plants. Life cycle of these obligate angiosperm parasitic plants depends upon the
presence of a suitable host and chemical signal released by the hosts (Table 1)
[55]. Therefore, chemical cue plays an important role in life cycle of these parasitic
plants and shows co-evolution of chemical biology of host and parasite (see
▶ Chap. 5, “Field Dodder: Life Cycle and Interaction with Host Plants” in this
book). More information will be available with development of sensitive tools of
molecular biology showing gene expression in parasites.
4
Competition for Survival
Plants, cultivated crop plants and weeds, compete with each other in field for
available resources for survival. Rhizosphere is a biologically active soil zone
where different plant roots compete for water, nutrients, and space. Here the roots
communicate with neighboring plants and symbiotic and pathogenic organisms by
the release of root exudates. It is suggested that these exudates play an active role in
root-root and root-microbe communication by manipulating the biological and
physical interactions between these interacting organisms. These chemicals can
change the physical, chemical, and biological properties of the soil and inhibit the
growth of competing plant species [56]. Thus, these exudates mostly play a role of
phytotoxins in the process of allelopathy.
Allelopathy was defined as influence of one plant on another through releasing of
chemicals into the environment [57]. Although the definition of allelopathy is not
confined to positive or negative aspect in particular, most of the studies are done on
the harmful effect of allelochemicals/phytotoxins. Plants release phytotoxins in
decomposing plant tissue, in green leafy volatiles, in leachates from live tissue,
and in root exudates [58, 59]. Sometimes these phytotoxins change the chemistry of
the soil for quite a long time. For example, the phenolic compounds and constituent
diterpenes from Cistus ladanifer L. exudates are toxic and harmful to germination
and growth of herbaceous plants. These allelochemicals are incorporated into the soil
through leaching of leaves and litter, whereas the flavonoids enter the soil through
litter degradation. The long retention of these compounds in litter will maintain their
phytotoxic levels for prolonged periods of time, without the need for continuous
supply of these compounds from the plant [60, 61].
Allelochemicals are mostly a wide variety of secondary metabolites such as
phenolics, cyanogenic glycosides, quinones, lactones, organic acids, and volatile
terpenes. The rich diversity of secondary metabolites evolves because of selection
for improved defense mechanisms against a broad range of microbes, herbivores,
and plants. These allelochemicals affect the other organisms by inducing a secondary
oxidative stress via producing reactive oxygen species (ROS). This as a result,
increases antioxidant enzyme activities and synthesis of molecular antioxidants
(glutathione, ascorbate, tocopherol, (À)-catechin). The allelochemical from root
exudates of Centaurea maculosa triggers a wave of reactive oxygen species (ROS)
in Arabidopsis thaliana, which leads to a Ca2 + signaling cascade triggering
genome-wide changes in gene expression and, ultimately, death of the root system
[11, 62]. Similarly, a study on Lactuca sativa suggested that β-cembrenediol
10
K. G. Ramawat and S. Goyal
presence of a suitable host and chemical signal released by the hosts (Table 1)
[55]. Therefore, chemical cue plays an important role in life cycle of these parasitic
plants and shows co-evolution of chemical biology of host and parasite (see
▶ Chap. 5, “Field Dodder: Life Cycle and Interaction with Host Plants” in this
book). More information will be available with development of sensitive tools of
molecular biology showing gene expression in parasites.
4
Competition for Survival
Plants, cultivated crop plants and weeds, compete with each other in field for
available resources for survival. Rhizosphere is a biologically active soil zone
where different plant roots compete for water, nutrients, and space. Here the roots
communicate with neighboring plants and symbiotic and pathogenic organisms by
the release of root exudates. It is suggested that these exudates play an active role in
root-root and root-microbe communication by manipulating the biological and
physical interactions between these interacting organisms. These chemicals can
change the physical, chemical, and biological properties of the soil and inhibit the
growth of competing plant species [56]. Thus, these exudates mostly play a role of
phytotoxins in the process of allelopathy.
Allelopathy was defined as influence of one plant on another through releasing of
chemicals into the environment [57]. Although the definition of allelopathy is not
confined to positive or negative aspect in particular, most of the studies are done on
the harmful effect of allelochemicals/phytotoxins. Plants release phytotoxins in
decomposing plant tissue, in green leafy volatiles, in leachates from live tissue,
and in root exudates [58, 59]. Sometimes these phytotoxins change the chemistry of
the soil for quite a long time. For example, the phenolic compounds and constituent
diterpenes from Cistus ladanifer L. exudates are toxic and harmful to germination
and growth of herbaceous plants. These allelochemicals are incorporated into the soil
through leaching of leaves and litter, whereas the flavonoids enter the soil through
litter degradation. The long retention of these compounds in litter will maintain their
phytotoxic levels for prolonged periods of time, without the need for continuous
supply of these compounds from the plant [60, 61].
Allelochemicals are mostly a wide variety of secondary metabolites such as
phenolics, cyanogenic glycosides, quinones, lactones, organic acids, and volatile
terpenes. The rich diversity of secondary metabolites evolves because of selection
for improved defense mechanisms against a broad range of microbes, herbivores,
and plants. These allelochemicals affect the other organisms by inducing a secondary
oxidative stress via producing reactive oxygen species (ROS). This as a result,
increases antioxidant enzyme activities and synthesis of molecular antioxidants
(glutathione, ascorbate, tocopherol, (À)-catechin). The allelochemical from root
exudates of Centaurea maculosa triggers a wave of reactive oxygen species (ROS)
in Arabidopsis thaliana, which leads to a Ca2 + signaling cascade triggering
genome-wide changes in gene expression and, ultimately, death of the root system
[11, 62]. Similarly, a study on Lactuca sativa suggested that β-cembrenediol
10
K. G. Ramawat and S. Goyal