropicrin,
desacylcynaropicrin, and pinoresinol having maximum activity [47].
One research reported ecological characteristics of terpenoids and their
allelopathic effects on plants and conclude that terpenoids can cause inhibition,
promotion, and autotoxic action on seed germination and seedling growth [48].
Toxicity of terpenoids may be due to different factors: (1) inhibition of ATP
formation, (2) disruption of hormonal activity, (3) alkylation of nucleophiles, (4)
complexation with protein, (5) binding with free sterols, and (5) inhibition
of respiration [49]. Some terpenoids (essential oil and monoterpenoids) affect seed
germination and plant growth [50, 51]. Allelopathic effects of leaf oil emulsion
of Eucalyptus grandis  Eucalyptus urophylla were tested against proliferation
19 Plant Allelochemicals and Their Various Applications
447