field conditions, respectively [69, 90]. The hormetic response of allelochemicals
varied depending on the type of allelochemicals, source of allelochemicals, time of
application, and crop trait [90]. For example, aqueous extracts of sorghum, maize,
and rice at low concentrations caused up to 35% increase in maize grain yield; each
extract caused different levels of enhancement [70]. Similarly, the sorghum extract
at 3% w/v concentration caused up to 42% increase in canola and maize yield,
respectively [71]. Allelochemicals from various sources have been reported for
their hormetic effect on different crop species in field conditions both under
normal and stress environments [90].
In addition to growth enhancement of crop plants, allelochemicals can
suppress the weed growth directly by acting as herbicide. The selectivity can be
achieved by applying allelochemicals at crop tolerant stage and weed sensitive
stage (early growth stage) [90]. Therefore, hormetic potential of allelochemicals
can be used to suppress weeds by providing crop plants competitive advantage
over the weeds.
4
Challenges in Implementing Allelopathic Weed Control
Establishing allelopathic weed control as tool for weed management in field
crops might be a difficult task as other interference appliances (competition
for inputs, soil microbial impact, and nutrient immobilization) work in parallel [72]. Estimation of herbicidal potential of allelochemicals after their entry
in soil is an important task because various allelochemicals only showed
inhibitory effect in bioassays, but no inhibition occurs when applied with soil
[72]. Moreover, various types of stresses in the ecosystem also influence
the allelopathic effect [73]. Hence, it is difficult to prove the mechanism of
allelopathy [74]. Type and concentration of allelochemicals released by any
specific plant species depend both on plant factors (species, growth stage,
and plant part) and environmental factors (soil fertility, moisture level, temperature, climatic conditions, etc.) [75]. Furthermore, fortune of allelopathic effect in
soil is not well known [76]. Soil environment affects the activity of
allelochemicals due to various physical, chemical, and biological interactions
[77, 78]. Furthermore, in the complex agroecosystem, allelochemicals do not
reproduce and are susceptible to chemical and microbial degradation. Herbicidal
potential of allelochemicals is a collective/synergistic response of various
chemicals in the mixture and not due to any particular chemical [75]. Thus, type
of allelochemicals and their integrated effects in the mixtures is important
to be considered.
High production cost (e.g., tentoxin), low efficacy, and poor selectivity are also
major limitations in using allelochemicals as potential weed control agents [4].
These herbicides might be toxic to nontarget crop species, for example, a natural
plant-released phytotoxin alpha-terthienyl extracted and isolated from common
marigold (Tagetes erecta L.) roots for use as a herbicide was equally toxic to
crop plants in addition to weeds [77]. Generally, allelochemicals have short half512
N. Farooq et al.
varied depending on the type of allelochemicals, source of allelochemicals, time of
application, and crop trait [90]. For example, aqueous extracts of sorghum, maize,
and rice at low concentrations caused up to 35% increase in maize grain yield; each
extract caused different levels of enhancement [70]. Similarly, the sorghum extract
at 3% w/v concentration caused up to 42% increase in canola and maize yield,
respectively [71]. Allelochemicals from various sources have been reported for
their hormetic effect on different crop species in field conditions both under
normal and stress environments [90].
In addition to growth enhancement of crop plants, allelochemicals can
suppress the weed growth directly by acting as herbicide. The selectivity can be
achieved by applying allelochemicals at crop tolerant stage and weed sensitive
stage (early growth stage) [90]. Therefore, hormetic potential of allelochemicals
can be used to suppress weeds by providing crop plants competitive advantage
over the weeds.
4
Challenges in Implementing Allelopathic Weed Control
Establishing allelopathic weed control as tool for weed management in field
crops might be a difficult task as other interference appliances (competition
for inputs, soil microbial impact, and nutrient immobilization) work in parallel [72]. Estimation of herbicidal potential of allelochemicals after their entry
in soil is an important task because various allelochemicals only showed
inhibitory effect in bioassays, but no inhibition occurs when applied with soil
[72]. Moreover, various types of stresses in the ecosystem also influence
the allelopathic effect [73]. Hence, it is difficult to prove the mechanism of
allelopathy [74]. Type and concentration of allelochemicals released by any
specific plant species depend both on plant factors (species, growth stage,
and plant part) and environmental factors (soil fertility, moisture level, temperature, climatic conditions, etc.) [75]. Furthermore, fortune of allelopathic effect in
soil is not well known [76]. Soil environment affects the activity of
allelochemicals due to various physical, chemical, and biological interactions
[77, 78]. Furthermore, in the complex agroecosystem, allelochemicals do not
reproduce and are susceptible to chemical and microbial degradation. Herbicidal
potential of allelochemicals is a collective/synergistic response of various
chemicals in the mixture and not due to any particular chemical [75]. Thus, type
of allelochemicals and their integrated effects in the mixtures is important
to be considered.
High production cost (e.g., tentoxin), low efficacy, and poor selectivity are also
major limitations in using allelochemicals as potential weed control agents [4].
These herbicides might be toxic to nontarget crop species, for example, a natural
plant-released phytotoxin alpha-terthienyl extracted and isolated from common
marigold (Tagetes erecta L.) roots for use as a herbicide was equally toxic to
crop plants in addition to weeds [77]. Generally, allelochemicals have short half512
N. Farooq et al.